Tyler's Astronomy Glossary (F-N)
Kaluza-Klein theory
"(a) Class of theories incorporating extra curled-up dimensions, together with quantum mechanics. [G99]
Isotones
"(a) Nuclei with the same number of neutrons but with different A and Z numbers. [H76]
Lutetium
"A silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. Lutetium is a very rare lanthanoid and has few uses.
Iridium
"A white transition metal that is highly resistant to corrosion. It is used in electrical contacs, in spark plugs, and in jewelry.
Giga-
A prefix meaning 109. [H76]
kayser
A wavenumber. (= cm-1) [H76]
Fermi Interaction
A weak interaction causing beta-decay. [H76]
Lambda Boo Stars
A-type stars with weak metallic lines, low rotational velocity and low radical velocity. [JJ95]
Missing Matter
Alternate term for dark matter. [F88]
Gauge Field
An energy field that permits a gauge symmetry. (See gauge symmetry.) [LB90]
Green's Theorem
An identity that connects line integrals and double integrals. [H76]
Keyhole Nebula
An old name for Eta Carinae. [H76]
Harmonic Overtone
Any integral multiple of the fundamental frequency (q.v.). [H76]
Harmonic Oscillator
Any oscillating particle in harmonic motion. [H76]
IRAS Samples
Astronomical objects detected in infrared radiation by the Infrared Astronomical Satellite launched in 1983. Infrared light has longer wavelengths than visible light. [LB90]
Line Wings
Broad "wings" that appear on either side of a spectral line when the number of atoms producing the line is very great. [H76]
Large-Scale Motions
Bulk motions of distant galaxies deviating from the Hubble flow. see Hubble Law [LB90]
Height
Elevation above ground or distance upwards from a given level (especially sea level) to a fixed point. (See altitude.) [S92]
Heat of Formation
Energy which would be required to form a molecule from dissociated atoms. If positive, the structure will not be formed spontaneously. Lower heats of formation indicate more stable molecules, which are formed preferentially. [SEF01]
Nuclear Statistical Equilibrium
Equilibrium with respect to strong and electromagnetic interactions. [H76]
FP
Fabry-Perot [LLM96]
FK
Fundamental Katalog [LLM96]
IPCS
Image Photon Counting System. A form of very low light level detector used in astronomy. By means of an image intensifier the IPCS is capable of counting individual photons of light. [McL97]
H and K Emission Line Stars
Late objects (F4 to M), which exhibit emission features in their H and K lines of Ca II. [JJ95]
MS Stars
M-type stars with ZrO bands. [H76]
MIR
Mid Infra-Red
Megaflops
Millions of floating-point operations per second. A computer benchmark. [McL97]
MMT
Multi-Mirror Telescope. [LLM96]
NASDA
National Air and Space Development Agency (Japan). [LLM96]
NBS
National Bureau of Standards (USA). [LLM96]
NOAO
National Optical Astronomy Observatory (USA). [LLM96]
Radio Galaxy Optical Types.
Qs, N, cD, db, d, E. [H76]
Hyperfine Structure
Splitting of spectral lines due to the spin and consequent magnetic moment of an atomic nucleus. It can be observed only at very high resolution. [H76]
Fibrilles
Striations or streaks which are observed to form whirls in the Solar chromosphere. [H76]
kinetic energy
The energy associated with motion; the work that must be done to change a body from a state of rest to a state of motion, equal to 1/2 mv2 for a body of mass m moving at velocity v. [Silk90]
Far Field
The field of a pulsar beyond the velocity-of-light radius. [H76]
Merger
The formation of a galaxy from the collision of two or more separate galaxies. [C95]
Ganymede (J III)
The largest satellite of Jupiter. Radius 2635 km (slightly larger than Mercury). Mass about 1.65 × 1026 g; period 7.155 days; e = 0.0015. [H76]
Fermi Constant
The parameter that fixes the strength with which the weak force couples to particles of matter in Fermi's original theory of the weak interaction. [D89]
Nadir
The point on the celestial sphere diametrically opposite to the zenith. [S92]
Galactic Equator
The primary circle defined by the central plane of the Galaxy. [H76]
Moment of Inertia
The product of the mass of a body and the square of its radius of gyration. [H76]
Non-Relativistic Zone
The region far from a pulsar where r M. [H76]
Hydrodynamics
The study of how gases and fluids flow under applied forces. [LB90]
Lifetime
The time it takes for a sample of identical particles to decay to 1/e of its initial population (e 2.718). A related concept is "half-life", being the time it takes for the number of particles to halve. Half-life, 1/2, is related to lifetime, , by 1/2 = (ln 2). [CD99]
M-Theory
Theory emerging from the second superstring revolution that unites the previous five superstring theories within a single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its detailed properties have yet to be understood. [G99]
Higgs Physics
This is the combined physics that explains the origin of the Higgs field, the reason the Higgs mechanism applies, and the properties and study of the Higgs bosons. [K2000]
Hypercharge
Twice the charge of a charge multiplet (q.v.). [H76]
Lyrae
see Vega [H76]
Neutron Star
"(a) A dead, collapsed star that consists mostly of neutrons and is only about 20 kilometers across. Neutron stars are much denser than white dwarfs. [C95]
Interferometer
"(a) A device for observing the interference of waves of light or similar emanations caused by a shift in the phase or wavelength of some of the waves. [F88]
Lepton
"(a) A fermion which is not made of quarks. [C97]
Ideal Gas
"(a) A nondegenerate gas in which the individual molecules are assumed to occupy mathematical points and to have zero volume, and in which the mutual attraction of neighboring molecules is zero. (also called Perfect Gas) [H76]
Field
"(a) A physical quantity, like the electric or magnetic field, which varies from point to point in space. [D89]
Magellanic Stream
"(a) A strand of gas spanning 300,000 light-years that the Milky Way has ripped out of the Magellanic Clouds. [C95]
Geometrodynamics
"(a) A theory which attempts to attribute all physical phenomena to properties of spacetime. (word coined by John Wheeler) [H76]
Normal Modes
"(a) All the characteristic frequencies of an oscillating body. [H76]
Luminosity Distance
"(a) Any distance to a celestial object which has been calculated using a standard candle. [C97]
Maxwell's Equations
"(a) Equations governing the varying electric and magnetic fields in a medium. [H76]
Messier Catalogue
"(a) List of the locations in the sky of more than 100 galaxies and nebulae, compiled by Charles Messier between 1760 and 1784. Some designations he originated are still used in identification; M1 is the Crab Nebula (in Taurus). [A84]
Halo
"(a) Nebulous quality round a celestial body (particularly round a red giant); the galactic halo, however, describes the spherical collection of stars forming a surrounding ""shell"" for our otherwise compact, discoid Galaxy. [A84]
Loschmidt Number (constant)
"(a) Number of molecules of an ideal gas per unit volume (2.687 × 1019 molecules per cm3). [H76]
Meteorite
"(a) Object that enters the Earth's atmosphere and is too large to be totally destroyed by friction before it hits the surface. Meteorites may in some way be connected with asteroids. see also Meteor [A84]
North Polar Sequence
"(a) Or circumpolar stars, comprises those stars which never set, from the viewpoint of an observer on Earth. [A84]
Families
"(a) Organization of matter particles into three groups, with each group being known as a family. The particles in each successive family differ from those in the previous by being heavier, but carry the same electric and nuclearforce charges. [G99]
Milky Way
"(a) Our own galaxy, the second largest in the local group. [A84]
Flatness Problem
"(a) Poses the question: Why, out of an infinite number of possibilities, is our Universe so close to the one special case: the ""flat"" Universe? [C97]
Helium Abundance
"(a) Presence - and dominance - of helium atoms in the Universe. The fact that about 8% of all atoms are helium can be traced, through the -- theory, to the primordial big bang. [A84]
Hysteresis
"(a) The ability to follow two different branches of states, as a parameter built in the system varies first in a monotonic fashion and subsequently comes back to its initial value by varying in the opposite direction. [D89]
Luminescence
"(a) The emission of radiation from a substance in which the particles have absorbed energy and gone into excited states. They then return to lower energy states with the emission of electromagnetic radiation. If the luminescence persists after the source of excitation is removed it is called phosphorescence; if not, it is called fluorescence.
Gravitino
"(a) The fermion partner of the graviton predicted by the supergravity extension of Einstein's theory of general relativity. [D89]
New Inflation
"(a) The first fully successful version of the inflationary theory, discovered independently by Andrei Linde in the Soviet Union, and by Andy Albrecht and Paul Steinhardt in the U.S. see Inflation; False Vacuum; Decay of the False Vacuum [G97]
Frequency
"(a) The frequency of a periodic or harmonic motion which repeats itself in equal time units is the number of oscillations or cycles per unit of time. Its unit is the hertz (Hz). [D89]
Hadron
"(a) The generic name for any particle which experiences the strong nuclear force. [CD99]
Nucleon
"(a) The generic name for the proton and the neutron. [CD99]
Interval
"(a) The quantity in Minkowski space-time which replaces length in ordinary space. [D89]
Mascons
"Abbreviated form of mass concentrations: apparent regions on the lunar surface where gravity is somehow stronger. The effect is presumed to be due to localized areas of denser rock strata. [A84]
Numbers (Dirac):
"Big Number Hypothesis: In 1937 Dirac pointed out that the ratio of the largest to the smallest natural units of length, of force and of time, each came to about 1040; a number which he called a cosmological constant; they have subsequently been often referred to as big numbers. Thus
Horizon Distance
"the maximum distance, at any given time, that a light signal could have travelled since the beginning of the Universe. [D89]
Newton's Laws
(1) A body remains in a state of rest or uniform motion when left to itself. (2) The net force on a body is equal to the product of its mass and its acceleration. (3) When two bodies interact, the force on the first due to the second is equal and opposite to the force on the second due to the first. [H76]
Hawking's Theorem
(1) A stationary black hole must be either static (i.e., nonrotating) or axisymmetric. (2) In interactions involving black holes, the surface area of the event horizon can never decrease. [H76]
Kirchhoff's laws
(1) To each chemical species there corresponds a characteristic spectrum. (2) Every element is capable of absorbing the radiation which it is able to emit; this is the phenomenon of the reversal of the lines. [H76]
Greenwich Sidereal Date
(GSD) The number of sidereal days elapsed at Greenwich since the beginning of the Greenwich sidereal day that was in progress at Julian date 0.0. [S92]
Half-Power Beamwidth
(HPBW) The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system. [H76]
Lattice
(a) A regular solids are characterized by the arrangement of the atoms on a set of regularly spaced points known as the lattice sites. [D89]
Jeans Length
(a) The critical wavelength (J = cs ( / G0)1/2, where cs is the isothermal sound speed in the medium) at which the oscillations in an infinite, homogeneous medium become gravitationally unstable. Any disturbance greater than the Jeans length will decouple by self-gravitation from the rest of the medium to become a stable, bound system. In general, J 1020 cm. [H76]
Gaussian Gravitational Constant
(k = 0.01720209895) the constant defining the astronomical system of units of length (astronomical unit), mass (solar mass) and time (day), by means of Kepler's third law. The dimensions of k2 are those of Newton's constant of gravitation: L3M-1T-2. [S92]
Frame Transfer
A CCD construction in which one half of the imaging area of the device is purposely covered with a mask opaque to light to provide a temporary charge storage section. [McL97]
Hierarchical Cosmology
A cosmology characterized by a system of clusters within clusters within clusters. [H76]
Lowell's Band
A dark border sometimes found on the Martian polar cap. [H76]
Feautrier's Method
A difference-equation method of solving transfer equations. [H76]
J-file
A group of lines of a supermultiplet having a common lower level. [H76]
Light Clock
A hypothetical clock that measures elapsed time by counting the number of round-trip journeys completed by a single photon between two mirrors. [G99]
Normalization
A mathematical technique for eliminating divergent terms or for making them converge. [H76]
Noether's Theorem
A mathematical theorem that states that for every symmetry of the Lagrangian of a physical system (i.e. for every set of transformations under which the Lagrangian is invariant), there will be some quantity which is conserved by the dynamics of the system. [CD99]
Mass Absorption Coefficient
A measure of the fraction of radiation absorbed at a certain wavelength per unit mass. [H76]
Julian Year
A period of 365.25 days. This period served as the basis for the Julian calendar. [S92]
Kerr solution
A solution to Einstein's equations describing the gravity produced by a spinning point of mass. [LB90]
Image Dissector Scanner
A specialized television camera used as a light detector (instead of a photographic plate) in the 19705. [LB90]
Free Spectral Range
A term used in spectrometers to indicate the wavelength interval between occurrences of the same wavelength produced in the next order of interference or diffraction. [McL97]
Gödel's Theorem
A theorem discovered and proved by the mathematician Kurt Gödel in 1931. Gödel's theorem says that there are certain propositions in each branch of mathematics (such as arithmetic or geometry) whose truth or falsity cannot be proven using methods and results only from that branch of mathematics. [LB90]
Hamiltonian Theory
A theory for calculating the trajectory of a particle under an applied force. Hamiltonian theory, developed in the nineteenth century, is equivalent to Newton's laws of mechanics but is reformulated in a mathematically elegant way to allow easier solutions to some problems. [LB90]
Galilean Telescope
A type of refracting telescope having a converging objective and a diverging eyepiece. The Galilean arrangement produces an upright final image. However the field of view is smaller than that of the Keplerian telescope (which gives an inverted image). The arrangement is used in cheap binoculars (opera glasses). The normal adjustment of this telescope has the intermediate image in the focal planes of both lenses. These are therefore separated by a distance fO - fE (New Cartesian sign convention). This was the first kind of telescope to be used in astronomy. With its aid, Galileo (1610) discovered the four largest moons of Jupiter. [DC99]
Minute of Arc
A unit of angle equal to 1/60 of a degree. [LB90]
Megaparsec
A unit of distance equal to a million parsecs, or 3.2616 million light-years. [C95]
Micron
A unit of length equal to 10-4 cm, or 104 Angstroms. [H76]
Longitudinal Wave
A wave vibrating along the direction of propagation - e.g., a sound wave. [H76]
Inverse Bremsstrahlung
Absorption (free-free absorption) of a photon by an electron in the field of a nucleus.[H76]
Nernst Theorem
All substances have zero entropy at 0 K. (Third law of thermodynamics)[H76]
Faraday Effect
An effect occurring in HII regions in which a magnetic field causes a change in the polarized waves passing through (see Faraday Rotation). [H76]
K electron
An electron in the K shell. [H76]
Image Tube
An electronic camera in which electrons, emitted from a photocathode surface exposed to light, are focused electronically onto a phosphor or photographic plate. (also called image intensifier) [H76]
Noise Source
An electronic device designed to generate known amounts of radio noise in order to test and calibrate the receivers of radio telescopes. [H76]
M32
An elliptical galaxy that orbits the Andromeda galaxy. [C95]
North America Nebula
An emission nebula (NGC 7000) in Cygnus. [H76]
f-Electron
An orbital electron whose l quantum number is 3. [H76]
Helium-Weak Stars
B-type stars in which the helium lines are weaker than in normal stars. Also called Bp helium-weak stars. [JJ95]
Kuiper bands
Bands in the spectra of Uranus and Neptune at wavelengths of 7,500 Å, indicating the presence of methane. [A84]
Granules
Convective cells (about 1000 km in diameter) in the solar photosphere. Each granule lasts about 5 minutes on the average and represents a temperature roughly 300° higher than the surrounding dark areas. At any one time, granules cover about one-third of the solar photosphere. [H76]
Fraunhofer Diffraction
Diffraction observed with incident parallel light. In Fraunhofer diffraction the wavefronts are parallel. Although a special case of Fresnel diffraction, it is far more important in most practical cases. Thus it is used to explain single - and multiple - slit patterns, as well as those produced by circular holes. [DC99]
Isomer Shift
Displacement of an absorption line due to the fact that the absorbing nuclei have a different s-electron density from that of the emitting nuclei. [H76]
Hydrogen-Deficient Early-Type Stars
Early type stars of type O, B or A in which the hydrogen lines are very weak or absent. [JJ95]
Many-Worlds
Everett-Wheeler interpretation of quantum mechanics: The view of quantum mechanics holding that a physical system simultaneously exists in all of its possible states prior to and after a measurement of the system. (compare with the copenhagen interpretation of quantum mechanics.) In the many-worlds interpretation, each of these simultaneous existences is part of a separate Universe. Every time we make a measurement of a physical system and find it to be in a particular one of its possible states, our Universe branches off to one of the Universes in which the system is in that particular state at that moment. The system, however, continues to exist in its other possible states, in parallel Universes. see Copenhagen Interpretation of Quantum Mechanics; Quantum Mechanics [LB90]
FOC
Faint Object Camera (Hubble Space Telescope). [LLM96]
Fano-Noise
Fano-noise-limited CCDs used for X-ray detection are limited by intrinsic noise due to the absorption of energy and vibration of the crystal lattice itself. [McL97]
Lyman Alpha Clouds
Gas lying between us and quasars that absorbs some of the radiation from those quasars. see Quasars [LB90]
Metal-Rich
Having a high metallicity with respect to olar. [C95]
HAC
Hydrogenated Amorphous Carbon
Multiverse
Hypothetical enlargement of the cosmos in which our Universe is but one of an enormous number of separate and distinct Universes. [G99]
Infrared Slavery
Inability of quarks to escape the bonds of the strong force that confines them to the company of other quarks. [F88]
Low Surface Brightness Galaxy
LSBG -- A galaxy which is very faint because it contains a very limited number of stars. [C97]
Local Thermodynamic Equilibrium
LTE -- The assumption that all distribution functions characterizing the material and its interaction with the radiation field at a point in the star are given by thermodynamic equilibrium relations at local values of the temperature and density. [H76]
LHEA
Laboratory for High Energy Astrophysics
LIGO
Large Interferometric Gravitational Wave Observatory
MCMC
Markov Chain Monte Carlo -- Iterative simulations such as Markov Chain Monte Carlo make it possible to fit complex and more realistic Bayesian models to large and/or incomplete datasets.
NELG
Narrow Emission Line Galaxy
NLXG
Narrow Emission Line X-ray Galaxy
NLR
Narrow Line Region
NLRG
Narrow-Line Radio Galaxy
NASA
National Aeronautics and Space Administration US government body set up in 1958, under which the Space Center at Houston, Texas, and the Space Center at Cape Canaveral, Florida, are responsible for manned and unmanned space flights. [A84]
NIR
Near InfraRed
Neutrino Process
Nucleosynthesis induced by neutrinos. It may create fluorine and boron. [C95]
Hermetic
Of or relating to Hermes Trismegistus, a mythical philosopher beloved of the Neoplatonists and usually identified with ancient Egypt. [F88]
Keller-Meyerott opacity
Opacity taken from the tables derived by G. Keller and R. Meyerott for assumed mixtures of hydrogen, helium, and heavy elements ranging from X = 99%, Y = 1%, Z = 0 to X = 0, Y = 50%, Z = 50%. [H76]
North Star
Polaris. [C95]
Missing Mass Problem
Poses the question: why does the Universe seem to have much more mass in it than can be seen with a telescope? Dynamical and theoretical constraints place the proportion of missing mass to be somewhere between 90-99 per cent of the total mass of the Universe. [C97]
Halley's Comet
Probably the best known of all comets. Its orbit was computed by Edmund Halley in 1704, at which time he predicted that the bright comet of 1682 would return in 1758 (Halley died in 1742, before he could see his prediction verified). Records of Halley's comet (a = 17.8 AU, e = 0.967, i = 162°.3, P = 76.2 yr perihelion distance 0.587 AU) have been traced back to 240 B.C. Last appearance 1910, next appearance 1986. [H76]
Geometric Albedo
Ratio of the flux received from a planet to that expected from a perfectly reflecting Lambert disk of the same size at the same distance at zero phase angle (cf. Bond albedo). [H76]
High-Energy Physics
See particle physics. [F88]
Induced Emission
See stimulated emission. [H76]
Manganese Stars
Stars with an anomalously high mn-fe ratio, which show deviations from the odd-even effect for phosphorus, gallium, and yttrium. [H76]
K edge
The absorption edge of the K shell (see absorption edges). [H76]
Many-Body Problem
The difficulty of calculating the interactions - e.g., the newtonian gravitational interactions - of three or more objects. [F88]
Maxwell-Boltzmann Distribution
The distribution function that any species of particle will have if it is in thermodynamic equilibrium. This distribution function describes both the equilibrium in velocity space or kinetic energy, and the equilibrium in potential energy. [H76]
Large-Scale Structure
The distribution of galaxies and other forms of mass on large distance scales, covering hundreds of millions of light years and larger. A perfectly homogeneous and isotropic Universe would have no large-scale structure; a Universe with all the galaxies lined up in single file would have enormous large-scale structure. [LB90]
Joule Dissipation
The heat produced when a current is passed through an electrically resisting medium. [H76]
Local Hypothesis (of quasars)
The hypothesis that quasars are not at the distances inferred from their redshifts. [H76]
Miranda
The innermost satellite of Uranus, discovered by Kuiper in 1948. P = 1d10h, diameter about 500 km. [H76]
K shell
The innermost shell, or energy level, of an atom. All elements heavier than hydrogen have a filled K shell, which consists of two 1s electrons orbiting the nucleus. [H76]
Fechner's Law
The intensity of a sensation increases as the logarithm of the stimulus. (see Pogson's Ratio.) [H76]
Geodetic Coordinates
The latitude and longitude of a point on the Earth's surface determined from the geodetic vertical (normal to the specified spheroid). (See zenith; latitude, terrestrial; longitude, terrestrial.) [S92]
Gould Belt
The local system of stars and gas within about 300 pc of the Sun. It is a belt inclined about 10°-20° to the galactic plane in which the greatest concentration of naked-eye O and B stars occurs. [H76]
K magnitude
The magnitude derived from observations at 2.2 microns. [H76]
Host Computer
The main or master computer in an instrumentation system. The computer responsible for interacting with the user. [McL97]
Mean Solar Day
The mean length of time (24h00m00s) between two successive culminations of the Sun (i.e., the mean period from apparent noon to apparent noon). [H76]
Mean Life
The mean time before decay of a large number of similar particles. It is equal to 1.44 times the half-life. [H76]
Granulation
The mottled appearance of the solar photosphere, caused by gases rising from the interior of the Sun (see granules). [H76]
Galilean Transformation
The non-relativistic method of relating observations from one frame of reference to another. [C97]
Israel-Robinson Theorem
The only locally stationary empty space-time which is asymptotically flat with a nondegenerate event horizon is the |a| < m Kerr metric, where a is angular momentum per unit mass. [H76]
Kramers's opacity
The opacity of stellar material derived by Kramers, who in 1923 carried out theoretical calculations of stellar opacity as a function of chemical composition. [H76]
Keplerian orbit
The orbit of a spherical particle of a finite mass around another spherical particle, also of finite mass, by virtue of the gravitational attraction between them. In the Bohr-Sommerfeld picture of atoms, the electrons are considered as describing Keplerian orbits in the field of the positive nucleus by virtue of the inverse square electric attraction between the electrons and the nuclei. [H76]
Nereid
The outer satellite of Neptune (radius 150-250 km). Period about 360 days direct. It has the most eccentric orbit (e = 0.76) of any natural satellite. Discovered by Kuiper in 1950. [H76]
Geocorona
The outermost part of Earth's atmosphere, a hydrogen halo extending out to perhaps 15 Earth radii, which emits Lyman- radiation when it is bombarded by sunlight. [H76]
Mesosphere
The part of Earth's atmosphere immediately above the stratosphere, where the temperature drops from about 270 K to 180 K. [H76]
Moiré Fringes
The pattern obtained when two regular sets of lines or points overlap. The effect can be seen through folds in netting drapes. Moiré patterns can be used as models of interference patterns. Another application is in comparing two diffraction gratings by superimposing them and observing the moir° pattern produced. [DC99]
kurtosis
The peakedness or flatness in the graphical representation of a statistical distribution. [H76]
Month
The period of one complete synodic or sidereal revolution of the Moon around the Earth; also a calendrical unit that approximates the period of revolution. [S92]
Isostasy
The plasticity of the surface layer of a planet - e.g., the ability of the surface layer to adjust its level according to the load (such as ice caps) that it has to carry. [H76]
Metallicity Gradient
The progressive change in metallicity from the center of a galaxy to its edge. A galaxy exhibiting a metallicity gradient is more metal-rich at its center than at its edges. [C95]
Monodromy
The property that all paths of points of a body simply rotating about an axis shall return into themselves. [H76]
Mean Profile
The relatively stable curve obtained by synchronously averaging together many pulses of a pulsar together. (also called Integrated Profile or Pulse Window [H76]
Harkins's Rule
The rule that atoms of even atomic number are more abundant than those of odd atomic number. [H76]
Mean Distance
The semi-major axis of an elliptic orbit. [S92]
Nucleus (of a comet)
The stellar-appearing frozen core, containing almost the entire cometary mass, in the head of a comet. [H76]
Invisible Astronomy
The study of celestial objects by observing their radiation at wavelengths other than those of visible light. [F88]
Neutron Lifetime
The time it takes an isolated neutron at rest to disintegrate into other elementary particles, about equal to 15 minutes. [LB90]
Forbidden Energy Gap
The unoccupiable interval of electron energy levels which forms in a crystalline substance (that is one having a periodic atomic formation) between the valence band (bound electrons) and the conduction band (free electrons). The forbidden energy gap is very large in insulators, non-existent in metals, and small but finite in a semiconductor. [McL97]
Line Width
The width of a spectral line in wavelength terms. The usual measure is the half-width of the line. see Monochromatic Radiation [DC99]
Gravity Waves
Traveling waves of energy that transmit a gravitational force whose strength is changing in time. Gravitational waves are analogous to the electromagnetic waves that transmit the electromagnetic force. [LB90]
Future Light Cone
see Light Cone [H76]
Fireball
see Meteor; see also Primeval Fireball. [H76]
Local Arm
see Orion Arm [H76]
Flocculus
see Plage [H76]
Gauge Group
the mathematical group associated with a particular set of gauge transformations. [D89]
Hawking Radiation
the radiation produced by a black hole when quantum effects are taken into account. It can be viewed as a type of virtual pair production in which one of the particles falls through the event horizon of the black hole and hence cannot escape to rejoin its partner. [D89]
Neutral Current
weak interaction where no change takes place in the charges of the participants. [D89]
Flat Universe
"(a) A Universe in which there is no curvature to the spacetime continuum. This means that the kinetic energy of the expansion is exactly balanced by the potential gravitational energy of the matter. Thus, after an infinite amount of time the Universe will stop expanding. [C97]
Hyperon
"(a) A baryon with non-zero strangeness. [CD99]
Metastable State
"(a) A condition of a system or body in which it appears to be in stable equilibrium but, if disturbed, can settle into a lower energy state. For example, supercooled water is liquid at below 0°C (at standard pressure). When a small crystal of ice or dust (for example) is introduced rapid freezing occurs. [DC99]
Magnetic Monopole
"(a) A hypothetical particle that carries an isolated north or south magnetic pole. This is in contrast to magnets which are north-south pole pairs. If magnetic monopoles exist, they must be very massive. [CD99]
Manifold
"(a) A mathematical concept used to describe the geometry of spacetime. [H76]
Kapteyn's Star
"(a) A nearby star discovered in 1897 by Jacobus Kapteyn. It is a red subdwarf that lies 13 light-years away and is both the nearest halo star to the Sun and the nearest star that orbits the Galaxy backward. (HD 33793, CD -45°1841) [C95]
Lyman Series
"(a) A series of lines in the ultraviolet spectrum emitted by excited hydrogen atoms. They correspond to the atomic electrons falling into the lowest energy level and emitting energy as radiation. The wavelength () of the radiation in the Lyman series is given by 1/ = R(1/12 - 1/n2), where n is an integer and R is the Rydberg constant. see also Spectral Series [DC99]
Mercury
"(a) A transition metal that is liquid at room temperatures (bromine is the only other element with this property). The vapor is very poisonous. Mercury is used in thermometers, special amalgams for dentistry, scientific apparatus, and in mercury cells.
Fermi
"(a) A unit of length equal to 10-13 cm. [H76]
Microwave
"(a) An electromagnetic wave (in the radio region just beyond the infrared) with a wavelength of from about 1 mm to 30 cm (about 109-1011 Hz). [H76]
Meson
"(a) Any particle made of two quarks. Examples are the pion (pi-meson) and the kaon. [C97]
Invariant
"(a) Any physical property which does not change under the transformation from one frame of reference to another. [C97]
Gregorian Calendar
"(a) Calendar established with the authority of the Roman Catholic Church by Pope Gregory XIII in 1582. Correcting at a stroke the 10-day accumulated margin of error of the Julian calendar, the main difference was in fact that century years were discounted as leap years unless they were divisible by 400. [A84]
Gluon
"(a) Carrier of interquark force. Plays a role in QCD analogous to that played by the photon in QED. [D89]
Gold
"(a) Element with atomic number 79. It is produced entirely by the r-process, in supernovae. [C95]
Mars
"(a) Fourth major planet out from the Sun. [A84]
Ionized
"(a) Having lost one or more electrons from an atom. [McL97]
Geocentric
"(a) Having the Earth at the centre. [A84]
Glueball
"(a) Hypothesized form of matter consisting entirely of gluons. [D89]
Magic Number
"(a) In nuclear physics, one of several numbers of neutrons or protons that characterize especially stable atomic nuclei. They are 2, 8, 20, 28, 50 and 82. [DC99]
Fusion
"(a) In nuclear physics, the combining of the atomic nuclei of lighter elements to form nuclei of a heavier element. Such a process involving the atomic nuclei of elements lighter than iron is accompanied by the emission of energy; for fusion of heavier elements, energy must be supplied. The process is thought to contribute to the condensation of stars from interstellar gas and dust. (see also Nuclear fusion) [A84]
Initial Condition
"(a) In physics, the state of a system at the time at which a given interaction begins - e.g., the approach of two electrons that are about to undergo an electromagnetic interaction.
Local Standard of Rest
"(a) LSR -- An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the Local Standard of Rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the Local Standard of Rest around the Galaxy is about 220 kilometers per second. [C95]
LSR
"(a) Local Standard of Rest -- An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the Local Standard of Rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the Local Standard of Rest around the Galaxy is about 220 kilometers per second. [C95]
Ionization
"(a) Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one. High temperature is particularly conducive to ionization. [A84]
Higgs Field
"(a) Mechanism operating in symmetry-breaking events; in electroweak theory, the Higgs field is said to have imparted mass to the W and Z particles. [F88]
Longitude, Terrestrial
"(a) On Earth, distance east or west of Greenwich, England, measured along lines drawn parallel to the equator. [F88]
Gravity
"(a) One of the four fundamental forces of nature, and the one most different from the other three. [C97]
GeV
"(a) One thousand million electron-volts. Sometimes called BeV: B for billion (US variety). [D89]
Look-Back Time
"(a) Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago. [F88]
Gravitational Radiation
"(a) Propagating waves of gravitational tidal force that are emitted by dynamical systems such as collapsing stars or binary star systems, and move with the speed of light. [D89]
No Hair Theorem
"(a) Proposed by John Wheeler, it states that the only properties of matter conserved after entering a black hole are its mass, its angular momentum and its electrical charge; it thus becomes neither matter nor antimatter. [A84]
Isotropy
"(a) Quality of being the same in all directions. Compare anisotropy. [F88]
Nutation
"(a) Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial ""bulge"". [A84]
Noise
"(a) Sound composed of a random mix of different frequencies. White noise is a completely random mix over a wide frequency range; it has a confusing effect on the listener. Pink noise - random frequencies in a selected range - is often used as a background to mask other sounds.
Fluctuations
"(a) Spontaneous deviations of the macroscopic variables from a certain `reference' state, arising from the thermal motion and the interactions of the molecules. [D89]
N Star
"(a) Stars of spectral type N are very red stars similar to M stars except that bands of C2, CN, and CH are present instead of those of TiO. N stars are strongly concentrated toward the Galactic plane. [H76]
Late Type Stars
"(a) Stars of spectral type later than the sun (G2). [JJ95]
Gregorian Telescope
"(a) Telescope devised - but never constructed - by James Gregory, in which an auxiliary concave mirror reflects the magnified image, the right way up, through a hole in the centre of the main objective mirror, i.e., through the end of the telescope itself. The Cassegrain telescope is similar but produces an inverted image. [A84]
Libration
"(a) The ""turning"" of the Moon so that although the same face is presented to Earth at all times, the overall surface of the Moon visible is 59% of the total. Libration is described as latitudinal, longitudinal and diurnal. [A84]
Hubble Time
"(a) The Hubble time is one divided by the Hubble constant, which gives a number from 10 to 20 billion years. For a flat universe with no cosmological constant, the age of the universe is two-thirds of the Hubble time. [G97]
Joule
"(a) The SI unit of energy, work, or quantity of heat. 1 J is equal to a force of 1 newton acting over a distance of 1 meter. 1 J = 107 ergs. [H76]
Fluorescence
"(a) The absorption of energy by atoms, molecules, etc., followed by immediate emission of electromagnetic radiation as the particles make transitions to lower energy states. Fluorescence is a type of luminescence in which the emission of radiation does not persist after the exciting source has been removed. The excited states have very short lifetimes.
Fomalhaut
"(a) The brightest star in the constellation Piscis Austrinus, Fomalhaut is a white A-type main-sequence star 21 light-years away. [C95]
Nucleus
"(a) The central part of an atom, composed of protons and neutrons (which are made of quarks) and containing nearly all of each atom's mass. [F88]
Gauss
"(a) The cgs unit of magnetic flux density. 1 gauss = 1 maxwell per square centimeter = 10-4 tesla. [H76]
Jeans Mass
"(a) The critical mass a volume of space must contain before it will collapse under the force of its own gravity. [C97]
Ionization Potential
"(a) The energy required to remove an electron from an atom or molecule to form a positive ion. [DC99]
Geoid
"(a) The equipotential surface (""mean sea level"") of Earth's gravitational field. [H76]
Magnetosphere
"(a) The extent of a planet's magnetic field. the earth's magnetosphere is shaped roughly like a teardrop, with the point opposite the sun; this is due to the effect of the Solar Wind. [A84]
Local Group
"(a) The gravitationally bound collection of nearby galaxies ruled by the Andromeda Galaxy and the Milky Way, which are its largest members. The Local Group has about 30 known galaxies. [C95]
Light Cone
"(a) The history, in space-time, of a light flash. [D89]
Inflation
"(a) The idea that, when it was a fraction of a second old, the universe expanded dramatically. If inflation is correct, then the mass density of the universe () should be 1.0, if there is no cosmological constant; if there is a cosmological constant and inflation is correct, the sum of and the cosmological constant () should be 1.0. [C95]
Fringes
"(a) The light and dark bands obtained by interference or diffraction of light. [DC99]
Ground State
"(a) The lowest energy state of an atom, molecule, or other system. [DC99]
Mass Defect
"(a) The mass equivalent of the binding energy of a nucleus. See binding energy. [DC99]
Horizon
"(a) The maximum distance that an observer can see. In cosmology, our horizon is the distance from us that light has traveled since the beginning of the universe. Objects more distant than our horizon are invisible to us because there hasn't been enough time for light to have traveled from there to here. [LB90]
Metric
"(a) The metric gives the spacetime interval de between two neighboring events. [H76]
Lunation
"(a) The period of time between two successive new Moons (cf. synodic month). [H76]
Node
"(a) The point at which a standing wave pattern intersects the horizontal axis and at which the wave consequently has zero amplitude. [H76]
Hubble Constant
"(a) The present expansion rate of the universe, in units of kilometers per second per megaparsec. The larger the Hubble constant, the younger the universe. [C95]
Lorentz Invariance
"(a) The principle that physics in an inertial frame is independent of the velocity of the frame relative to any other frame. [D89]
Helicity
"(a) The projection of a particle's spin along its direction of motion. The helicity of a particle is described as being either left-handed or right-handed depending on whether its spin projection is in the direction of motion or against it. [CD99]
Nebula
"(a) The term ""nebula"" was previously applied to all kinds of hazy patches in the sky, many of which are now recognized to be clusters or galaxies. (The plural is nebulae) [H76]
Flavor
"(a) The term used to describe different quark types. There are six quark flavors: up, down, strange, charm, bottom and top. [CD99]
Half-Life
"(a) The time it takes for half of a given quantity of radioactive material to decay. [F88]
Nucleosynthesis
"(a) The transformation of one element or isotope into another. Nucleosynthesis occurred just after the big bang, but today most nucleosynthesis takes place in stars - for example, the Sun presently converts hydrogen into helium. [C95]
Lorentz Transformation
"(a) The transformation which keeps the speed of light invariant between relativistic frames of reference. [C97]
Gravitation
"(a) The universal ability of all material objects to attract each other; F = Gm1m2 / r2. [H76]
Meridian
"(a) Theoretical north-south line on the Earth's surface, or an extension of that line onto the night sky, connecting the observer's zenith with the celestial pole and the horizon. The meridian is used to state directional bearings. Devices and structures - such as meridian arcs - marking the meridian were once common in observatories. [A84]
Metal
"(a) To an astronomer, a metal is any element heavier than hydrogen and helium; thus, not only are iron and copper metals, but so are elements like oxygen and neon. [C95]
Magellanic Clouds
"(a) Two relatively small, nebulous stellar systems visible only in the southern hemisphere; the larger is, however, the brightest ""nebular"" object in the sky. both are members of the local group of galaxies, and in fact seem to be associated, though detached, parts of the milky way system. [A84]
Neutron
"(a) Uncharged particle in the nucleus of all atoms except hydrogen. Through beta decay, a neutron may become a proton and an electron; the process occurs in reverse during the formation of a neutron star. [A84]
Flux Unit
"(a) Unit of flux density. 1 f.u. = 10-26 watts per square meter per hertz (see Jansky). [H76]
Galaxy
"(a) Vast system of celestial objects, typically consisting of between 106 and 1012 stars, plus interstellar gas and dust. There are three basic types: spiral (further subdivided into normal spirals and spirals with a ""bar"" at the centre, and yet further subdivided according to the ""openness"" of the spiral arms), elliptical (subdivided according to ellipticity) and irregular (subdivided according to whether they are made up of Population I or Population II stars). Another not uncommon type of galaxy is a lenticular form mid-way between the spiral and the elliptical. [A84]
Latitude, Terrestrial
"(b) On Earth, distance north or south on the equator along a line connecting the poles. [F88]
Krypton
"A colorless odorless monatomic element of the rare-gas group, known to form unstable compounds with fluorine. It occurs in minute quantities (0.001% by volume) in air. Krypton is used in fluorescent lights.
Iodine
"A dark-violet volatile solid element belonging to the halogens.
Ghost
"A faint image near the image required, caused by radiation that has taken a different path. In the case of traditional `silvered' glass mirrors, the main images are caused by light from the backing of the glass. Ghost images, formed by reflection from the front surface of the glass, may be a tenth as bright as the main images. Other `ghost' images may arise following reflections inside the glass.
Germanium
"A hard brittle gray metalloid element. Most germanium is recovered during zinc or copper refining as a by-product. Germanium was extensively used in early semiconductor devices but has now been largely superseded by silicon. It is used as an alloying agent, catalyst, phosphor, and in infrared equipment.
Francium
"A radioactive element of the alkali-metal group. It is found on Earth only as a short-lived product of radioactive decay, occurring in uranium ores in minute quantities. A large number radioisotopes of francium are known.
Fermium
"A radioactive transuranic element of the actinoid series, not found naturally on Earth. It is produced in very small quantities by bombarding 239Pu with neutrons to give 253Fm (half-life 3 days). Several other short-lived isotopes have been synthesized.
Mendelevium
"A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several short-lived isotopes have been synthesized.
Lawrencium
"A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several very short-lived isotopes have been synthesized by bombarding 252Cf with boron nuclei or 249Bk with 18O nuclei.
Nobelium
"A radioactive transuranic element of the actinoid series, not found naturally on Earth. Several very short-lived isotopes have been synthesized.
Fluorine
"A slightly greenish-yellow highly reactive gaseous element belonging to the halogens. It is slightly more abundant than chlorine, accounting for about 0.065% of the Earth's crust. Fluorine is the most reactive and most electronegative element known; in fact it reacts directly with almost all the elements, including the rare gas xenon.
Holmium
"A soft malleable silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. It has few applications.
Gallium
"A soft silvery low-melting metallic element. Gallium is used in low-melting alloys, high-temperature thermometers, and as a doping impurity in semiconductors. Gallium arsenide is a semiconductor used in light-emitting diodes and in microwave apparatus.
Indium
"A soft silvery metallic element. It is found in minute quantities, primarily in zinc ores, and is used in alloys, in several electronic devices, and in electroplating.
Neptunium
"A toxic radioactive silvery element of the actinoid series of metals that was the first transuranic element to be synthesized (1940). Found on Earth only in minute quantities in uranium ores, it is obtained as a by-product from uranium fuel elements.
Neodymium
"A toxic silvery element belonging to the lanthanoid series of metals. It occurs in association with other lanthanoids. Neodymium is used in various alloys, as a catalyst, in compound form in carbon-arc searchlights, etc., and in the glass industry.
Hassium
"A transactinide element formed artificially.
Molybdenum
"A transition element used in alloy steels, lamp bulbs and catalysts.
Hafnium
"A transition metal found in zirconium ores. Hafnium is difficult to work and can burn in air. It is used in control rods for nuclear reactors and in certain specialized alloys and ceramics.
Nickel
"A transition metal that occurs naturally as the sulfide and silicate. It is used as a protective coating (on other metals) and in the manufacture of various alloys, such as Nichrome and stainless steel.
Monochromatic Radiation
"Electromagnetic radiation of an extremely narrow range of wavelengths. (The word means `of one color'.) It is impossible to produce completely monochromatic radiation, although the output of some lasers is not far off. The `lines' in line spectra produced even in the most ideal circumstances have some width in wavelength terms. The half-width is the measure used. It is the range of wavelengths defined in the figure, and contains almost 90% of the energy emitted. The half-width of a sharp line in an optical spectrum is typically 10-6 to 10-7 of the wavelength. Using lasers, half-widths of the order 10-12 can be obtained. Low-quantum-energy gamma rays emitted by atoms bound in crystals may have values of the order 10-13 .
HIPPARCOS
"HIgh-Precision PARallax COllecting Satellite. [LLM96]
Ionization Radiation
"Radiation such that the individual particle or quantum has sufficient energy to ionize substances. Electrons with kinetic energy just greater than the ionization potential will cause ionization, but other particles (e.g. molecular positive ions) require higher energies. Gamma-rays and x-rays ionize indirectly by means of the electrons they eject from substances by the photoelectric effect or Compton scattering. Short-wavelength ultraviolet quanta may ionize individual molecules by the photoelectric effect, but the ejected electrons have insufficient kinetic energy to cause further ionization unless an electric field is applied.
Group Theory
"The branch of mathematics which describes symmetry. A mathematical group G is defined as a collection of elements {a, b, c,....} with the properties:
Keplerian telescope (astronomical telescope)
"The most common type of refracting telescope arrangement, consisting of converging objective and eyepiece. Unlike the Galilean telescope, it provides an inverted image and has a greater length. However the field of view is larger and image quality is higher.
Kepler's Laws
(1) Each planetary orbit is an ellipse with the Sun at one focus. (2) (law of areas) Equal areas are swept out in equal times. (3) (harmonic law) The square of the period is proportional to the cube of the distance. Newton's generalized formula for the third law is P2 = 42 a3 / [G (m1 + m2)]. [H76]
Hertz
(Hz) A unit of frequency equal to one cycle (or wave) per second. [F88]
Mean Solar Second
1/86,400 of a Mean Solar May (cf. Ephemeris Second ). [H76]
Lemaître Universe
A big-bang cosmology proposed by the Belgian Abbey Lemaître in 1929 in which the Universe is assumed to have exploded from a primeval "atom". In the Lemaître Universe the rate of expansion steadily decreases. [H76]
Nanosecond
A billionth of a second. [LB90]
Luyten 726-8
A binary (M5.5e V, M6e V) (component B is UV Cet) with a very small mass (total mass of system [1974] 0.3 M ). It is about 2.7 pc distant. [H76]
Hulse-Taylor Pulsar
A binary pulsar discovered in 1974, probably consisting of a neutron star and an even more compact object in an eccentric orbit, with an orbital period of 0.3230 days and a pulsation period of 59 milliseconds. (PSR 1913+16) [H76]
Gray Body
A body whose emissivity is constant and less than unity. [H76]
Getter
A chemical absorption method of removing (pumping) gas from a chamber by tying up molecules on a surface. Activated charcoal. Molecular sieve. See zeolite. [McL97]
Heavy-Metal Stars
A class of peculiar giants that includes the Ba II stars and the S stars. [H76]
Henry Draper system
A classification of stellar spectra into the sequence O, B, A, F, G, K, M, in order of decreasing temperature. [H76]
17 Leporis
A close binary system (A0 V, M1 III) with a shell-like spectrum indicating that mass transfer may be occurring from the late-type companion onto the A0 primary. [H76]
Molecular Clouds
A cloud of interstellar gas and dust that consists mostly of molecular hydrogen. [C95]
Free-Fall
A collapse in which gas clouds do not hit or impede one another. According to ELS (Eggen, Lynden-Bell & Sandage), the Galactic halo formed in a free-fall collapse. [C95]
Interstellar Cloud
A collection of gas and dust that lies between the stars. [C95]
High-Luminosity Early Type Objects
A collective designation for some early type stars with very peculiar spectra, like S Dor and P Cyg. [JJ95]
Hubble nebula
A cometary nebula whose apex star is R Mon. (NGC 2261) [H76]
Hourglass Nebula
A compact H II region in the center of M8. [H76]
4-kpc Arm
A component of the Sagittarius arm with non-circular gas motions. (see 3-kpc Arm) [H76]
Giant Branch
A conspicuous sequence of red stars with large radii in the H-R diagram of a typical globular cluster that extends from the main-sequence turnoff point upward and redward to the red-giant tip. [H76]
Honeycomb Mirrors
A construction method for a large mirror in which the back is hollowed-out to leave a ribbed structure that resembles a honeycomb. [McL97]
Kleinmann-Low nebula
A cool (< 600 K) extended infrared source in the Orion Nebula, about 1' NW of the Trapezium and about 12" south of the BN nebula, discovered in 1967. It dominates the infrared emission at 20 µ, and a CO cloud is centered on it. It is probably a collapsing cloud of 102-103 M in which protostars are embedded. (KL nebula) [H76]
Global Inertial Frame
A coordinate system or frame of reference anchored with respect to the overall distribution of matter in the universe. [Silk90]
Local Inertial Frame
A coordinate system or frame of reference defined in the vicinity of the earth in which Newtons first law of motion is valid; that is, a nonrotating and nonaccelerating reference frame. [Silk90]
Faint Blue Galaxy
A distant, irregularly shaped galaxy in which a large amount of star formation is taking place. [C97]
Mizar
A double star in Ursa Major. [A84]
Fornax System
A dwarf spheroidal galaxy, about 190 kpc distant, in the Local Group (Mv -12, M 2 × 107 M). [H76]
Guillotine Factor
A factor that measures the sharp reduction in the opacity of a gas when the temperature is high enough to have ionized the atoms down to their K shells. [H76]
Mean Sun
A fictitious body that moves eastward in a circular orbit along the celestial equator, making a complete circuit with respect to the vernal equinox in a tropical year. [H76]
Finesse
A figure of merit for the reflectance of a Fabry-Perot etalon. It is given by R / (1 - R) where R is the reflectance (R < 1). [McL97]
Monoceros Loop
A filamentary loop nebula about 1 kpc distant, the remnant of a supernova that occurred about 300,000 years ago. [H76]
Maclaurin Spheroid
A form which a homogeneous self-gravitating mass can take when in a state of uniform rotation. its eccentricity varies from zero (when it is not rotating) to 1 in the limit of infinite angular momentum. [H76]
Klein-Gordon equation
A fundamental equation of relativistic quantum field theory. [G99]
Markarian Galaxy
A galaxy in Markarian's list of galaxies with abnormally strong ultraviolet continua. they have broad emission lines arising in a bright, semi-stellar nucleus. Markarian 231 is the most luminous galaxy known if it is at its Hubble distance. [H76]
Irregular Galaxy
A galaxy with amorphous structure and with relatively low mass (108-1010 M). Fewer than 10% of all galaxies are classified as irregular. [H76]
Golay Cell
A gas bulb used to detect infrared radiation. [H76]
Multi-Dimensional Hole
A generalization of the hole found in a doughnut to higher-dimensional versions. [G99]
Frequency Standard
A generator whose output is used as a precise frequency reference; a primary frequency standard is one whose frequency corresponds to the adopted definition of the second (see Second; Système International), with its specified accuracy achieved without calibration of the device. [S92]
Gum Nebula
A giant H II region 30°-40° in diameter in which the Vela pulsar and the Vela X supernova remnant are embedded. It appears to be a fossil Strömgren sphere produced by an outburst of ionizing radiation that accompanied the Vela X supernova remnant. (Named for the Australian astronomer Colin Gum.) [H76]
Fermi-Dirac-Sommerfeld Law
A law which gives the algebraic number of a quantized system of particles which have velocities within a small range. [H76]
N-Magnitude
A magnitude derived from observations made at a wavelength of 10 microns. [H76]
Goldstone Boson
A massless spin-0 particle which arises whenever a (continuous) global symmetry is spontaneously broken. [CD99]
Lumogen
A material used as a down-converter. [McL97]
Lattice Gas
A model of a condensed system in which atoms may be present on or absent from the sites of a lattice, but no movement of the sites or distortion of the lattice is allowed.[D89]
Hierarchical Clustering Model
A model of galaxy clustering in which different patterns appear at different scales of distance and in which the "average" density of matter depends on the size of the volume over which the average is performed. In a homogeneous model, on the other hand, the average density is independent of the size of the volume over which the average is performed. (See pancake model.) [LB90]
Helix Nebula
A planetary nebula about 140 pc distant in Aquarius with the largest known angular diameter of any planetary. (NGC 7293) [H76]
Line Profile
A plot of intensity versus wavelength across a spectral line. [H76]
North Galactic Pole
A point in the constellation Coma Berenices where we look perpendicular to and above the Galactic Plane. The nearest bright star to the North Galactic Pole is Arcturus, in the neighboring constellation Bootes. [C95]
Femto
A prefix meaning 10-15. [H76]
milli-
A prefix meaning 10-3. [H76]
micro-
A prefix meaning 10-6. [H76]
nano-
A prefix meaning 10-9. [H76]
kilo-
A prefix meaning 103. [H76]
mega-
A prefix meaning 106. [H76]
Filament
A prominence seen in projection on the Solar disk. [H76]
Inelastic Collision
A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system. [H76]
Hoyle-Narlikar Theory
A reformulation of the general theory of relativity that incorporates and extends Mach's principle (q.v.). In this theory, the inertial mass of a particle is a function of the masses of all other particles, multiplied by a coupling constant which is a function of cosmic epoch. In cosmologies based on this theory, the gravitational constant G decreases strongly with time. [H76]
Instability Strip
A region in the Hertzsprung gap (q.v.) occupied by pulsating stars in a post-main-sequence stage of stellar evolution. Stars traverse the instability strip relatively quickly at least once on their way to their final evolutionary configuration. [H76]
Lindblad Resonance
A resonance hypothesized by Lindblad in the 1920s in his attempt to explain the existence of spiral arms (see Density Wave Theory). It is a resonance which occurs when the frequency at which a star encounters the galactic spiral wave is a multiple of its epicyclic frequency. The inner Lindblad resonance occurs whenever the ratio of the frequency of the radial oscillation to that of the rotational motion around the center of the galaxy (in a frame of reference rotating together with the spiral pattern) is 2:1. [H76]
Mariner Spaceprobes
A series of US spaceprobes launched to explore the planets of the Solar System, particularly Mercury, Venus and Mars. [A84]
Meteoroid
A small particle orbiting the Sun in the vicinity of Earth. [H76]
Geodetic Precession
A small, relativistic, direct motion of the equinox along the ecliptic, amounting to 1".915 per century. [H76]
Nuclide
A species of atomic nucleus, analogous to the word "isotope" for a species of atom. The word is also used to distinguish between atomic nuclei that are in different energy states. [H76]
K
A spectral line in the X-ray region ( = 0.334 Å), produced by the transition between the lowest level of the K shell and the lowest level of the L shell. [H76]
K line
A spectral line of singly ionized calcium at 3933 Å (see Fraunhofer lines). [H76]
Galactocentric Distance
A star's distance from the Galactic center. The Sun's Galactocentric distance is about 27,000 light-years. [C95]
Metastable
A state which is not stable, but which lives long enough to have significance, is called metastable. [G97]
Frequency Distribution
A statistical arrangement of numerical data according to size or magnitude (see also Distribution Function). [H76]
Gaussian Distribution
A statistical distribution defined by the equation p = c exp(-k2x2), in which x is the statistical variable. It yields the familiar bell-shaped curve. Accidental errors of measurement and similar phenomena follow this law. [H76]
Metagalaxy
A synonym for the Universe. [H76]
Hypergalaxy
A system consisting of a dominant spiral galaxy surrounded by a cloud of dwarf satellite galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are hypergalaxies. [Silk90]
Jodrell Bank
A telescope in England designed to detect radio waves emitted by astronomical objects. [LB90]
Kelvin scale
A temperature scale with the same divisions as the Celsius (centigrade) scale and with the zero point at 0° absolute. (Room temperature is about 295 K). [H76]
Forward (or Reverse) Bias
A term applied to an electronic device known as a diode - usually formed by a junction of p-type and n-type semiconductor material - in which current flows easily if the externally applied voltage has the correct polarity or direction. If the opposite polarity is applied there is almost no current flow and the device is said to be reverse biased. [McL97]
Implicate Order
A term coined by the physicist David Bohm to describe the sort of enfolded order that is characteristic of quantum theory. It is to be contrasted with the explicate orders of Newtonian physics. Bohm believes that this implicate order has a universal importance and will be useful in understanding the nature of consciousness. [P88]
Franck-Condon Principle
A theoretical interpretation of the relative intensity of vibrational transitions in an electronic band on the assumption that the intense transitions correspond to situations where an endpoint in the lower vibrational level is vertically below the corresponding endpoint in the upper vibrational level. [H76]
Nordtvedt Effect
A theoretical violation of the principle of equivalence for massive, self-gravitating bodies. [H76]
Mass-Luminosity-Radius Relation
All nondegenerate stars with the same mass and the same chemical composition will have the same radius and the same luminosity. see Vogt-Russell Theorem [H76]
Field Galaxy
An isolated galaxy which does not belong to any cluster of galaxies. The ratio of galaxies in clusters to field galaxies is about 23:1. [H76]
Metallicity
An object's abundance of metals. In practice, this usually means the abundance of iron, which is easy to measure. [C95]
Herbig Ae, Be Stars
Be or Ae stars associated with nebulosity. [JJ95]
K capture
Capture by a nucleus of one of the electrons in its innermost shell, accompanied by the emission of X-rays. [H76]
Matter Waves
Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See wave-particle duality. [F88]
N-body Simulations
Computer simulations of the behavior of a large number of bodies under their mutual interactions. In cosmological N-body simulations, the bodies are usually galaxies and the interactions are gravitational. Thus, the computer simulates how a group of galaxies should behave under their mutual gravitational attraction. The law of gravity and the initial positions and velocities of the hypothetical galaxies and other masses are fed into the computer. The computer then calculates the evolution of the system. [LB90]
Isochoric
Constant volume. [H76]
Mira Variables
Cyclic variables with cycles 100-500 days, and of spectral types K, M, S and C. (Also called long-period variables.) [JJ95]
Lunar Phases
Cyclically recurring apparent forms of the Moon. New Moon, First Quarter, Full Moon, and Last Quarter are defined as the times at which the excess of the apparent celestial longitude (see Longitude, Celestial) of the Moon over that of the Sun is 0°, 90°,180°, and 270°, respectively. [S92]
Landau Damping
Damping caused by electrons that are moving at the phase velocity of the wave. It is analogous to a surfer who will be carried along by a wave if he is already moving at the velocity of the wave when it hits him. [H76]
Limb Darkening
Decrease in, and reddening of, the optical brightness of the Sun or other star from its center to its limb. [H76]
Interstellar Square Law
Decreasing as one over distance squared (1/r2), where r is the distance from the source. Light and gravity both have this property. [McL97]
Intelligence
Defined in SETI as the ability and willingness to transmit electromagnetic signals across interstellar space. [F88]
Grotrian Diagram
Energy-level diagram. [H76] LEVEL5: Grotrian Diagrams
Nautical Mile
Equals 1.15 statute miles. [F88]
Flop Transition
Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and acceptable physical consequences in the context of string theory. [G99]
Full Width at Half-Maximum
FWHM -- The full width of a spectral line at half-maximum intensity. [H76]
FIR
Far Infrared
FIRAS
Far-Infrared Astronomical Spectrometer [LLM96]
FIRST
Far-Infrared Space Telescope [LLM96]
FFT
Fast Fourier Transform [LLM96]
Non-Perturbative
Feature of a theory whose validity is not dependent on approximate, perturbative calculations; an exact feature of a theory. [G99]
FOV
Field of View
FTP
File Transfer Protocol
Morgan's Classification
First, the galactic spectral type a, af, f, fg, g, gk, k (corresponding to the integrated stellar types); then the form type S (spiral), B (barred spiral), E (elliptical), I (irregular), Ep (elliptical with dust absorption), D (rotational symmetry without pronounced spiral or elliptical structure), L (low surface brightness), N (small bright nucleus); finally a number from 1 (face-on) to 7 (edge-on). The Andromeda Galaxy (M31) is kS5. [H76]
FIFO
First-In-First-Out buffer. Used in many microprocessor-controlled systems to smooth the flow of data. [McL97]
FITS
Flexible Image Transport System A method for saving image data which has become standard in astronomy. [McL97]
Flux Density
Flux of radiation through a unit surface; the strength of an electromagnetic wave, defined as the amount of power incident per unit area. In radio astronomy, the brightness temperature integrated over the solid angle of the source yields the flux density. [H76]
Longitude of the Perihelion
For a Solar System body, the longitude of the ascending node plus the angle along the orbit from the node to the perihelion point. () [H76]
Mass-Radius Relation (Chandrasekhar)
For any given mass less than the Chandrasekhar limit, there is a unique radius for a totally degenerate star. [H76]
Kennelly-Heaviside layer
Former name for the D and E layers (q.v.). [H76]
Fossils
Geological remains of what was once a living thing. [F88]
Non-Euclidean Geometry
Geometry that does not follow the postulates and results of Euclidean geometry. For example, in a non-Euclidean geometry, the sum of the interior angles of a triangle differs from 180 degrees. According to Einstein's general relativity theory, gravity distorts space into a non-Euclidean geometry. [LB90]
Heterotic String
Gross's version of string theory in which space-times of different dimensions are associated with the same closed loop. [P88]
HRS
High-Resolution Spectrograph (Hubble). [LLM96]
HST
Hubble Space Telescope. A space-based reflecting telescope with a primary mirror diameter of 2.4 m (94 in) capable of high-resolution imaging from the far ultraviolet to the near infrared. A joint NASA/ESA mission. Launched in 1990 with a planned lifetime of 15 years. Encountered reduced performance when the mirror was found to have spherical aberration. Solved by the installation of corrective optics (COSTAR) in 1994. [McL97]
h
Hubble's constant in units of 100 km s-1 Mpc-1. [H76]
IBC
Impurity Band Conduction. [LLM96]
Hardness Ratio
In high-energy (x-ray and gamma-ray) astronomy the hardness ratio is defined as the number of counts in a high-energy band minus those in a lower-energy band, normalized by the sum of the counts in both bands. For X-ray sources the hardness ratio can provide a rough indication of the source temperature, spectral index, or associated column density. Terms such as "hard", "soft" and "supersoft" are often used in the literature to qualitatively characterize the hardness ratio. [BFM2004]
Neumann Lines
In iron meteorites, groups of very fine parallel lines that cross each other at various angles. Irons containing Neumann lines can easily be cleaved in three mutually perpendicular directions. [H76]
Minimum Resolvable Angle
In radians, 1.22 divided by the aperture of the telescope. [H76]
ISAS
Institute of Space and Astronautical Science.
Heliometer
Instrument to measure the apparent diameter of the Sun at different seasons, also used to measure angular distances between stars. [A84]
Fission, Nuclear
Interaction in which nucleons previously united in an atomic nuclei are disjoined, releasing energy. Fission powers "atomic" bombs. Compare with Fusion. [F88]
IDL
Interactive Data Language. [LLM96]
IF
Intermediate Frequency. The beat frequency between the signal and the local oscillator in a radio detection system. [McL97]
ILR
Intermediate Line Region
IAU
International Astronomical Union. [LLM96]
IUE
International Ultraviolet Explorer. [LLM96]
Luminous
Intrinsically bright, as opposed to being just apparently bright. [C95]
LIDAR
LIght Detection And Ranging [LLM96]
LED
Light-Emitting Diode -- A semiconductor diode, made from certain materials (e.g. Gallium Arsenide), in which light is emitted in response to the forward-bias current. The light results from the recombination of electrons and positive holes, with a transition to a lower energy state. see also Diode [DC99]
Isophotes
Lines connecting points of equal light intensity. [H76]
Nebular Lines
Lines that originate from forbidden transitions. [H76]
Median
Literally the middle value in a sequence of values arranged in increasing size order. A useful mathematical estimator of the true value from a set of values when one of these values is contaminated, i.e. known to be much larger than the average. [McL97]
LAN
Local Area Network -- A means of interlinking computers. [McL97]
LTE
Local Thermodynamic Equilibrium -- The assumption that all distribution functions characterizing the material and its interaction with the radiation field at a point in the star are given by thermodynamic equilibrium relations at local values of the temperature and density. [H76]
LPV
Long-Period Variables -- Pulsating disk-population red giants or supergiants with periods of 100 to 1000 days (Population I typically have periods greater than 200 days; Population II, periods less than 200 days). Typical is Mira (o Ceti), which has a period of 331 days. Long-period variables may vary by as much as 9 magnitudes in the visible, but in the integrated spectrum (most of their radiation is in the infrared) they vary by only 2 or 3 magnitudes. They are usually of spectral type M, R, or N. (sometimes called Red Variables) [H76]
LUT
Look-Up Table [McL97]
LSBG
Low Surface Brightness Galaxy
LERG
Low-Excitation Radio Galaxy
LINER
Low-Ionization Nuclear Emission-line Region
LBG
Lyman Break Galaxy
LEDA
Lyon/Meudon Extragalactic Database
MACHO
Massive Compact Halo Object -- These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter. [C97]
Non-Baryonic Matter
Material that consists of exotic subatomic particles. These subatomic particles can move slowly (cold dark matter) or fast (hot dark matter). Cosmologists who believe in inflation - or anyone else who thinks the mass density of the Universe () is 1.0 - believe that most of the Universe consists of non-baryonic matter. [C95]
Nuclear Matter
Matter in which the numbers of protons and neutrons are roughly equal, as in atomic nuclei. Nuclear matter is probably in a liquid or a solid state. [H76]
Intergalactic Gas
Matter that is present in the region between the galaxies. It has been detected in considerable amounts in great clusters of galaxies, where the intergalactic gas is so hot that it emits copious amounts of x-radiation. In several groups of galaxies, including the Local Group, the evidence for the presence of intergalactic gas is controversial; clouds of atomic hydrogen may be present. [Silk90]
MEM
Maximum Entropy Method: An image reconstruction methodology which defines a measure of information content and seeks to maximize it. [McL97]
Geodesy
Measurement and study of the Earth's size and shape. [A84]
Mer-Cad-Tel
Mercury-Cadmium-Telluride (HgCdTe) -- A semiconductor alloy useful as an infrared photoelectric detector. Also known as CMT (Cad-Mer-Tel) [McL97]
MIS
Metal Insulator Semiconductor. [LLM96]
Mass Models
Models that attempt to infer the distribution of mass in an astronomical system by comparing the observed properties of the system (such as the distribution of light) with those properties predicted by various theoretical distributions of mass. [LB90]
Inflationary Cosmology
Modification to the earliest moments of the standard big bang cosmology in which universe undergoes a brief burst of enormous expansion. [G99]
MTF
Modulation Transfer Function [LLM96]
Meinel Bands
Molecular bands of the N2+ radical near 8000 Å. [H76]
Hydrocarbon
Molecule which contains only hydrogen and carbon. Type of organic molecule. [SEF01]
H I
Neutral hydrogen gas. It emits radio waves that are 21 centimeters long. [C95]
NGC
New General Catalogue -- A catalogue of 7,840 nebulae, star clusters, and galaxies that was published in 1888 by John Dreyer. [C95]
NTT
New Technology Telescope [LLM96]
Newton's Constant G
Newton's law of gravitation says that the gravitational for ce between any two bodies is proportional to the product of their masses and decreases as the square of the distance between them. This statement is turned into an equation by inserting the constant G, so the force F = Gmm'/r2. Because all of the particles feel the gravitational force, G is universal, so G can be used to form quantities with dimensions, giving the Planck scale. G is measured by finding the force between two objects of known masses separated by a known distance. [K2000]
NEC
Noise Equivalent Charge [LLM96]
NEF
Noise Equivalent Flux [LLM96]
NEP
Noise Equivalent Power [LLM96]
Free (of a particle)
Not bound to a nucleus. [H76]
Fermi-Dirac Nuclei
Nuclei of odd A-number (i.e., nuclei that do not have integral spin) (cf. Bose-Einstein nuclei). Fermi-Dirac nuclei therefore obey the Exclusion Principle. [H76]
4N Nuclei
Nuclei possessing equal and even numbers of neutrons and protons. 4N nuclei are formed in supernova envelopes at temperatures of at least 2 × 109 K and are very stable. (or Even-Even Nuclei) [H76]
Lunar
Of the Moon. [A84]
Force Carriers
Particles that act as the transmitters of forces. The best known example is the photon, which transmits electromagnetic forces. The gluons are the transmitters of the strong interactions, and the W+, W-, and Z0 particles are the transmitters of the weak interactions. [G97]
Lithium Stars
Peculiar giant stars (spectral types G-M) whose spectra show high abundances of lithium. They are primarily S stars and carbon stars, although Li is also found in T Tauri stars, and is sometimes observed in normal late-type giants. Interstellar Li / H 2 x 10-10. (Solar System Li / H 10-9.) Lithium is destroyed rapidly (in about 7500 years) at typical nuclear burning temperatures. Spallation is the only production mechanism known for 6Li, but 7Li can be transported from the core in the form of 7Be and converted in the envelope to 7Li by electron capture. Observed 7Li / 6Li > 10. [H76]
Fundamental Constants
Physical quantities, like the speed of light or the mass of an electron, that enter into the laws of physics in a basic way and are believed to be the same at all times and everywhere in the Universe. Most physicists take the fundamental constants of nature as given properties of the Universe. [LB90]
Hubble Diagram
Plot of galaxy redsifts against their distances. This was the first evidence of the expansion of the universe. [F88]
Indeterminacy Principle
Quantum precept indicating that the position and trajectory of a particle cannot both be known with perfect exactitude. Indeterminacy thus indicates the existence of a basic quantum of knowledge of the particle world. And, since information about one quantity can be extracted at the expense of another, it demonstrates that the answer events result to some extent from the questions we choose to ask about them. [F88]
Magnetic-Dipole Radiation
Radiation emitted by a rotating magnet. [H76]
Gyrosynchrotron Radiation
Radiation emitted by mildly relativistic electrons. [H76]
g-factor
Ratio of a particle's magnetic moment to its spin angular momentum. [H76]
f Number
Ratio of the focal length to the diameter of a lens. [H76]
F Region
Region of the ionosphere above the F layers. [H76]
Kirkwood gaps
Regions in the asteroid zone which have been swept clear of asteroids by the perturbing effects of Jupiter. They were named for the American astronomer Daniel Kirkwood. [H76]
Mini-Quasar
Sandage's term for the blue nucleus of an N galaxy. According to Sandage, N galaxies can be understood as ordinary giant elliptical galaxies with "mini-quasars" embedded in them. [H76]
Geology
Scientific study of the dynamics and history of the earth, as evidenced in its rocks, chemicals, and fossils. [F88]
Great Looped Nebula
See 30 Doradus. [H76]
Gem
See Castor. [H76]
YY Gem
See Castor. [H76]
Krzeminski's star
See Cen X-3. [H76]
Heaviside Layer
See E layer. [H76]
Harvard Classification
See Henry Draper system.
K corona
See K component. [H76]
Harmonic Law
See Kepler's third law. [H76]
KL nebula
See Kleinmann-Low nebula. [H76]
Gem
See Pollux. [H76]
Galactic Tide
See tide. [C95]
Gravity Darkening
See von Zeipel's theorem. [H76]
Interstellar Lines
Sharp, distinct absorption lines superposed on stellar spectra, produced by the interstellar gas located between the source and the observer. Strongest are the D lines, followed by the H and K lines, and the K I doublet at 7699 and 7644 Å.
Interstellar Grains
Small needle-shaped particles in the interstellar gas with dimensions from 10-6 to 10-5 cm. They are primarily composed of silicates and strongly absorb, scatter, and polarize visible light at wavelengths comparable to their size, reemitting the light in the far-infrared region of the spectrum. The amount of visual extinction is wavelength-dependent and leads to a dimming and reddening of starlight. [Silk90]
Fines, Lunar
Small particles of rock or powdered rock on the Moon. [H76]
Forbidden Lines
Spectral lines emitted from metastable states, or those which have a very low probability (10-9-10-10) of occurrence. They appear at particle densities 108 cm-3. All forbidden lines have low excitation potentials. Forbidden lines are designated by enclosing them in brackets, e.g., [O II]. [H76]
Intercombination Lines
Spectral lines emitted in transitions between two levels with different values of S. [H76]
K
Spectral type for orange stars, such as Arcturus, Aldebaran, and Alpha Centauri B. K type stars are somewhat cooler than the Sun. [C95]
Natural Selection
Tendency of individuals better suited to their environment to survive and perpetuate their species, leading to changes in the genetic makeup of the species and, eventually, to the origin of new species. see Evolution [F88]
Lunisolar Precession
That component of general precession that is caused by the gravitational coupling between the Moon and the Earth and between the Sun and the Earth. Lunisolar precession causes the equinox to move westward along the ecliptic about 50" per year (cf. planetary precession). [H76]
Horizontal Branch
That part of the H-R diagram of a typical globular cluster that extends shortward from the asymptotic branch at an approximately constant absolute bolometric magnitude of about 0.3. A star appears on the horizontal branch after it has undergone the helium flash and begins to burn helium quietly in its core and hydrogen in a surrounding envelope. [H76]
Gravitational Mass
That property of matter which makes it create a gravitational field and attract other particles (cf. inertial mass; equivalence principle). [H76]
Microwave Background
The 2.7 degree Kelvin radiation that pervades the Universe and is believed to be the afterglow of the Big Bang. [C95]
M31
The Andromeda galaxy, the largest member of the local group. It is a giant spiral galaxy that lies 2.4 million light-years away. [C95]
Inner Lagrangian Point (L1)
The Lagrangian point (q.v.) through which mass transfer occurs. [H76]
M42
The Orion Nebula, a star-forming region in the constellation Orion. [C95]
M33
The Pinwheel Galaxy, the third largest member of the Local Group, after Andromeda and the Milky Way. It is a spiral galaxy that lies 2.6 million light-years away. [C95]
M45
The Pleiades, a beautiful open star cluster in the constellation Taurus. It is 410 light-years away. [C95]
Galactic Latitude
The angle between the line of sight to a star and the Galactic plane. Galactic latitude ranges from +90 degrees to -90 degrees; the Galactic plane has a Galactic latitude of 0 degrees. Regions north of the Galactic plane have positive Galactic latitude; regions south have negative Galactic latitude. The point with a Galactic latitude of +90 degrees is called the north Galactic pole, and the point with a Galactic latitude of -90 degrees is called the south Galactic pole. [C95]
Isoplanatic Patch
The angular region on the sky over which the wavefront correction applied by an adaptive optics system remains valid. It is relatively small and therefore a nearby reference star is also required. [McL97]
Huyghenian Region
The brightest portion of the Orion Nebula. [H76]
Mean Equator and Mean Equinox
The celestial reference system determined by ignoring small variations of short period in the motions of the celestial equator. Thus the mean equator and equinox are affected only by precession. Positions in star catalogs are normally referred to the mean catalog equator and equinox (see Catalog Equinox) of a standard epoch. [S92]
Maxwell
The cgs unit of magnetic flux through 1 cm2 normal to a field of 1 gauss. 1 Mx corresponds to 10-8 Wb. [H76]
Lyman Alpha Line
The characteristic spectral line of atomic hydrogen associated with its lowest excited state. The corresponding wavelength is 1216 Å in the far ultraviolet, and so the Lyman alpha line can only be studied from spacecraft or in the spectra of highly redshifted quasars. [Silk90]
Inverse Compton Effect
The collision between a photon and an energetic (cosmic-ray) electron, in which some of the energy of the electron is transferred to the photon. [H76]
Line Blanketing
The combined effects of spectral lines upon the emergent energy distribution from and the temperature distribution in a stellar or planetary atmosphere. [H76]
Funneling
The concentration of stars from different parts of the main sequence in the red-giant region. [H76]
Gravitational Equilibrium
The condition in a star in which at each point the weight of the overlying layers is balanced by the total pressure. [H76]
Jump Conditions
The conditions for jumps in pressure and density (or temperature or energy) across a shock wave. These are the Hugoniot conditions. [H76]
Littrow
The configuration of a diffraction grating spectrograph in which the diffracted ray returns along the same direction as the incident ray. [McL97]
Landé Factor
The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1. [H76]
Inner Bremsstrahlung
The continuous electromagnetic radiation that accompanies the -decay of nuclei. [H76]
Kelvin contraction
The contraction of a star contemplated by Kelvin and Helmholtz as a consequence of a star's radiating its thermal energy. It is currently believed that the contraction of a star occurs in this manner in its pre-main-sequence evolution. (also called Kelvin-Helmholtz contraction) [H76]
Missing Mass
The cosmic mass that some scientists hypothesize so that the Universe will have the critical density of matter, with an exact balance between gravitational energy and kinetic energy of expansion. Such mass is called missing because it represents about 10 times as much mass as has actually been detected. see Closed Universe; Critical Mass Density; Dark Matter [LB90]
Long Distance Scale
The cosmological distance scale which uses a Hubble constant of approximately 50 km/s/Mpc. [C97]
Mixer
The critical element of a radio detection system which allows the incoming wave to be combined with the reference frequency from the local oscillator. Usually a diode. [McL97]
Lenz's Law
The current induced by an electromotive force will appear in such a direction that it opposes the charge that produced it. [H76]
Light Cylinder
The cylinder whose radius is that at which the rotational velocity of a neutron star would equal the speed of light. RL = cT / 2, where RL is the radius of the light cylinder and T is the period. [H76]
Horizontal Parallax
The difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon. [S92]
Immersion
The disappearance of a celestial body due to eclipse or occultation. [H76]
Nucleosynthetic Era
The era following the Leptonic Era, between 1 second and 1,000 seconds after the Big Bang, when neutrons were produced and helium and deuterium were synthesized. [Silk90]
Neutron Excess ()
The excess of neutrons over protons in an atomic nucleus. = (Nn - Np) / (Nn + Np). [H76]
Halation
The formation of a halo around bright star images by light reflected from the back of the photographic plate or emulsion. [H76]
Galilean Satellites
The four largest satellites of Jupiter - Io (J I), Europa (J II), Ganymede (J III), and Callisto (J IV) - discovered by Galileo in 1610. All are locked in synchronous rotation with Jupiter. [H76]
Magnitude of a Solar Eclipse
The fraction of the Solar diameter obscured by the moon at the greatest phase of a Solar eclipse (see Eclipse, Solar), measured along the common diameter. [s92]
Magnitude of a Lunar Eclipse
The fraction of the lunar diameter obscured by the shadow of the Earth at the greatest phase of a lunar eclipse (see Eclipse, Lunar), measured along the common diameter. [s92]
Mass Fraction
The fractional amount (by mass) of a given element or nuclide in a given composition. [H76]
Gyrofrequency
The frequency with which an electron or other charged particle executes spiral gyrations in moving across a magnetic field. [H76]
Hydrogen Burning
The fusion of hydrogen into helium and the process by which all main-sequence stars generate energy. Every star born with more than 0.08 solar masses burns hydrogen. [C95]
Inhomogeneous Early Universe
The idea that during the first few minutes after the big bang, the universe had regions of different density. An inhomogeneous early universe can produce elements different from those of the standard homogeneous early universe. [C95]
K component
The inner part of the Solar corona (the gaseous phase) which emits a continuous spectrum without absorption lines. Physically, the K component results from Thomson scattering of photospheric radiation by free electrons in the corona. The K component is polarized and decreases rapidly with distance from the Sun. (from the German Kontinuum) [H76]
Greenwich Sidereal Day Number
The integral part of the Greenwich sidereal date. [S92]
Nodes, Line of
The intersection between the orbital plane of the Moon or a planet and the plane of the ecliptic. [H76]
Hadron Barrier
The interval (t 10-43 [10-23] s after the big bang, when = 1093 [1052] g cm-3) during which quantum and general-relativistic effects are expected to modify each other in an unknown way. The quantities in brackets are for a different equation of state. [H76]
Hadron Era
The interval (t 10-5 s after the big bang) when the Universe was matter-dominated and when kT m. It was followed by the lepton era (q.v.). [H76]
Nodical Month
The interval of time (27.2122 days) between two successive transits of the Moon through its ascending node. (also called Draconic month)[H76]
Light-Time
The interval of time required for light to travel from a celestial body to the Earth. During this interval the motion of the body in space causes an angular displacement of its apparent place from its geometric place. see Geometric Position; Aberration, Planetary [S92]
Integration Time
The interval of time used to collect photons of light on a detector and build-up a strong signal. [McL97]
Hund's Rule
The larger the value of S (total spin angular momentum), the lower the value of the average perturbation energy <V>SL. [H76]
Main Beam
The lobe of maximum sensitivity in a radio telescope. [H76]
Mass Defect
The mass equivalent of the binding energy of a nucleus. see Binding Energy [DC99]
l-number
The orbital quantum number, which determines the magnitude of an electron's angular momentum. [H76]
Neon Burning
The stage in which a star burns neon into oxygen and magnesium. [C95]
Higgsino
The superpartner of the Higgs boson. [K2000]
H and K lines
The two closely spaced lines of singly ionized calcium at 3968 and 3934 Å, respectively. [H76]
Interpulse
The weaker component of a pulsar pulse when its period is roughly half that of the main pulse. [H76]
2MASS
Two-Micron All-Sky Survey. [LLM96]
Infinities
Typical nonsensical answer emerging from calculations that involve general relativity and quantum mechanics in a point-particle framework. [G99]
Heavy-Fermion Systems
a class of recently discovered materials, usually rare-earth or actinide compounds, in which the `effective mass of the electrons appears to be hundreds or thousands of times the real electron mass. [D89]
Ferromagnet
a material such as iron in which there may be a permanent magnetic moment. In a ferromagnet the spins of the atoms are aligned parallel to one another. [D89]
Information
a measure of the delocalization of the state of the system in the space of all possible events. [D89]
Maclaurin Series
a power series expansion of f(x) of the form f(x) = f(0) + f'(0) x + [f"(0)/2!] x2 + . . . + [f(n) (0)/n!] xn + . . .(= taylor series at x = 0). [H76]
Ising Model
a simplified version of the Heisenberg model in which the atomic spins must be aligned parallel or antiparallel to a given direction. [D89]
Master Equation
an equation describing the evolution of the probability of a state at a given time as the balance between transitions leading to this state, and transitions removing the system from this state. [D89]
Hubble Radius
c/H The radius of the observable universe (> 1027 cm). [H76]
Logistic Equation
models the growth of a population as a competition between self-reproduction on the one side and inhibition arising from density-dependent effects on the other side. [D89]
J Meson
name for the J- or -meson, mass 3 GeV, composed of a charmed quark and charmed antiquark. Its discovery in 1974 instigated a scientific revolution in the later half of the decade. [D89]
KeV
one thousand electron volts. [D89]
Magneton
see Bohr Magneton [H76]
Lambda ()
see Cosmological Constant [C95]
Lambda Term
see Cosmological Constant [LB90]
F Corona
see F Component. [H76]
FDS Law
see Fermi-Dirac-Sommerfeld Law. [H76]
Lensing
see Gravitational Lensing [C95]
Minkowski Space-Time
space and time considered together, with special importance attached to the progress of a light flash, and to the light-cone and the `interval'. [D89]
Liquid Crystal
substances intermediate in their properties between liquids and crystals. There is considerable variety in the type of structure that they can have, but in general they have the anisotropy of a complex crystalline solid but no crystalline long range order, and they can flow like a liquid. [D89]
ft-Values
t = half-life of the -unstable nucleus, and f stands for an integral which depends on the -decay energy and the type of transition. [H76]
Kelvin scale
the `natural' or `absolute' scale of temperature, on which the value of temperature corresponds roughly to the typical thermal energy. The Kelvin temperature is approximately the Celsius temperature plus 273 degrees. [D89]
Meissner Effect
the phenomenon in which a metal cooled through its superconducting transition temperature in the presence of a magnetic field completely expels the field. [D89]
Localisation
the wave-function of an electron is said to be localized if it is confined to a small region of a large system rather than being extended through the system. [D89]
Magnetic Susceptibility
when a magnetic field is applied to a material, magnetisation is induced. The ratio of the magnetisation induced to the applied magnetic field is the magnetic susceptibility. [D89]
False Vacuum
"(a) A region of space that appears to be empty (a vacuum), but actually contains stored energy. When this stored energy is released, the false vacuum is said to decay. (see Vacuum) [LB90]
Fall
A "fall" as opposed to a "find" is a meteorite whose arrival on Earth is witnessed. Stones constitute 92% of the observed falls. [H76]
F
Spectral type for yellow-white stars, which are slightly hotter than the Sun. The brightest F-type stars in Earth's sky are Canopus and Procyon. [C95]
f-Values
see Oscillator Strengths [H76]
F-Spot
see Sunspots [H76]
Fermi Statistics
the form of statistics applicable to fermions. It forbids two particles to occupy the same state. [D89]
Field Rotation
The rotation of a star field about the center which occurs in an alt-az telescope because the motion is not about the polar axis. [McL97]
Field Star
"(a) A star that is not part of any star cluster. Most stars, including the Sun, are field stars. [C95]
Fermion
"(a) Any particle with half-integer spin: 1/2 , 3/2 , 5/2 , etc. All fermions obey Pauli's exclusion principle. [CD99]
FET
Field Effect Transistor A tiny transistor amplifier in which the current flow between two terminals, called the source and the drain, is controlled by the electric field generated inside the silicon by an external voltage on a surface called the gate electrode. {McL97]
Faraday Rotation
Rotation of the plane of polarization of linearly polarized radiation when the radiation passes through a plasma containing a magnetic field having a component in the direction of propagation. [H76]
Fermilab
The Fermi National Accelerator Laboratory, in Batavia, Illinois, USA. Fermilab is the home of the Tevatron, the world's most powerful accelerator, a p collider with a maximum collision energy of 1.8 Te V (= 1800 Ge V = 1.8 × 1012 eV). [CD99]
Fermi Level
The maximum energy of any particle in a group of low-temperature subatomic particles called fermions. Fermions, such as electrons, cannot occupy the same space at the same energy. Thus, if many fermions are placed close together, their energies must all be different. The energy of that particle with the largest energy is the Fermi energy of the system. [LB90]
Fermi's Question
The question of why, if spacefaring extraterrestrial civilizations exist, their representatives haven't visited Farth. [F88]
Field Emission
The release of electrons from a surface as a result of a strong external electric field. Very high electric fields are necessary; these are obtained at sharp points. [DC99]
f-Sum Rule
The sum of the f-values for all the transitions from a given state (positive for absorption and negative for emission) is unity. [H76]
Ion
"(a) A charged particle consisting of an atom, or group of atoms, that has either lost or gained electrons. Sodium chloride (salt), for example, is made up of positive sodium ions - atoms that have each lost an electron - and negative chlorine ions - atoms that have each gained an electron. See ionization. [DC99]
Interference Filter
"(a) A filter used to shut out all light except the desired wavelengths. [H76]
Horizon Problem
"(a) A quandary in standard big bang theory, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion. It appears to have been resolved in the inflationary universe theory. [F88]
Maxwell Distribution
"(a) An expression for the statistical distribution of velocities among the molecules of a gas at a given temperature. [H76]
Neptune
"(a) Eighth major planet out from the Sun, discovered in 1846 by Johann Galle and Louis d'Arrest following predictions calculated by Urbain Le Verrier. Similar predictions had been made a year earlier by John Couch Adams but were not followed up. [A84]
Great Rift
A "split" in the Milky Way between Cygnus and Sagittarius caused by a succession of large, overlapping dark clouds in the equatorial plane of the Galaxy. It is about 100 pc distant. [H76]
M87
A giant elliptical galaxy in the Virgo cluster. [C95]
Harmonic Motion
A motion that repeats itself in equal intervals of time. (also called periodic motion) [H76]
Lommel-Seeliger Surface
A surface with large-scale roughness where shadowing effects are important. [H76]
Law
A theory of such wide and invariable application that its violation is thought to be impossible. [F88]
Gaseous Nebula
An H II region, a supernova remnant, or a planetary nebula. H II regions have an emission-line optical spectrum, and a thermal continuous spectrum declining in intensity as the wavelength increases (from maximum in the ultraviolet) through infrared and radio. Supernova remnants have an emission-line optical spectrum and a nonthermal radio spectrum. Temperatures of planetary nebulae are much higher than those of H II regions. [H76]
h-line
An Mg II resonance line at 2803 Å. [H76]
Field Curvature
An aberration in an optical instrument, common in Schmidt telescopes, in which the light rays come to a focus on a curved surface instead of on a plane. [H76]
Island Universe Hypothesis
Assertion that the sun belongs to a galaxy and that the spiral nebulae are other galaxies of stars, which in turn are separated from one another by vast voids of space. Compare nebular hypothesis. [F88]
Multiplex
Combining many signals into one or a small number of signals. [McL97]
Heliograph
Device for recording the positions of SUNSPOTS. [A84]
Micrometer
Device used in conjunction with a telescope in order to measure extremely small angular distances. [A84]
Hyperion
Eighth satellite of Saturn about 160 km in diameter. P = 21d6h38m. Discovered by Bond in 1848. [H76]
Monotonic
Either continuously increasing or decreasing. [SILK90]
Gluon Lattice
Force field generated by the strong nuclear force that holds quarks together. See gluons. [F88]
GFF
Giga French Francs. [LLM96]
HEASARC
High Energy Astrophysics Science Archive Research Center
M Numbers
Refer to the Messier Catalogue. [A84]
Fundamental Frequency
The lowest characteristic frequency of oscillation of a dynamical system. [H76]
Hubble Flow
The movement of the galaxies away from us caused by the expansion of the Universe. [C97]
kernel
The set of points mapped into zero. [H76]
Near Zone
The zone of a pulsar within the velocity-of-light radius. [H76]
DDO (or van den Bergh) Classification
This contains two parameters: (1) the galactic type (Sa, Sb, Sc, Ir) and (2) the luminosity class (I, II, III, IV, V), similar to the MKK system of stellar luminosity class. The notations S- and S+ are used to denote subgiant species with low and high resolution, respectively. The notation S(B) has been introduced to denote objects intermediate between true spirals and barred spirals. [H76]
F Layers
Two layers in the Earth's ionosphere (F1 and F2 at about 200 and 300 km, respectively) immediately above the E layer. (also called Appleton Layers) [H76]
Lambda Doublet
Two lines in the microwave region of the spectrum of the OH molecule caused by splitting of electronic levels. [H76]
Metric Tensor
the mathematical object that describes the deviation of Pythagoras's theorem in a curved space. [D89]
Isospin
"(a) A quantum number which arises from regarding different members of a charge multiplet (q.v.) as different states of a single particle. (also called Isotopic Spin ) [H76]
Mira
"(a) A red giant in the constellation Cetus that varies in brightness as it pulsates. When brightest, Mira is visible to the naked eye; when dimmest, Mira can be viewed only with optical aid. Mira is the prototype of all pulsating red giants, which are called Miras in its honor. [C95]
Muon
"(a) A second-generation lepton. It is essentially a more massive electron. [CD99]
Line Spectrum
"(a) A spectrum composed of a number of discrete lines corresponding to single wavelengths of emitted or absorbed radiation. Line spectra are produced by atoms or simple (monatomic) ions in gases. Each line corresponds to a change in electron orbit, with emission or absorption of radiation. [DC99]
Galactic Halo
"(a) A spherical aggregation of stars, globular star clusters, and thin gas clouds, centered on the nucleus of the galaxy and extending beyond the known extremities of the galactic disk. [F88]
Nova
"(a) A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for ""new"". see Supernova [F88]
High-Velocity Star
"(a) A star whose U and/or V and/or W velocities are much greater or much less than zero. Such stars usually have eccentric orbits around the Galaxy. [C95]
Gauge Theory
"(a) A theory whose dynamics originate from a symmetry. That is, the formulae describing the theory (in particular, the Lagrangian) are unchanged under certain symmetry transformations, called ""gauge"" transformations. For example, the equations of classical electrodynamics are invariant under local redefinitions of the electrostatic potential. This symmetry is ultimately responsible for the conservation of electric charge. However, in quantum electrodynamics this gauge symmetry is reinterpreted as invariance under local redefinitions of the phase of the electron wave function. The term ""gauge theory"" is an archaic one, coming from earlier theories which were based on invariance under transformation of scale (i.e. gauge). [CD99]
Fabry-Perot Interferometer
"(a) A type of interferometer wherein the beam of light is passed through a series of pairs of partly reflecting surfaces set at various angles to it and spaced at certain prechosen numbers of the wavelength to be examined. It differs from the Michelson interferometer in that it has only one arm. [H76]
Gauge Symmetry
"(a) Abstract mathematical symmetry of a field related to the freedom to re-gauge, or re-scale, certain quantities in the theory (potentials) without affecting the values of the observable field quantities. [D89]
Mössbauer Effect
"(a) After an atomic nucleus has emitted a gamma-ray photon (during radioactive decay), the absorption of the momentum of the atom by the whole of its crystal lattice because it is so firmly bound that it cannot recoil. The same effect occurs with absorption of gamma rays. [DC99]
Force
"(a) Agency responsible for a change in a system. In Newtonian mechanics, gravitational force bends the moon away from the straight trajectory it would otherwise pursue. [F88]
H II Region
"(a) An area of ionized hydrogen. Most H II regions are red and arise from hot blue O and B stars, whose ultraviolet light can ionize all the hydrogen for dozens or even hundreds of light-years in every direction. The most famous H II region is the Orion Nebula. [C95]
Isotope
"(a) An atomic nucleus having the same number of protons as a more commonly found atomic nucleus but a different number of neutrons. For example, the hydrogen nucleus has a single proton; deuterium has one proton and one neutron and would be called an isotope of hydrogen. (see neutron; proton.) [LB90]
Grand Unification
"(a) An attempt to produce a unification of all the forces of nature. While some success was made in unifying the gluon force between quarks with the electroweak force, problems always arose when gravity was included. Grand unification eventually gave way to superstring theory. [P88]
Neutrino
"(a) An electrically neutral, very weakly interacting particle, with a rest energy which is either zero or very small. The particle was predicted in 1931 as a means of reconciling the measurements of beta decays with the conservation of energy, but it was not directly detected until 1956. [G97]
Friedmann Equation
"(a) An equation for the evolution of the Universe. The Friedmann equation can be derived from Einstein's theory of gravity, plus the assumptions that the Universe is homogeneous (looks the same at every point) and isotropic (looks the same in every direction). The solution of the Friedmann equation tells, among other things, how the distance between galaxies changes with time. (see Homogeneity; Isotropy) [LB90]
Irradiation
"(a) An optical effect of contrast that makes bright objects viewed against a dark background appear to be larger than they really are. [S92]
Limb
"(a) Apparent edge of the disk of a Solar System body as projected on the sky. see Terminator [H76]
Main Sequence
"(a) Band that runs from top left to bottom right on the hertzsprung-russell diagram representing the majority of stars. Stars off the main sequence are in some way uncharacteristic and include red giants, blue dwarfs, Cepheids and novae. [A84]
Luminosity
"(a) Brightness of a celestial body, measured in terms of (apparent) magnitude, absolute magnitude, or using the Sun's brightness as 1.0 on a Solar scale. The luminosity of a star corresponds with its internal radiation pressure, which in turn depends on its mass. [A84]
Julian Calendar
"(a) Calendar established by Julius Caesar in 46 BC, which overestimated the duration of the sidereal year by 11 minutes and 14 seconds. It was replaced, from 1582, by the Gregorian calendar, by which time it was inaccurate by a total of 10 days. [A84]
Flare Star
"(a) Dim red dwarf star that suddenly lights up with great - but brief - luminosity, corresponding to an equally powerful but short-lived burst of radio emission. The cause is thought to be a sudden and intense outburst of radiation on or above the star's surface. [A84]
Lorentz Contraction
"(a) Diminution in the observed length of an object along the axis of its motion, as perceived by an external observer who does not share its velocity. [F88]
Light-Year
"(a) Distance traveled at the speed of light after one Earth-year: 9.46 million million km. [A84]
General Relativity
"(a) Specific theory of gravitation in terms of curved space-time developed by Einstein; provides field equations to determine the space-time metric for a given distribution of matter. [D89]
M
"(a) Spectral type for red stars, such as Betelgeuse, Antares, and Proxima Centauri. [C95]
Globular Cluster
"(a) Spherical, densely populated cluster of older stars. There are a number of such clusters round the edge of our Galaxy. [A84]
Flux
"(a) Total radiation going out from the 2 solid angles of a hemisphere. If the radiation is uniform, the flux is 2 times the intensity. Measured in ergs cm-2 s-1. [H76]
Light Elements
"(a) Usually, hydrogen, helium, and lithium, which have atomic numbers of one, two, and three; sometimes also beryllium and boron, which have atomic numbers of four and five. [C95]
Hysteresis Cycle
"A closed loop obtained by plotting the flux density, B, of a ferromagnetic substance against the magnetizing field strength, H. The substance is first brought to magnetic saturation from an unmagnetized state - this produces curve OA (see illustration). As the field strength is taken through one cycle of reductions, reversals, and increases, the curve follows the path ACDEFGA. This is known as a hysteresis loop.
Gadolinium
"A ductile malleable silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. Gadolinium is used in alloys, magnets, and in the electronics industry.
Intensity
"A measure of the rate of energy transfer by radiation. The unit of intensity is the watt per square meter (W m-2).
Interaction
"A mutual effect between two or more systems or bodies, so that the overall result is not simply the sum of the separate effects. There are four separate interactions distinguished in physics:
Froude Number (Fr)
"A non-dimensional parameter, sometimes called a Froude constant, used in fluid dynamics to describe the flow of a fluid in which there is a free surface. It is defined by the relationship Fr = v/(gl)1/2, where Fr is the Froude number, v is the velocity of the liquid, g the acceleration due to gravity and l is a parameter representing length. The flow of an inviscid, incompressible fluid in two geometrically similar flow systems is dynamically similar when their Froude numbers are the same. The number is named after William Froude (1810-1879), who derived it in 1869. It is used extensively by naval architects. In France it is called the Reece number after F. Reece (1805-1884), a French naval officer from Alsace. All Froude's original notebooks, together with those of his son R. E. Froude (1846-1924), are in the Admiralty Establishment in Gosport, Hampshire.
Interline Transfer
A CCD construction consisting of vertical strips which are alternately opaque and light sensitive. The opaque strips conceal charge transfer registers. [McL97]
Milne Cosmological Model
A Friedmann model of the Universe in which matter does not exist. Only radiation is present in a Milne Universe.
k line
A Mg II resonance line at 2795.5 Å. [H76]
Kepler's Supernova
A Type I supernova (SN Oph 1604) whose light reached Earth in 1604. If H0 = 50, then Kepler's supernova is out in the galactic halo at a distance of 12.1 kpc and 1.4 kpc above the galactic plane, according to van den Bergh. Kepler's supernova is the prototype of Type I supernovae; at its brightest it reached an apparent magnitude of about -2.2. (3C 358) [H76]
Hydrostatic Equilibrium
A balance between the gravitational force inward and the gas and radiation forces outward in a star. [H76]
G-band
A band of CH at 4303 Å. It is conspicuous in the spectra of G-K stars. [H76]
False Vacuum Bubble
A bubble which has false vacuum on the inside, and true vacuum on the outside. In principle, the creation of such a bubble offers the possibility of creating a new Universe in a hypothetical laboratory. [G97]
Lyrae Stars
A class of eclipsing binaries whose secondary minima are intermediate between those of Persei and those of W UMa. The prototype Lyr (B8.5 II, F V) is a complex eclipsing system and is presently in a state of rapid mass transfer. The spectrum of one companion is invisible; it may be a black hole. Beta Lyrae is also a weak radio source. [H76]
Newtonian
A class of reflecting telescope developed by Sir Issac Newton with a paraboloidal primary mirror and a small, plane secondary mirror at 45° to deflect the focus of the primary to a position outside the tube near the top of the telescope. [McL97]
Gregorian
A class of reflecting telescope which uses a concave secondary mirror placed after the prime focus is reached instead of a convex secondary placed before the prime focus. [McL97]
Head-Tail Galaxies
A class of relatively weak radio sources associated with clusters of galaxies and characterized by a high-brightness "head" close to the optical galaxy and a long low-brightness "tail". [H76]
MKK System
A classification of stellar spectra according to luminosity, devised by Morgan, Keenan and Kellman (see Luminosity Class). The MKK system uses two parameters (Spectral Type and Luminosity Class) to describe a system with three variables: temperature, luminosity, and abundance. [H76]
Minimal Surface
A mathematical term referring to surfaces that satisfy a minimization procedure. Soap bubbles, for example, minimize their energy by forming shapes with the minimum possible area. The world surface of a string is likewise a minimal surface. [P88]
Hilbert Space
A mathematical tool used in the formalism of quantum mechanics. The dimensions of a Hilbert space consist of wave functions, instead of length, width, and breadth. (See quantum mechanics; wave function.) [LB90]
Hermitian Matrix
A matrix which remains unchanged if each element is replaced by its complex conjugate and the rows and columns are interchanged. In quantum mechanics all matrices corresponding to observables have this property. [H76]
Iron Peak
A maximum on the element-abundance curve near atomic mass number 56. [H76]
Non-Destructive Readout
A means of reading out an infrared array or a CCD with a skipper output in which the output voltage is sampled without resetting the detector. The output voltage can be sampled a large number of times to improve noise performance. [McL97]
Mass Function
A numerical relation between the masses of the two components of a spectroscopic binary when the spectral lines of only one component can be seen: f (MpMs) = (Ms3 sin3 i) / (Mp + sM)2, where Mp = mass of primary, Ms = mass of secondary, and i = inclination of the orbit. [H76]
Flattening
A parameter that specifies the degree by which a planet's figure differs from that of a sphere; the ratio f = (a - b) / a, where a is the equatorial radius and b is the polar radius. [S92]
First Order Transition
A phase transition is called first order if it occurs in a manner similar to the way water boils. Bubbles of the new phase (steam) form in the midst of the old phase (water), so that temporarily the two distinct phases (steam and water) coexist. In a second order phase transition, by contrast, one phase evolves into the other as the temperature changes, so the two phases never coexist. [G97]
Fine-Tuning
A phrase meaning a highly constrained and implausible adjustment of the parameters of a theory. [LB90]
Light Curve
A plot of magnitude or intensity versus time for a variable star. [H76]
Least Squares, Principle of
A principle which states that the best estimate of an experimental quantity, deducible from a number of observations, is that for which the sum of the weighted squares of the residuals is least. [H76]
Graceful Exit Problems
A problem of the original formulation of the inflationary theory, in which the formation of bubbles at the end of inflation destroys the homogeneity of the universe. See decay of the false vacuum, and percolation. [G97]
Magnetic Monopole Problem
A problem, discovered by John Preskill in 1979, concerning the compatibility of grand unified theories with standard cosmology. Preskill showed that if standard cosmology were combined with grand unified theories, far too many magnetic monopoles would have been produced in the early Universe. [G97]
Heliarc
A process of joining two metals using an electric arc in an atmosphere of a noble gas. [McL97]
Inverse P Cygni Profile
A profile in which the emission is on the violet side of the absorption. It is usually interpreted to mean infall of matter. [H76]
Force Charge
A property of a particle that determines how it responds to a particular force. For instance, the electric charge of a particle determines how it responds to the electromagnetic force. [G99]
Galaxy Counts
A quantitative measure of how many galaxies there are in each range of luminosity and at each range of distance from earth. [LB90]
Four-Vector
A quantity that has four components which, under the Lorentz transformation, transform like space and time. Instead of locating a point in three-dimensional space, a spacetime four-vector locates a point in four-dimensional spacetime. (four-dimensional vector) [H76]
K
A quantum number which refers to the component of angular momentum around a molecule's axis of symmetry. [H76]
Gaunt Factor (gbar)
A quantum-mechanical correction factor applied to the semiclassical Kramers formula for photon absorption. [H76]
North Polar Spur
A radio continuum feature extending from the galactic plane to the vicinity of the North Galactic Pole. It is probably about 50-200 pc distant, and is believed to be a supernova remnant. It is also an X-ray source. [H76]
Lupus Loop
A radio source, a large broken shell 4°.5 in diameter, identified as a prehistoric supernova remnant. [H76]
Meitnerium
A radioactive metallic element not found naturally on Earth. Only a few atoms of the element have ever been detected; it can be made by bombarding a bismuth target with iron nuclei. The isotope 266Mt has a half -life of about 3.4 × 10-3s. Symbol: Mt; p.n. 109. [DC99]
Gaussian
A random distribution of initial conditions is often referred to as a Gaussian distribution. Also, a certain kind of bell-shaped curve is called a Gaussian. (See boundary conditions.) [LB90]
Matrix
A rectangular array of numbers or algebraic quantities representing a system of entities related in a systematic manner. Matrices do not obey the commutative laws of multiplication. An m × n matrix has m rows and n columns. Matrices may often be abbreviated to A = [aij]. By convention, the first subscript (i) gives the number of the row; the second (j) gives the number of the column. [H76]
Hind's Nebula
A reflection nebula (q.v.) discovered by Hind in 1852, which is illuminated by the star T Tauri. It is remarkable for its changes in brightness. (NGC 1554-5) [H76]
Maksutov Telescope
A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration. [H76]
Molecular Cloud Complex
A region of extensive emission of molecular line radiation by dense, cold interstellar gas. There are often several distinct intensity peaks, each representing individual clumps or clouds of gas and dust in a region that characteristically extends for 50 light-years and is often associated with T-Tauri stars - young, pre-main-sequence stars - and also hot massive stars and the ionized gas around them. [SILK90]
Neutral Region (solar)
A region where the magnetic field strength approaches zero. Generally, neutral regions occur between regions of opposite polarity. [H76]
Fossil Strömgren Sphere
A relict HII region which remains after the evolution of its exciting star. [H76]
Nuclear Disk
A rotating disk of about 106 M of neutral hydrogen in the inner 800 pc of our Galaxy. [H76]
Kerr black hole
A rotating, axisymmetric black hole based on Kerr's 1963 solution to Einstein's field equations. [H76]
Keplerian rotation curve
A rotation curve is a plot of speed versus distance from the center of an astronomical system. If most of the mass of the system is concentrated at the center, as in a solar system, then the speed of any orbiting body, such as a planet, is inversely proportional to the square root of its distance from the center. This last situation produces a Keplerian rotation curve. (See rotation curve.) [LB90]
Fixed Point
A scale-invariant limit point of the flow of configurations or coupling constants generated by a coarse-graining operation. [D89]
Hypothesis
A scientific proposition that purports to explain a given set of phenomena; less comprehensive and less well established than a theory. [F88]
Leap Second
A second (see Second, Système International) added between 60s and 0s at announced times to keep UTC within 0s.90 of UT1. Generally, leap seconds are added at the end of June or December. [S92]
Lane-Emden Equation
A second-order nonlinear differential equation describing the structure of polytropes. [H76]
Mixing-Length Theory
A semiempirical theory used to describe convection phenomena in stars. [H76]
Isoelectronic Sequence
A sequence of ions which have the same number of electrons but different atomic numbers. [H76]
Frame of Reference
A set of axes to which positions and motions in a system can be referred. [H76]
Great Wall
A sheet of galaxies which stretches more than 500 million light-years across the sky. [G97]
Hyad
A single member of the Hyades. [H76]
Naked Singularity
A singularity that will be visible and communicable to the outside world. [H76]
DQ Herculis
A slow nova. It is an eclipsing binary with an orbital period of only 4h39m. It also has a regular flickering period of 71 seconds, the shortest period of regular variations known, except for pulsars and compact X-ray objects. It is probably composed of an M dwarf and a white dwarf with an accretion disk. (Nova Herculis 1934) [H76]
Integrated Circuit
A small electronic component made of semiconductor silicon on which an entire electronic circuit of numerous microscopic transistor amplifiers, diodes and resistors has been constructed. [McL97]
Group
A small gathering of galaxies, smaller than a cluster. The Milky Way is part of the Local Group, which contains about thirty galaxies. [C95]
Metal-Rich Stars
A small subgroup of A-type stars in which the lines of MgII are very strong. Also called metal-strong stars or CN-strong stars or super-metal-rich Mg-strong stars. [JJ95]
Kruskal diagram
A spacetime diagram exhibiting the properties of the Schwarzschild metric by eliminating the formal singularity that appears at the Schwarzschild radius in the form in which the metric is usually written. It exhibits the character of the horizon that appears at the Schwarzschild radius and illustrates the true nature of the singularity at the center. The Kruskal diagram fully describes spacetime in the vicinity of a black hole all the way down to the singularity. [H76]
knock-on spectrum
A spectrum of particles that are being built up - the inverse of a spallation spectrum. [H76]
G-Type Star
A star having a spectral type of G, that is, yellow, like the Sun. [C95]
F Star
A star of spectral type F with a surface temperature of about 60O0-7500 K, in which lines of hydrogen and Ca II are of about equal strength. Metal lines also become noticeable. Examples are Canopus, Procyon. [H76]
Low-Velocity Star
A star whose U, V, and W velocities are all near zero. Such stars have nearly circular orbits around the Galaxy. [C95]
Li-rich stars
A subgroup of C stars, with a very strong LiI 6078 line. [JJ95]
FU Ori Stars or Fuors
A subgroup of T Tau stars with considerable changes in brightness. The post-eruption spectrum is that of a late supergiant. [JJ95]
Hydrogen-Deficient C-type Stars
A subgroup of high-luminosity C stars with weak or absent hydrogen lines, mostly of types F and G. Variable stars having such characteristics are called R CrB stars. [JJ95]
Flat Space
A synonym for ordinary Euclidean space. [Silk90]
Kapteyn Universe
An incorrect model for the Galaxy proposed by Jacobus Kapteyn in which the Milky Way was small and the Sun located at or near the Galaxy's center. [C95]
No-Boundary Proposal
An initial (boundary) condition for the Universe proposed by James Hartle and Stephen Hawking. In this proposal, the mathematics of general relativity is reformulated so that time is replaced by a space-like coordinate, in effect representing the Universe as having 4 space dimensions instead of 3 space dimensions and a time dimension. (In such a formulation, "time" does not have its usual meaning.) Hawking and Hartle suggest that the geometry of this representation of the Universe should be analogous to the geometry of the surface of a sphere, that is, a shape with no edges-hence the name no-boundary proposal. When translated back into ordinary time and space, this suggested boundary condition takes the form of a specific initial condition for the Universe. The no-boundary proposal is formulated within a quantum mechanical calculation of the behavior of the early Universe. [LB90]
Hologram
An interferometric method of recording information about the three-dimensional nature of an object which relies on preserving both the amplitudes and phases of the wavefronts which reach the detector, instead of merely the amplitudes. Hologram means "whole record". The basic principle was outlined by D. Gabor in 1948. [McL97]
Herbig-Haro Object
An object with many of the characteristics of a T Tauri star (e.g., its spectrum shows a weak continuum with strong emission lines), believed to be a star in the very early stages of evolution. All known Herbig-Haro objects have been found within the boundaries of dark clouds. They are strong infrared sources and are characterized by mass loss. [H76]
Focal Reducer
An optical component or system for changing the image scale of a telescope to achieve a better match between the seeing disk and the pixel size. (see Optical Matching) [McL97]
Flash Gate
An ultra-thin transparent electrode across the entire back surface of certain CCDs used to control the charge on the back surface and hence the QE for blue and UV light. [McL97]
Hades
An unofficial name for Jupiter IX, the outermost satellite of Jupiter (P = 758 days retrograde, i = 156°, e = 0.28). Discovered by Nicholson in 1914. [H76]
Hercules Cluster
An unsymmetrical cluster of about 75 bright galaxies (z = 0.036) of which about half are spiral or irregular and about half elliptical or 50. It contains a rather large number of disturbed and peculiar galaxies. The "missing mass", if present, must constitute more than 95% of the total. (3U 1551 + 15) [H76]
Great Year
Ancient concept of a celestial and historical cycle, its duration roughly a thousand or ten thousand years, at the end of which there is universal destruction and a new year begins. [F88]
Longitude, Celestial
Angular distance on the celestial sphere measured eastward along the ecliptic from the dynamical equinox to the great circle passing through the poles of the ecliptic and the celestial object. [S92]
Latitude, Celestial
Angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object. [S92]
Hour Angle
Angular distance on the celestial sphere measured westward along the celestial equator from the meridian to the hour circle that passes through a celestial object. [S92]
Inertial Frame of Reference
Any "standard of rest" or coordinate frame for which Newton's first law is valid. [H76]
Fourier Theorem
Any finite periodic motion may be analyzed into components, each of which is a simple harmonic motion of definite and determinable amplitude and phase. [H76]
Hot Dark Matter
Any form of dark matter which was relativistic at its point of decoupling. [C97]
IRAS Galaxy
Any galaxy which was discovered by the Infra-Red Astronomical Satellite (IRAS) to possess an excessive amount of infrared emission. [C97]
Integrating Detector
Any imaging device, like a photographic emulsion or CCD, which can build up more signal and contrast by a longer exposure to light or other electromagnetic energy. [McL97]
Josephson Effect
Any of various electrical effects observed with pairs of superconductors. Josephson junctions (between semiconductors) are finding various applications in investigating fundamental constants, the phenomenon of electron tunneling, and in high-speed computers. [DC99]
Flickering
Aperiodic behavior in an oscillating system; rapid, large-amplitude variations in light. [H76]
Galactic Centers
Are now thought to comprise black holes - which would explain why the centre of our Galaxy appears strangely obscure, and emits only infrared radiation. [A84]
Hektor
Asteroid 624, the largest (about 100 km long) of the Trojans. Its shape is apparently as elongated as that of Eros. Rotation period 6.9225 hours. Its visual mean opposition magnitude is near +14.5, which makes it the brightest of the Trojans. Assumed albedo 0.28. It has a large obliquity. [H76]
Hidalgo
Asteroid 944, perhaps 20 km in diameter, with the largest known orbit (a = 5.8 AU), second highest inclination to the ecliptic (42°.5), and second highest eccentricity (e = 0.66) of any known minor planet. Period 13.7 years. Discovered by Baade in 1920. [H76]
Icarus
Asteroid No. 1566, 1.1 km in diameter, discovered by Baade in 1948. It has the smallest orbit and highest eccentricity (a = 1.07 AU, e = 0.827, i = 23°, P = 408d) of any known minor planet. It is the only asteroid known to come closer to the Sun than Mercury (perihelion distance 0.19 AU). Rotation period 2h16m. [H76]
Helium-Strong Stars
B-type stars in which the helium lines are stronger than in normal stars. One distinguishes usually the extreme helium stars (also called hydrogen-deficient stars), in which no trace of hydrogen is seen, and the intermediate helium-rich stars, in which the hydrogen lines are still visible, but weaker than in normal stars. Related to these objects are the hydrogen deficient C stars. [JJ95]
Materialism
Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism. [F88]
HZ Stars
Blue horizontal-branch stars, the first catalog of which was compiled by Humason and Zwicky. [H76]
Haro Galaxies
Blue objects whose spectra show sharp emission lines.
Helium Variable Stars
Bp stars in which the strength of the helium lines varies periodically. At the extreme phases the objects appear as helium-rich, whereas at other phases He can be very weak or absent. [JJ95]
Methylamine
CH3NH2 -- A molecule discovered in interstellar apace in 1974, in Sgr B2, at a frequency of 87.77 GHz. Methylamine can react with formic acid to produce glycine, the simplest amino acid. [H76]
Methanol
CH3OH -- An organic molecule (also called methyl alcohol) discovered in interstellar space in 1970. More rotational lines have been observed astronomically for it than for any other molecule. [H76]
Mass Discrepancy
In the study of clusters of galaxies, the difference between the mass of a cluster obtained by using the virial theorem and the mass (usually much smaller) inferred from the total luminosities of the member galaxies. Typically MVT / ML > 10. [H76]
Mean Motion
In undisturbed elliptic motion, the constant angular speed required for a body to complete one revolution in an orbit of a specified semi-major axis. [S92]
LBV
Luminous Blue Variables -- A variable-star designation for the high-luminosity early type objects. Also called S Dor variables or Hubble-Sandage Variables. [JJ95]
KREEP
Lunar basaltic material rich in radioactive elements. (from K for potassium, REE for rare-earth elements, P for phosphorus) [H76]
Maximum Entropy Method
MEM: An image reconstruction methodology which defines a measure of information content and seeks to maximize it. [McL97]
Magnetohydrodynamics
MHD The study of how magnetic fields interact with conducting fluids (e.g. plasmas or liquid metals).
Julian Date, Modified
MJD -- The Julian Date minus 2400000.5. [S92]
Hidden Mass
Matter whose presence is inferred from dynamical measurements but which has no optical counterpart. The luminous regions of galaxies have mass-luminosity ratios of about 10. However, the mass-uminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present. A similar situation prevails in galaxy clusters, where nonluminous matter must provide most of the self-gravitational attraction that holds the clusters together. The missing mass is not really missing; it is present but invisible (at least to current detectors). It is generally believed to consist either of the remnants of massive stars or of planetary-sized objects comparable in mass to Jupiter. [Silk90]
MOS
Metal Oxide Semiconductor -- A construction used to fabricate microelectronic components. including CCDs, which consists of three layers, namely a metal conductor. an insulating layer usually made from an oxide of silicon, and a semiconductor such as silicon. [McL97]
MIPS
Millions of Instructions Per Second -- A computer benchmark. [McL97]
Flare, Solar
Sudden and short-lived (as short as 300 s) brightening of a region of the Solar chromosphere in the vicinity of a sunspot, caused by the sudden release of large amounts of energy (up to 1032 ergs) in a relatively small volume above the Solar "surface." During an intense Solar flare (electron density 1011 compared with 108 in Solar quiet times) the ionization in Earth's atmosphere may increase by several orders of magnitude. Solar flares are classified on a scale of importance ranging from 3+ (largest area) to 1- (smallest area). The largest Solar flares eject a mass of about 1016 g at a speed of roughly 1500 km s-1. [H76]
Illumination
Symbol: E A measure of the visible-radiation energy reaching a surface in unit time. Once called `intensity of illumination', it is measured in lux (lx). One lux is an illumination of one lumen per square meter. [DC99]
Luminous Intensity
Symbol: Iv The luminous flux from a point source per unit solid angle. The unit is the candela (cd). [DC99]
Luminance
Symbol: Lv A measure of the brightness of an extended source (one that cannot be considered a point). In a given direction, it is the luminous intensity per unit area projected at right angles to the direction. The unit is the candela per square meter (cd m-2). [DC99]
Lumen
Symbol: lm -- The SI unit of luminous flux, equal to the luminous flux emitted by a point source of one candela in a solid angle of one steradian. 1 lm = 1 cd sr.[DC99]
Luminous Flux
Symbol: v The rate of flow of energy of visible radiation. It is the radiant flux corrected for the fact that the sensitivity of the eye is different for different wavelengths. The unit is the lumen (lm). [DC99]
Induction
System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them. [F88]
International Atomic Time
TAI The continuous scale resulting from analyses by the Bureau International des Poids et Mesures of atomic time standards in many countries. The fundamental unit of TAI is the SI second (see second, Système International), and the epoch is 1958 January 1. [S92]
Liouville's Theorem
The Equation of Continuity: For a general Hamiltonian system the volume of an element of phase space is invariant with respect to the equations of motion. As applied to astronomy, the difference between the number of the stars moving into a volume of six-dimensional phase space at a given time and those moving out of it at the same time must be equal to the increase in the distribution function for those stars. [H76]
kilogram
The SI basic unit of mass (not of weight or of force). kilogram is equal to the mass of 1.000028 cubic decimeters of water at the temperature of its maximum density. [H76]
Newton (N)
The SI derived unit of force, equal to the force necessary to give an acceleration of 1 meter per second2 to a mass of 1 kg. N = 105 dynes. [H76]
Mole
The SI unit of the amount of substance, defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12 [14th CGPM 1971, Resolution 3]. 1 mole, which is equal to gram multiplied by the molecular weight, contains 6.02 × 1023 molecules (see Avogadro's Number). In general, 1 mole of any gas occupies a volume of 22.4 liters. [H76]
M104
The Sombrero galaxy, in which galactic rotation was first detected. It lies in the constellation Virgo. [C95]
M51
The Whirlpool Galaxy, a stunning spiral in the constellation canes venatici. [C95]
Free-Free Radiation
The acceleration of an unbound (or free) electron by a proton or atomic nucleus results in the emission of electromagnetic radiation. [Silk90]
Intrinsic Brightness
The amount of light an object actually emits, as opposed to how bright the object looks from Earth. An apparently bright star can be intrinsically bright and far away or intrinsically faint and nearby. [C95]
Gain
The amplification factor. [McL97]
Fourier Analysis
The analysis of a periodic function into its simple harmonic components. [H76]
Fringing
The appearance of complex light and dark contours in a CCD image due to constructive and destructive interference effects of light reflected inside the detector. [McL97]
Lyman Forest
The appearance of many differentially redshifted Lyman- absorption lines in a quasar's spectrum, caused by intervening hydrogen clouds along our line of sight to the quasar. [C97]
Interarm Region
The area between a spiral galaxy's spiral arms. These areas look dark, not because they lack stars, but because they contain none of the young, luminous stars that light the arms. [C95]
Limit Cycle
The attractor describing a time-periodic regime of a dissipative dynamical system. In the phase space a limit cycle is represented by a closed curve. [D89]
Gravitational Lensing
The bending of light caused by the gravity of an object lying between us and the light source. This may cause the light source to look brighter than it normally does. [C95]
Light, Deflection of
The bending of the beam of light due to gravity. It is observable when the light from a star or planet passes a massive object such as the Sun. [S92]
kinematics
The branch of mechanics that studies bodies undergoing continuous change of position. Whereas dynamics takes into account mass, force, distance, and time, kinematics is concerned only with distance and time. [H76]
Natural Line Broadening
The broadening of spectral lines resulting from the fact that excited levels have certain mean lives, and these mean lives, by virtue of the uncertainty principle, imply a spread in the energy values. [H76]
Julian Proleptic Calendar
The calendric system employing the rules of the Julian calendar, but extended and applied to dates preceding the introduction of the Julian calendar itself. [S92]
NLS1
The class of Narrow-Line Seyfert 1 galaxies was first proposed by Osterbrock and Pogge (1985, Ap.J., 297, 166). These objects have permitted linewidths (on the order of 1,000 km/sec) which are much smaller than normal Seyfert 1 galaxies (which have linewidtths on the order of 10,000 km/s). However, they differ from Seyfert 2 galaxies in that their optical spectra show several characteristics normally associated with Seyfert 1 galaxies, such as [OIII]/H_beta < 3, permitted lines somewhat broader than forbidden lines, and either blends of Fe II or very high excitation lines of [Fe VII] or [Fe X], etc. About 10% of Seyfert 1 galaxies may be Narrow-Line Seyfert galaxies. [BFM06]
Mean Place
The coordinates, referred to the mean equator and equinox of a standard epoch, of an object on the celestial sphere centered at the Sun. A mean place is determined by removing from the directly observed position the effects of refraction, geocentric and stellar parallax, and stellar aberration (see Aberration, Stellar), and by referring the coordinates to the mean equator and equinox of a standard epoch. In compiling star catalogs it has been the practice not to remove the secular part of stellar aberration (see Aberration, Secular). Prior to 1984, it was additionally the practice not to remove the elliptic part of annual aberration (see Aberration, Annual; Aberration, E-terms of). [S92]
Mantissa
The decimal part of a common logarithm. [H76]
Nuclear Density
The density of an atomic nucleus (about 1014 g cm-3). [H76]
Hamiltonian Operator (H)
The dynamical operator in quantum mechanics that corresponds to the Hamiltonian function in classical mechanics. [H76]
M Star
Stars of spectral type M are cool red stars with surface temperatures of less than 3600 K whose spectra are dominated by molecular bands, especially those of TiO. Examples are Betelgeuse and antares. M dwarfs are the most numerous type in our galaxy. [H76]
Me Stars
Stars of spectral type M with emission lines in their spectra. [H76]
MS Stars
Stars sharing the M and S characteristics. They thus exhibit bands of both TiO and SrO. [JJ95]
Halo Stars
Stars that have high spatial velocity and low metallicity. This is not an observational definition. [JJ95]
Laminar Flow
Steady flow in which the fluid moves past a surface in parallel layers of different velocities. Compare turbulent flow. [DC99]
Lossy
Subject to absorption of light. If a material is not lossy, it means that light would be scattered or reflected off. [H76]
Flat
Subject to the rules of geometry codified by Euclid; a shape, like the surface of a perfectly smooth tabletop, and its higher-dimensional generalizations. [G99]
Magnification
The effect of an optical system on the apparent angular size of an object. An increase in angular size occurs if the magnification factor is greater than 1. If the factor is less than 1 then demagnification occurs. [McL97]
Gravitational-Lens Effect
The effect of matter in curved spacetime, which tends to focus any beam of radiation from a distant source. In effect, the spacetime curvature is a lens of great focal length. At z 1, the angular size of an object starts increasing with distance. [H76]
Gravitational Encounter
The encounter between two massive bodies which results in the deviation from their original directions of motion. [H76]
Intrinsic Luminosity
The energy per second emitted by an astronomical object, analogous to the wattage of a light bulb. [LB90]
Lorentz Force Equation
The equation relating the force on a charged particle to its motion in an electromagnetic field. [H76]
Leo I
The most distant galaxy that orbits the Milky Way. A dwarf galaxy, Leo I was discovered in 1950 and lies 890,000 light-years from the Galactic center. It is close to the bright star Regulus, whose glare interferes with the study of the galaxy. [C95]
Inflaton
The name given to whatever fields are responsible for driving inflation. [G97]
K meson or kaon
The name of particular spin-0 mesons with non-zero strangeness quantum numbers. [CD99]
Large Magellanic Cloud
The nearest and largest of the many galaxies that orbit the Milky Way. It is 160,000 light-years away. [C95]
Lyman Alpha
The spectral line at 1216 Å in the far ultraviolet that corresponds to the transition of an electron in the hydrogen atom between the two lowest energy levels. [McL97]
Helium Burning
The stage when a star fuses helium into carbon and oxygen. All stars born with more than half a solar mass eventually burn helium. [C95]
Maxwell's Theory
Theory uniting electricity and magnetism, based on the concept of the electromagnetic field, devised by Maxwell in the 1880s; shows that visible light is an example of an electromagnetic wave. [G99]
Maia Sequence
a hypothetical sequence of blue variable stars named for the B-type star Maia (20 tau) in the Pleiades. Maia has the lowest rotational velocity of any B star in the Pleiades. [H76]
Irreversibility
time-asymmetric evolution of an observable quantity of a physical system. For an isolated system irreversibility implies the monotonic evolution in the future toward the state of thermodynamic equilibrium. [D89]
Low-Temperature Physics
usually defined as the physics of matter below about 20 degrees absolute (-253 degrees Celsius). [D89]
kaon (K-meson)
variety of strange meson. [D89]
Inflationary Universe
"(a) A cosmological model in which the Universe underwent an epoch of extraordinarily rapid expansion within the first 10-30 s or so after the big bang; in a typical version, the diameter of the Universe increased by a factor at least 1025 times larger (or perhaps much larger still) than had been previously thought; the model was proposed by A.H. Guth in 1981, but the original formulation contained a crucial flaw which was remedied by the development of the new inflationary universe model in 1982 by A.D. Linde, and by A. Albrecht and P. Steinhardt (see new inflationary universe). [D89]
Hess Diagram
"(a) A diagram showing the frequencies with which stars occur at various positions in an H-R diagram. [H76]
Minkowski Space
"(a) A four-dimensional spacetime with a flat (i.e., Euclidean) geometry, which is used in the special theory of relativity. [H76]
Gravitational Lens
"(a) A galaxy that intervenes between us and a distant astronomical object and that gravitationally deflects the light from that distant object. (Light, like matter, is attracted by gravity.) Gravitational lenses can focus, distort, and split light beams in the same way that ordinary glass lenses do. [LB90]
Fractal
"(a) A geometric figure in which a pattern is repeated ad infinitum on smaller and smaller scales. A classic example is Von Koch's snowflake, for which the construction begins with an equilateral triangle. Trisect each side, and replace the middle section by two sides of a smaller equilateral triangle, bulging outward. The snowflake is obtained by repeating this process for each side of the resulting figure, then for each side of the subsequent figure, and continuing forever. [G97]
Higgs Boson
"(a) A hypothetical, spinless particle that plays an important role in the Glashow-Weinberg-Salam electroweak theory (and in other theories involving spontaneous symmetry breaking, e.g. GUTs). [CD99]
Magnitude
"(a) A logarithmic brightness scale for astronomical objects. [McL97]
Graviton
"(a) A massless spin-2 particle which is the hypothetical quantum of the gravitational field. It mediates the force of gravity in a similar way to that in which the spin-1 gauge bosons (i.e. the photon, W±, Z0, and gluons) mediate the other forces. [CD99]
Lagrangian
"(a) A mathematical expression summarizing the properties and interactions of a physical system. It is essentially the difference between the kinetic energy and potential energy of the system. Moreover, one can derive the system's dynamical equations of motion directly from the Lagrangian. [CD99]
Grand Unified Theory
"(a) A mathematical scheme in which the electromagnetic, weak and strong nuclear forces are unified into a consistent description. [D89]
Luminosity Class
"(a) A measure of a star's intrinsic brightness, as determined from the star's spectrum. Supergiants have luminosity class I, bright giants have luminosity class II, giants have luminosity class III, subgiants have luminosity class IV, and main-sequence stars have luminosity class V. [C95]
Magnetic Moment
"(a) A measure of the extent to which a physical system (e.g. an atom, or nucleus, or particle) behaves like a tiny magnet. It is generally measured in units of magnetons, i.e. e / 2mc. [CD99]
Higgs Mechanism
"(a) A mechanism by which gauge bosons acquire mass through spontaneous symmetry breaking. In the Glashow-Weinberg-Salam electroweak model, for example, Higgs fields are introduced into the theory in a gauge-invariant way. However, the state of minimum energy breaks the local gauge symmetry, generating masses for the W± and Z0 bosons, and giving rise to a real, observable Higgs boson, '. [CD99]
Lamb Shift
"(a) A minute correction to the energy levels of atoms (specifically the first excited state of the hydrogen atom) predicted by quantum electrodynamics, and confirmed to great accuracy by Willis Lamb. [D89]
Molecular Hydrogen
"(a) A molecule consisting of two hydrogen atoms (H2) and the most common molecule in space. [C95]
Isomer
"(a) A molecule having the same number of atoms of each element as another molecule. [SEF01]
Mixmaster Model
"(a) A non-Friedmannian cosmological model that begins with a highly anisotropic infant Universe and shows how anisotropies are reduced in time. (see also Friedmann Models) [LB90]
Hertzsprung-Russell Diagram
"(a) A plot of stellar color, temperature, or spectral type versus stellar luminosity. The H-R diagram segregates three principal types of stars: the main sequence, which forms a diagonal band from bright blue stars to faint red ones; red giants and supergiants, which appear in the upper right of the diagram; and white dwarfs, which lie below the main sequence. [C95]
Galactic
1. When capitalized, the word refers to our Galaxy. 2. When not capitalized, it refers to a galaxy. [C95]
Fornax A
A 10th-magnitude S0 galaxy (NG6 1316), which is a strong radio source. [H76]
Geocentric Cosmology
"(a) School of ancient theories that depicted the earth as standing, immobile, at the center of the universe. [F88]
Interstellar Dust
"(a) Dust particles in the space between the stars. These are responsible for the dark patches of obscuration seen on astronomical photographs. The particles are composed of common heavy elements such as carbon and silicon but there is no agreement about the exact composition of the dust grains. Typically, the particles have size about 1 µm but there must be a wide range of particle sizes present to explain the interstellar extinction curve. The dust plays a key role in giant molecular clouds in protecting the fragile molecules from intense interstellar ionising and dissociating radiation. The energy absorbed by the grains is emitted in the far-infrared waveband, and this form of dust emission is one of the most important energy loss mechanisms for regions of star formation. [D89]
Galactic Pole
"(a) Either of the two points in the sky where we look perpendicular to the disk of the Milky Way. The north Galactic pole is the Galactic pole located above the disk; the south Galactic pole is the Galactic pole located below the disk. [C95]
Gamma Ray
"(a) Electromagnetic radiation similar to X-radiation, although of shorter wavelength, emitted spontaneously by some radioactive substances from atomic nuclei during radioactive decay. [A84]
Hydrogen
"(a) Element that is the lightest and the most abundant in the Universe. Its atom comprises one proton and one electron. The element occurs both in stars and as interstellar clouds, in regions where it may be neutral (H I regions) or ionized (H II regions). [A84]
Helium
"(a) Element which, after hydrogen, is the second lightest and second most abundant in the Universe. Its atom comprises two protons, two neutrons and two electrons; its nucleus is sometimes called an alpha particle. Helium is the product of the nuclear fusion of hydrogen in most stars, but this does not explain the overall helium abundance. [A84]
Iron
"(a) Element with atomic number 26, created mostly by type Ia supernovae, with an additional contribution from type Ib, Ic, and II supernovae. It is the most stable element. [C95]
Nitrogen
"(a) Element with atomic number seven and the fourth most common metal in the Universe. It formed during hydrogen burning in main-sequence stars and red giants, via the CNO cycle. [C95]
Magnesium
"(a) Element with atomic number twelve. it is the fifth most common metal in the Universe and was produced by high-mass stars that exploded. [C95]
Gravitational Potential Energy
"(a) Energy that a body can acquire by falling through a gravitational field and that decreases as the kinetic energy increases. There is no general reference level (analogous to the state of rest of a body in defining kinetic energy), and so we customarily define the change in gravitational potential energy as the negative of the work done by the gravitational forces during the bodys change of position. [Silk90]
Gegenschein
"(a) Faint oval patch of light visible from Earth only at certain times of the year, opposite the Sun. Its nature and cause are still not known. It is sometimes known as ""counterglow"". [A84]
Lambert's Law
"(a) First proposed in 1760; then restricted to visible light, it is now used with all radiations. The law concerns the rate of absorption of radiation as it travels deeper into a medium. It states that equal thicknesses of the medium absorb equal proportions of the incident radiation. In other words, the intensity I of the transmitted radiation falls off exponentially with distance d in the medium:
Meteor
"(a) Fragment or particle that enters the Earth's atmosphere and is then destroyed through friction, becoming visible as this occurs as a momentary streak of light. At certain times of the year, meteors apparently emanating from a single area of the sky (a radiant) form meteor showers. They are thought to originate within the Solar System. see also Meteorite [A84]
Gravitational Constant
"(a) Fundamental constant with units of cm3 g-1 s-2 that determines the gravitational force between two bodies at a given separation. [D89]
Gravitational Redshift
"(a) Generic name for the shift in the frequency or wavelength of a signal that travels up or down in a gravitational field; effect is a redshift if signal travels upward, a blueshift if it travels downward. [D89]
Isotropic
"(a) Having equal and uniform properties at all points and in all directions. In astronomy the term describes microwave background radiation. [A84]
Intermediate Vector Boson
"(a) IVB Generic name for W and Z bosons - the carriers of the weak force. [D89]
Inclination
"(a) In astronomy, the angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic. [A84]
Homogeneity
"(a) In cosmology, the property that any large volume of the universe looks the same as any other large volume. Most cosmological models assume homogeneity. [LB90]
Julian Date
"(a) JD: The number of ephemeris days that have elapsed since 12h ephemeris time on January 1, 4713 B.C. JD for 1970 January 1 is 2440588. [H76]
Generation
"(a) Leptons and quarks come in three related sets, called generations, consisting of two leptons and two quarks. The first generation consists of (e-, e; u, d). The second and third generations consist of (µ-, µ;c, s) and (-, ; t, b) respectively. [CD99]
Lithium
"(a) Lightest of all solid elements, third in the periodic table after hydrogen and helium. Its atom comprises one proton and three electrons. One of the electrons is at a higher energy level than the other two. [A84]
MAMA
"(a) Multi-Anode Microchannel Analyzer . A detection system used with microchannel plates to detect events. Used as an imaging system in the ultraviolet. see Microchannel Plates. [McL97]
Microwave Radiation
"(a) Radiation in the electromagnetic spectrum between infrared and radio waves. This range has wavelengths of between about 20 cm and about 1 mm. Radiation of this type was detected as background radiation. [A84]
Heisenberg Uncertainty Principle
"(a) States that the position and momentum of a particle can only be known to a certain level of precision. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space. [C97]
Mean Free Path
"(a) Symbol: 1. The average distance traveled by the particles of a fluid between collisions. It is given by
Interferometry
"(a) Technique for studying sources of electromagnetic radiation (light or radio waves) through interference patterns caused when two waves are combined. [A84]
Infrared
"(a) That part of the electromagnetic spectrum that lies beyond the red, having wavelengths from about 7500 Å to a few millimeters (about 1011-1014 Hz). Infrared radiation is caused by atomic transitions, or by vibrational (near-IR) and rotational (far-IR) transitions in molecules. (IRe1, IRc1, IRs1: the e sources are extended; the c sources are unresolved; the s indicates an infrared nebula surrounding a visible star.) [H76]
Meter
"(a) The SI unit of length. The meter is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the Krypton-86 atom (11th CGPM, 1960, Resolution 6). [H76]
Mass-to-Light Ratio
"(a) The amount of mass in an object divided by its luminosity, both measured in Solar units. The Sun has a mass-to-light ratio of one, because it has one Solar mass and one Solar luminosity. Stars brighter than the Sun, such as upper main-sequence stars, giants, and supergiants, have low mass-to-light ratios, because most have somewhat more mass than the Sun but much more luminosity. Stars fainter than the Sun, such as red, orange, and white dwarfs, have high mass-to-light ratios, because most have smaller masses than the Sun but much smaller luminosities. Dark matter, by definition, has a high mass-to-light ratio: it has much mass but radiates little or no light. [C95]
Fraunhofer Lines
"(a) The dark lines in the spectrum of light from the Sun, caused by the absorption of particular wavelengths by certain elements in its cooler outer regions. The wavelengths of these lines are used as reference points in specifying quantities that vary with wavelength, e.g. refractive constants. [DC99]
Magnetic Moment
"(a) The intrinsic spins of the electrons in an atom or ion, together with the motion of the electrons round the nucleus, give rise to a magnetic field around the atom. The magnitude of this field is determined by the magnetic moment of the atom or ion. [D89]
Hubble Law
"(a) The law that recessional speed is proportional to distance for a homogeneous and isotropic universe. Galaxies moving away from us with a speed precisely following this law are said to follow the Hubble flow. Because the actual universe is not precisely homogeneous, with lumpiness arising from clustering of galaxies and voids of empty space, the motions of actual galaxies deviate somewhat from the Hubble flow. [LB90]
Luminosity Function
"(a) The number of stars in the Galaxy with a particular absolute magnitude. The luminosity function reveals that luminous stars are rare and intrinsically faint stars common.
Mach's Principle
"(a) The precept that the inertia of objects results not from their relationship to Newtonian absolute space, but to the rest of the mass and energy distributed throughout the Universe. though unproved and perhaps unprovable, mach's principle inspired einstein, who sought with partial success to incorporate it into the general theory of relativity. [F88]
Mass
"(a) The quantitative property of an object due to the matter it contains. (Weight, in contrast, describes a force with which a body is attracted towards a gravitational focus.) Units of mass are grams and kilograms. [A84]
Mass-Luminosity Ratio
"(a) The ratio of the mass of a system, expressed in Solar masses, to its visual luminosity, expressed in Solar luminosities. The Milky Way has a mass-luminosity ratio in its inner regions of 10, indicating that the typical star is a dwarf of mass about half that of the sun. A rich cluster of galaxies such as the Coma cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter. [SILK90]
Mach Number
"(a) The ratio of the speed of a moving object (e.g. a high-speed aircraft) to the speed of sound in the air or other medium through which the object is traveling. An aircraft passes through the `sound barrier' as the Mach number exceeds one; at this speed the air resistance increases sharply. [DC99]
Ionosphere
"(a) The region of Earth's atmosphere (80-500 km), immediately above the stratosphere. The ionosphere consists of the D layer, the E layer, and the F layers (q.v.). It is strongest at the end of the day. [H76]
Local Supercluster
"(a) The supercluster to which the Local Group belongs. It is shaped like a cigar, with the Virgo cluster of galaxies at its center and the Local Group near one end. [C95]
kelvin
"(a) The temperature scale that astronomers usually use. On the Kelvin scale, the coldest possible temperature is 0 degrees. This corresponds to -273° Celsius or -460° Fahrenheit. Room temperature is about 295 degrees Kelvin. [C95]
Gravitational Force
"(a) The weakest of the four fundamental forces of nature, the gravitational force between any two masses is proportional to the product of the masses and varies inversely as the square of the distance between them. The other three fundamental forces are the electromagnetic force and two kinds of nuclear forces. (See electromagnetic force; nuclear forces.) [LB90]
Nuclear Forces
"(a) There are two kinds of nuclear forces: the strong nuclear force and the weak nuclear force. These two forces, plus the gravitational and electromagnetic forces, comprise the four fundamental forces of nature. The strong nuclear force, which is the strongest of all four forces, is the force that holds protons and neutrons together in the atomic nucleus. The weak nuclear force is responsible for certain kinds of radioactivity; for example, the disintegration of a neutron into a proton, electron, and antineutrino. [LB90]
Isothermal Change
"A process that takes place at constant temperature. If work is done on a body in such a way as would otherwise increase its temperature (for example by compressing it) it is possible to keep the temperature constant by heat transfer from the body. Conversely a body may expand doing external work, while heat is transferred to it to prevent the temperature from falling. In the case of an ideal gas, for which the internal energy depends only on the temperature, the heat transferred from the substance must be exactly equal to the work done on it. For real substances the work and heat may not balance in an isothermal process since the internal energy may depend also on the volume.
Jansky
"A unit used in radio astronomy to indicate the flux density of electromagnetic radiation received from outer space, one jansky being equal to 10-26 W m-2 Hz-1. The unit is named after the American electrical engineer Karl G. Jansky (1905-1950), who became the first radio astronomer in the world in December 1930 when he detected electromagnetic radiation of wavelength 15 m coming from the Milky Way (see Flux unit). It was adopted in August 1973 by the International Astronomical Union. [JM92]
Multiples and Numbers
"In April 1795 the French Revolutionary Government introduced prefixes to represent the multiples and submultiples of the basic metric units. Those given below, with their recognized modern abbreviations, are still in use.
Mass-Shell
"In quantum mechanics, a particle's energy and momentum are essentially independent of each other. A particle is said to be ""on mass-shell"" when its energy and momentum satisfy the formula from special relativity:
MHD
"Magnetohydrodynamics -- (a) The study of the collective motions of charged particles in a magnetic field. (sometimes called hydromagnetics)[H76]
Irradiance
"Symbol: E The rate of energy reaching unit area of a surface; i.e. the radiant flux per unit area. Unlike illumination, irradiance is not restricted in use to visible radiation. The unit is the watt per square meter (W m-2).
kilogram (kg)
"The fundamental unit of mass in the metric system. It was originally supposed to represent the mass of a cubic decimetre of water at the temperature of its maximum density. Subsequent work showed that the cube used for the determination of the kilogram of 1799, known as the Kilogramme des Archives, had a volume of 1.000028 cubic decimetres, thereby making it too heavy. At the Convention du Mètre held in Paris in May 1875 it was decided to make a new international kilogram which would be an exact copy of the Kilogramme des Archives. The new standard, known as the International Prototype Kilogramme, was established in 1889 and copies of it were sent to the various bodies responsible for weights and measures throughout the world. Copy No. 18 was sent to England, No. 20 to the USA. The British copy is kept at the National Physical Laboratory. The standard itself is a platinum iridium cylinder preserved at the International Bureau of Weights and Measures at Sèvres, near Paris, and is unique in that it is now the only one of the seven basic SI units which is man-made. The vault in which it is housed is secured by three locks. A separate key is held by the director at Sèvres, the chairman for weights and measures and the archivist of France. However it seems probable that, early in the twenty first century, the mass of the international prototype kilogram will be redefined either in terms of the mass of a silicone crystal or by an appropriate combination of the values of some readily measurable and defined physical constants such as the fine structure constant Rydberg constant etc.
Laser
"The word laser stands for Light Amplification by Stimulated Emission of Radiation. Proposed by A. Schawlow and C. Townes, the first operating laser was constructed by T. Maiman. Lasers generate intense directional beams of coherent radiation through stimulated emission provided sufficient energy is provided to maintain a large number of radiating atoms in the laser. [D89]
interference
"When similar waves with a regular phase relationship pass through the same region they are said to interfere. The resultant displacement at any point is the sum of the individual displacements, taking into account their directions and phases. The waves emerge from the overlapping region unaffected. The combination of such coherent waves is to be contrasted with the combination of incoherent waves, for example light from two lamps falling on a surface, where the resultant intensity is just the sum of the separate intensities.
Meteor Shower
(a) A profusion of meteors that fall within a period of a few hours and that appear to radiate from a common point in the sky. Shower meteors are usually low-density material, have high eccentricities, and are known to be associated with comets (e.g., the orbit of the Leonids is identical with that of comet Tempel-Tuttle 1866 I). [H76]
Jeans Instability Criterion
(below) see Jeans Length
Maser
(microwave amplification by stimulated emission of radiation). A device that utilizes the natural oscillations of an atom or molecule to amplify electromagnetic radiation. Molecules are pumped into a metastable upper state by incident radiation of broad frequency via upper states that connect to the metastable state. They are then stimulated downward by radiation of a certain frequency connecting the metastable and ground states. When a bound electron in the metastable state is hit by a photon of the right frequency, the electron can return to a lower state by emitting a photon of exactly the same frequency as the incident photon; and it will emit it in exactly the same direction in which the incident photon is scattered. This means that the photons move off precisely in phase. If each hits another electron in the same state, there will be four photons in phase, etc. [H76]
Hubble Expansion
... of the Universe. There are billions of other galaxies and all except the closest ones (the Local Group) are receding from us as deduced from the Doppler redshift effect of their spectra. Thus the Universe appears to be expanding. Moreover, the greater the distance the faster the speed of recession. This is interpreted as the expansion of spacetime itself since an event called the Big Bang. [McL97]
Fornax
1. A faint constellation in the southern sky. 2. A dwarf galaxy that orbits the Milky Way and lies 440,000 light-years from the Galactic center. It was discovered in 1938. [C95]
Giant
1. A star that has evolved off the main sequence and is roughly a hundred times as luminous as the Sun. Giants can be of any color, but yellow, orange, and red giants are the most common. 2. A planet much more massive than Earth. The solar system has four giant planets, all far from the Sun: Jupiter, Saturn, Uranus, and Neptune. [C95]
Microchannel Plate
A compact electrostatic high-voltage electron multiplier with a very large number of narrow pores or channels. A photoelectron generated at the entrance face (photocathode) stimulates a cascade of secondary electrons down the nearest channel to produce a huge cloud of charge at the output face. The output pulse can be used in many different ways to record the event. If it impacts a phosphor screen then light emission can be detected with a CCD. Direct electrical detection can be obtained using a Multi Anode Microchannel Analyzer. [McL97]
Isotopic Spin
A concept introduced by Heisenberg in 1932 to describe the charge independence of the strong nuclear force. Since the strong force cannot distinguish between a proton and a neutron, Heisenberg proposed that these particles were actually different states of a single particle - the nucleon. He argued that just as the electron comes in two different spin states, so the nucleon comes in two different "isospin" states. So, isospin is a concept analogous to spin which is conserved by the strong interaction. The nucleon is an isospin-1/2 particle, and its third component of isospin determines whether we are talking about a proton (I3 = +1/2) or a neutron (I3 = -1/2). See Isospin [CD99]
LS Coupling
A condition in an atom in which the spins couple to spins and orbital angular momenta couple to orbital angular momenta to form total spin and total orbital angular-momentum vectors which then couple to form the total angular momentum of the atom. In this case spin-spin and orbit-orbit torques exceed spin-orbit torques; the opposite extreme results in j-j coupling. (also called Russell-Saunders coupling)[H76]
Ground Loop
A condition in which two pieces of apparatus are connected together while each has a separate earth connection and these are not at identical potentials. A current will flow and small signals will be affected by fluctuations, called noise. [McL97]
Galaxy Cluster
A conglomeration of hundreds or thousands of galaxies. The nearest large galaxy cluster is the Virgo cluster. [C95]
Malmquist Correction
A correction introduced into star counts distributed by apparent magnitude. [H76]
Number-Count Test
A cosmological test that involves counting all galaxies down to a certain limiting magnitude and repeating this procedure for fainter and fainter limiting magnitudes. Deviations from the relation expected in Euclidean space can help ascertain whether the Universe is open or closed. In practice, this test provides a strong constraint on models of galactic evolution and luminosity at past eras. [Silk90]
Klein-Alfvén cosmology
A cosmology in which the observed expansion of the Universe results from the bounce of an originally collapsing cloud of matter and antimatter. The bounce is caused by radiation pressure generated by annihilations when the cloud reaches high density (10-2 cm-3). [H76]
Forbush Decrease
A decrease in cosmic-ray intensity with an increase in olar activity (and vice versa). This phenomenon was first noted by Forbush in 1954. [H76]
Lead
A dense, dull, gray, soft metallic element; the end product of radioactive decay series. It occurs in small quantities in a wide variety of minerals but only a few are economically important. Lead is used in (lead-acid) accumulators, alloys, radiation shielding, and water and sound proofing. It is also used in the petrochemical, paint, and glass industries. Symbol: Pb; m.p. 327.5°C; b.p. 1830°C; r.d. 11.35 (20°C); p.n. 82; r.a.m. 207.2. [DC99]
Globule
A dense, spherical cloud of dust that absorbs radiation (see Bok globule). [H76]
Heterodyne
A detection method used extensively in radio astronomy in which the wave nature of light is used. The method usually involves combining the measured wave with a local oscillator or reference wave and looking for the signal at the difference frequency. [McL97]
Microphotometer
A device for measuring the variations in density in a photographic emulsion. [H76]
Nicol Prism
A device for producing plane polarized light, consisting of a crystal of calcite cut with a 68° angle, cleaved along the optic axis, and stuck together with a thin layer of Canada balsam. The Canada balsam, which is not birefringent, has the same refractive constant for both ordinary and extraordinary rays (n = 1.66). The latter ray passes through the prism (in calcite n = 1.66). However, the ordinary ray in calcite has a lower refractive constant (n = 1.48) than in the Canada balsam, and suffers total internal reflection at the interface. Nicol prisms are more transparent than Polaroid. [DC99]
Hybrid Array
A device in which the roles of radiation (infrared mostly) detector and signal multiplexer are separated. The device is a sandwich of two slabs. Other names include focal plane array (FPA) and sensor chip assembly (SCA). [McL97]
Nicol Prism
A device made from a split crystal of Iceland spar with which plane-polarized light can be detected. [H76]
Krüger 60 AB
A faint, twelfth-magnitude dM binary (P = 44.5 years) in the Solar neighborhood (3.93 pc distant). It may be a subluminous star. [H76]
Groombridge 1830
A famous halo star that lies 28 light-years away in the constellation Ursa Major. Its proper motion, discovered in 1841, was then the largest known, displacing that of 61 Cygni. [C95]
Milne-Eddington Approximation
A first approximation in the analysis of stellar spectra, in which a line is assumed to be formed in such a way that the ratio of the line absorption coefficient to the continuous absorption coefficient is constant with depth. It is used primarily in analyses of the lines of ionized metals, for which cases it is often an accurate approximation. [H76]
Inverse Square Law
A force law that applies to the gravitational and electromagnetic forces in which the magnitude of the force decreases in proportion to the inverse of the square of the distance. [Silk90]
HD
Henry Draper Catalogue, which lists over 200,000 stars. It was published in nine volumes between 1918 and 1924. [C95]
N Galaxy
A galaxy with a small, bright, blue nucleus superposed on a considerably fainter red background. (In the Yerkes 1974 system, a galaxy with a small nucleus containing a considerable fraction of the luminosity; N-, less pronounced N galaxies; N+, extreme examples of N galaxies.) Also, a type of radio galaxy having a brilliant, starlike nucleus containing most of the luminosity of the system. N galaxies are compact galaxies, and as a class are intermediate between Seyfert galaxies and quasars in properties of form, color, spectra, redshift, and optical and radio variability. [H76]
Hertzsprung Gap
A gap (from about A0 to F5) in the horizontal branch of the H-R diagram (see instability strip). The few stars that populate this gap are RR Lyrae and other variable stars. It is regarded as a region through which a star moves rapidly in its evolutionary track away from the main sequence. [H76]
Friedmann Models
A general class of cosmological models that assume the Universe is homogeneous and isotropic on large scales and that allow the Universe to evolve in time. Most calculation in the standard Big Bang model assume a Friedmann cosmology. (see Friedmann Equation; Homogeneity; Isotropy.) A cosmological model that has the same properties as a Friedmann model under some conditions is said to have a Friedmann limit. [LB90]
M81
A giant spiral galaxy 11 million light-years away in the constellation Ursa Major. It rules the M81 group, the second nearest galaxy group to the Local Group. [C95]
Internet
A global spider-web-like network of computers and computer systems with no central hub or single point of control. [McL97]
47 Tucanae
A globular cluster in the southern constellation Tucana and a member of the thick-disk population. [C95]
Hour Circle
A great circle passing through the celestial poles - i.e., perpendicular to the celestial equator. [H76]
M13
A great globular cluster in the constellation Hercules. [C95]
Northern Cross
A group of bright stars in the constellation Cygnus. [H76]
Multiplet
A group of spectral lines arising from transitions having a common lower energy level. The group of lines have the same values of L and S but different values of J. [H76]
Moving Cluster
A group of stars dynamically associated so that they have a common motion with respect to the local standard of rest. Examples are the Hyades and the Ursa Major group. [H76]
H II Condensation
A high-density H II region. [H76]
BL Lacertae
A highly variable object (the most rapid radio variable known, also an optically violent variable - mv = 12 to 15 mag - and an infrared source). Probably an exceedingly compact nonthermal object, and undoubtedly extragalactic. Its optical spectrum is characterized by an absence of lines, so its redshift cannot be measured. (In 1974 Oke and Gunn infer z = 0.07 from an H absorption line in the surrounding halo and conclude that it lies at the center of a bright (Mv = - 23) elliptical. If true, this would make BL Lac the nearest known quasar.) [H76]
Friedmann Universe
A homogeneous, isotropic model of the Universe involving nonstatic (i.e., expanding or contracting) solutions to Einstein's field equations (with zero cosmological constant) calculated by the Russian mathematician A. Friedmann in 1922. [H76]
Giant Molecular Cloud
A huge complex of interstellar gas and dust, consisting mostly of molecular hydrogen, that typically stretches over 150 light-years and contains 200,000 solar masses. Giant molecular clouds give birth to new stars. [C95]
Neutral Hydrogen
A hydrogen atom that has a proton and an electron and so is electrically neutral. Neutral hydrogen produces radio waves that are 21 centimeters long. [C95]
Ionized Hydrogen
A hydrogen atom that has lost its electron. Hydrogen gets ionized by hot O and B stars in H II regions. The most famous H II region is the Orion Nebula. [C95]
Metal-Enhanced Star Formation
A hypothesis according to which stars form preferentially from regions of above-average Z in a chemically inhomogeneous interstellar medium. [H76]
Great Attractor
A hypothesized large mass, some hundred million light years from earth, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity. [LB90]
Metallic Hydrogen
A hypothetical form of hydrogen in which the molecules have been forced by extremely high pressures to assume the lattice structure typical of metals. It is estimated that as much as 40% of Jupiter's mass (but not more than 3% of Saturn's) may be in the form of metallic hydrogen. [H76]
Galactic Wind
A hypothetical outflow of tenuous material from a galaxy, analogous to the solar wind. [H76]
Monopole
A hypothetical quantum object being a single, isolated magnetic pole. Normally, magnetic poles, the sources of a magnetic field, occur in pairs as north and south poles. [P88]
RR Lyrae Stars
A large class of pulsating (amplitude variation about 1 mag) blue giants of anomalous spectral type (A2-F6) with periods of less than 1 day. Their average absolute magnitude is about +0.8. which makes them almost 50 times more luminous than the Sun. They are Population II objects often (but not always) present in globular clusters. RR Lyrae stars are valid distance indicators out to more than 200 kpc. (also called Cluster Variables) [H76]
Luminiferous Ether
A material that, prior to special relativity, was believed to permeate all of space, allowing the propagation of light. With the advent of relativity, we now believe that light can propagate without a medium. [G97]
Galactic Longitude
A measure of a star's position with respect to the Sun and Galactic center. Galactic longitude ranges from 0 degrees to 360 degrees. Imagine the Sun at the center of a giant clock, with the Galactic center located in the direction of six o'clock. A Galactic longitude of 0 degrees would correspond to the direction of six o'clock, a Galactic longitude of 90 degrees to the direction of three o'clock, a Galactic longitude of 180 degrees to the direction of twelve o'clock, and a Galactic longitude of 270 degrees to the direction of nine o'clock. [C95]
kiloparsec
A measure of distance equal to 1,000 parsecs, or about 3,000 light years. [LB90]
kinetic temperature
A measure of the average random motion of the particles in a system. [H76]
Galaxy Correlation Function
A measure of the degree of galaxy clustering in a large sample of galaxies. The 2-point correlation function is the probability that there will be a second galaxy at a certain distance from any one galaxy. [Silk90]
Impact Parameter
A measure of the distance by which a collision misses being head-on. In astronomy, usually the distance, at closest approach, between the centers of two particles in a collision if there were no attractive force acting between them. [H76]
Fourier Component
A measure of the fluctuations of some physical quantity on a particular length scale. The density of sand on a beach, for example, would have a small Fourier component on the scale of a few feet, where the beach appears smooth, but a large Fourier component on the scale of a hundredth of an inch, where the beach appears grainy owing to individual sand particles. [LB90]
Love Number
A measure of the rotational deformation of a rotating body in hydrostatic equilibrium. [H76]
Fried Parameter
A measure of the scale of the turbulence in the atmosphere. The length over which a disturbance to a wavefront is well-correlated. [McL97]
Mean Solar Time
A measure of time based conceptually on the diurnal motion of the fictitious mean sun, under the assumption that the Earth's rate of rotation is constant. [S92]
Inverse Maser
A mechanism that absorbs radiation and cools a gas so that the number of molecules in an upper level falls below that expected in a condition of thermal equilibrium. The effect is inverse to that found in a maser, where there is an overpopulated upper level. [H76]
BL Lac Object
A member of a class of astronomical objects with the following characteristics: (1) rapid variations in intensity at radio, infrared, and optical wavelengths; (2) energy distributions such that most of the energy is emitted at infrared wavelengths; (3) absence of discrete features in low-dispersion spectra; and (4) strong and rapidly varying polarization at visual and radio wavelengths. [H76]
Horizontal Branch Star
A metal-poor star, similar in mass to the Sun, that fuses helium into carbon and oxygen at its core. Such stars range in color from blue to yellow. RR Lyrae stars are horizontal-branch stars. Stars bluer than RR Lyraes are called blue horizontal-branch stars; stars redder are called red horizontal-branch stars, even though they are actually yellow. All other things being equal, the more metal-poor a globular cluster, the bluer its horizontal branch; the older a globular cluster, the bluer its horizontal branch, too. [C95]
Nuclear Chronology
A method of dating an object by measuring how many atomic nuclei have disintegrated and changed into other nuclei. Uranium dating of the earth is an example of nuclear chronology. [LB90]
Gravitational Field Lines
A method of depicting a gravitational field by drawing lines. The direction of the field is indicated by the direction of the lines, and the strength of the field is indicated by how closely the lines are spaced. [G97]
Hanning Method
A method of smoothing out the noise in radio data. For each data point, one-half the value of that point is taken, plus one-quarter the value of the point on each side. The result is usually a smoother curve. [H76]
Non-Gray Atmosphere
A model atmosphere constructed by letting the absorption coefficient vary with frequency. [H76]
Gray Atmosphere
A model atmosphere in which the continuous absorption coefficient is assumed to be independent of frequency. [H76]
Heisenberg Model
A model of magnetic systems in which each magnetic atom has a spin which is free to point in any direction in space. Neighboring atoms are coupled by a force which tends to align the spins in parallel (for a ferromagnet) or opposite (for an antiferromagnet) directions. [D89]
Fokker-Planck Equation
A modified form of the Boltzmann equation allowing for collision terms in an approximate way. It is used in the problem of charged-particle transport in fluctuating electromagnetic fields. [H76]
Hubble Classification
A morphological classification sequence of galaxies devised by Edwin Hubble. It splits galaxies into ellipticals, lenticulars, spirals, barred spirals and irregulars. [C97]
IC 4182
A nearby galaxy in which a type Ia supernova exploded in 1937. [C95]
Hayashi Track
A nearly vertical track of stellar evolution toward the main sequence during phases when the star is largely or completely in convective equilibrium. The luminosity, originally very high, decreases rapidly with contraction, but the surface temperature remains almost constant. [H76]
Negatron
A negatively charged electron, as opposed to a positron. [H76]
Kolmogorov-Smirnov test
A nonparametric test used in statistics. The Kolmogorov statistic is simply the magnitude of the maximum deviation between the integral distribution function of a sample and the theoretical distribution one wishes to test. [H76]
Free Parameter
A number which is needed to define a theory well enough so that predictions can be made, but which must be determined by experiment or observation. [G97]
Hz
Hertz A unit of frequency equal to one cycle (or wave) per second. [F88]
Galactic Coordinates
A system of coordinates based on the mean plane of the Galaxy, which is inclined about 63° to the celestial equator. Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane from the galactic center. In 1958, because of increased precision in determining the location of the galactic center, a new system of galactic coordinates was adopted, with the origin at the galactic center in Sagittarius at (1950) = 17h42m.4, (1950) = - 28°55'. The new system is designated by a superior roman numeral II (i.e., bII, lII) and the old system by a superior roman numeral I: lII lI + 32°.31. Galactic coordinates are independent of precession. [H76]
Grazing-Incidence Telescope
A telescope used in X-ray and gamma-ray astronomy. It focuses these rays by making use of the fact that they behave like light rays if they strike surfaces at a shallow enough angle. [H76]
Glitch
A term used in rocketry to describe a malfunction (or "slide") of the stylus on a chart recorder; also, a sudden change in frequency, as in a pulsar. (Yiddish) [H76]
Natural Philosophy
A term widely employed in the seventeenth century to mean what today is encompassed in the word science. [F88]
Mie Theory
A theory of the diffraction of light by small spherical particles. [H76]
Million
A thousand thousand (106). [F88]
Hyperbolic Space
A three-dimensional space whose geometry resembles that of a saddle-shaped surface and is said to have negative curvature. [Silk90]
Graticule
A transparent plate or cell bearing a grid, cross-wire, or scale, designed to be used with an optical instrument or cathode-ray oscilloscope for the purpose of positioning, measuring, or counting objects in the field of view; the scale, grid, etc., on such a plate.
Monte Carlo Method
A trial-and-error technique used on computers to solve complex problems. [H76]
Flux Tube
A tube of magnetic field lines. [H76]
Murray Meteorite
A type II carbonaceous chondrite that fell near Murray, Kentucky, in 1950. [H76]
Murchison Meteorite
A type II carbonaceous chondrite which fell in 1969 near Murchison, Australia, and which was found to contain at least 17 amino acids. Left-handed and right-handed forms were present in roughly equal quantities. [H76]
U Geminorum Star
A type of dwarf nova (q.v.). All U Geminorum stars are binaries containing a white dwarf and a red dwarf with total masses of roughly 1-2 M and with periods of less than 12 hours (period of U Gem, 1.5 × 104 seconds). About 150 are known. [H76]
klystron
A type of electron tube used in radar and high-frequency radio work. [H76]
Fresnel Lens
A type of lens with one surface cut in steps so that transmitted light is refracted just as if by a much thicker (and heavier and more expensive) conventional lens. Lighthouse lamp lenses have long been of this type. Very cheap plastic Fresnel lenses now have many uses, for example in overhead projectors, flashlamps, etc. The angle of each step is made to produce the desired effect. [DC99]
kaon (K-meson)
A type of meson. There are four types of meson: positively charged, negatively charged, and two types of neutral particle. They are strongly interacting bosons of zero spin with several alternative modes of decay. [DC99]
Gamma Decay
A type of radioactive observer decay in which gamma rays are emitted by the specimen. Gamma decay occurs when a nuclide is produced in an excited state, gamma emission occurring by transition to a lower energy state. It can occur in association with alpha decay and beta decay. [DC99]
Neutral Current
A type of reaction between particles subject to the weak nuclear force. In this reaction, all intermediary particles that transmit the forces are electrically neutral, which is the origin of the word "neutral". [LB90]
Homogeneous
A universe is called homogeneous if it would look the same to all observers, no matter where they were located. The real universe is not precisely homogeneous, but it appears to be homogeneous on large scales. That is, if we averaged the mass density or other property of the matter in the universe over cubes of a few hundred million light-years on each side, all such cubes would be very similar to each other. See also isotropic. [G97]
Lallemand Camera
A very early form of image tube.
Microprocessor
A very large silicon integrated circuit with essentially all the functions of a computer on a single chip. [McL97]
Meniscus Mirror
A very thin mirror with a high curvature. A method of constructing very large mirrors which assumes from the outset that the mirror is too thin to hold its shape against gravity and will require an active control system. [McL97]
Hyades
A young (5 × 108 yr) moving cluster (radial velocity, + 36 km s-1) of more than 200 stars (spectral types A1-K) visible to the naked eye in Taurus, about 40 pc distant. Aldebaran is a foreground star in that region of the sky. [H76]
Non-Coherent Scattering
Absorption of a photon and reemission at a different frequency (as seen by an observer) by scattering atoms. The natural width of the lines, Doppler broadening, and pressure broadening are the main processes that give rise to noncoherent scattering. [H76]
Nebular Variable Stars
Also called T Tauri variables, a type of variable star of spectral classification F, G or K (giants above the main sequence on the Hertzsprung-Russell diagram) that loses an appreciable proportion of its mass in its (irregular) more luminous periods, and is thus surrounded by volumes of gas and dust. [A84]
Multi-Pinned-Phase
Also multi-phase-pinned. The method of driving all CCD phases (gates), including the integrating phase, into inversion and thereby greatly reducing the dark current. The penalty is a slightly poorer well-depth for charge storage. The advantage is a much higher operating temperature. [McL97]
Multi-Doughnut
Also, Multi-Handled Doughnut. A generalization of a doughnut shape (a torus) that has more than one hole. [G99]
Insolation
Amount of radiation received from the Sun per unit area on the Earth's surface per unit time. (The word is a contraction of "incoming solar radiation"). [H76]
R Monocerotis
An A-F pec variable star that illuminates the variable cometary nebula NGC 2261. Its temperature is about 810 K, and it is a source of CO emission. [H76]
H- ion
An H ion with an extra electron in its shell. It is an important source of stellar opacity in stars whose spectral types are later than about A5. [H76]
Hercules X-1
An X-ray pulsar probably about 5 kpc distant, a member of an occulting binary system with an orbital period of 1.7 days. The visible component has been identified as the blue variable HZ Herculis, whose spectrum varies from late A or early F to B. Her X- l has a pulsation period of 1.2378 seconds, presumably its rotation period, and exhibits a 35-day quasi-periodicity in the X-ray region (but not in the optical). It is probably a rotating neutron star in a circular orbit (e < 0.1) with a mass of about 0.7 M, which is accreting matter from HZ Her. The orbital period is stable, but the pulsation period is speeding up at a rate of about 1 part in 105 per year. The X-ray eclipse lasts 0.24 days. (3U 1653+35) [H76]
Horsehead Nebula
An absorption nebula in the middle of Orion. See Omega Nebula. (NGC 2024) [H76]
Gibbous
An adjective applied to the Moon or Venus when it is more than half full (but not full). [H76]
Frozen-In
An adjective which applies to the abundance of elements produced by the r-process in a supernova when the temperature has dropped below the point at which they can serve as seed nuclei for further nucleosynthesis. It also applies to the magnetic field lines of a star in gravitational collapse. [H76]
Henyey Track
An almost horizontal track of stellar evolution between the Hayashi track and the main sequence. [H76]
Mottle
An alternative word for spicule. [H76]
Mills Cross
An antenna array consisting of two antennas oriented at right angles to each other. It produces a single narrow pencil beam. [H76]
Holtsmark Approximation
An approximation in which the lines emitted and absorbed by atoms are subject to the fluctuating electrostatic fields to which the atom is subject in an ionized atmosphere. [H76]
Lennard-Jones Potential
An approximation of the interaction between two atoms or molecules. [H76]
Mare
An area on the moon that appears darker and smoother than its surroundings. Lunar maria are scattered basaltic flows. (plural is Maria)[H76]
Iris
An arrangement able to vary the amount of light that enters an optical instrument. In the mammalian eye, this is a circular muscle that changes the size of the pupil. In many optical instruments, a similar effect is obtained with a diaphragm. In either case the aperture is varied. [DC99]
Juno
An asteroid 250 km in diameter (P = 1,594 days; a = 2.67 AU: e = 0.256; i = 13°.0) with a relatively large albedo (0.2). Rotation period 7h.21. [H76]
Image Intensifier
An electronic device for increasing the brightness of a faint optical image. The image is first formed on a thin metallic surface called a photocathode from which electrons are then ejected. The stream of electrons is accelerated and focussed onto a phosphorescent screen which glows brightly as a result of the impact. [McL97]
Lagoon Nebula
An emission nebula in Sagittarius 2 kpc distant. (M8, NGC 6523) [H76]
Flash Spectrum
An emission spectrum of the olar chromosphere, obtained by placing an objective prism in front of the telescopic lens the instant before (or after) totality in a olar eclipse. [H76]
Hagedorn Equation of State
An equation of state for extremely degenerate matter (density greater than about 1015 g cm-3). [H76]
Harman-Seaton Sequence
An evolutionary sequence of hot subdwarfs and nuclei of planetary nebulae. [H76]
Microwave Background Anisotropy Experiment
An experiment designed to measure the intensity of the cosmic microwave background radiation in different directions. A fundamental prediction of the cosmological origin of this radiation is that the earth's motion relative to the distant regions of the Universe should be detectable. The effect amounts to an increase of about 10-3 K in brightness in the direction we are traveling and a similar decrease in the opposite direction. [SILK90]
Klein-Nishina formula
An expression for the total or differential cross section for the Compton scattering of a photon by a free electron according to Dirac's electron theory. [H76]
Ney-Allen Nebula
An extended infrared source in the Trapezium region of Orion, which shows a strong 10-µ emission feature assumed to result from circumstellar shells of silicate dust. [H76]
H
Chemical symbol for hydrogen. The most abundant chemical in the universe. H2 is the symbol for the molecular hydrogen molecule which is abundant in giant clouds in our galaxy and can be detected by its infrared spectrum. The symbols HI and HII are used to indicate neutral and ionized hydrogen respectively. HII regions are usually associated with star formation, e.g. the Orion Nebula, and are detected by their emission lines such as H-alpha at 656.3 nm. Radio emission at 21 cm wavelength can be detected from neutral hydrogen. [McL97]
Higher-Dimensional Supergravity
Class of supergravity theories in more than four spacetime dimensions. [G99]
Laplacian Determinism
Clockwork conception of the Universe in which complete knowledge of the state of the Universe at one moment completely determines its state at all future and past moments. [G99]
Limb Correction
Correction that must be made to the distance between the center of mass of the Moon and its limb. These corrections are due to the irregular surface of the Moon and are a function of the librations in longitude (see Longitude, Celestial) and latitude (see Latitude, Celestial) and the position angle from the central meridian. [S92]
Neutron Matter
Degenerate matter in which the electron:proton:neutron ratio is about 1:1:8 (or perhaps 1:1:10 or 1:1:12). [H76]
J H K L M N Q
Designations for parts of the infrared waveband transmitted by the Earth's atmosphere in the wavelength range 1 to 20 µm. [McL97]
Fresnel Diffraction
Diffraction observed when either source or screen (or both) are close to the diffractor. In Fresnel diffraction, the wavefronts are not plane (as in Fraunhofer diffraction) and analysis is difficult. The approach is useful in explaining (for instance) diffraction around a circular obstacle. [DC99]
Gravitational Waves
Disturbances or ripples in spacetime predicted by the General Theory of Relativity due to changing distributions of mass such as the spindown of a neutron star binary system or the implosion of a star during a supernova. No confirmed direct detections so far. [McL97]
Light
Electromagnetic radiation with wavelengths of or close to those detectable by the eye. [F88]
Neon
Element with atomic number 10, and the third most common metal in the Universe. Neon is produced by carbon burning in high-mass stars and released into the Galaxy when they explode. [C95]
Mean Elements
Elements of an adopted reference orbit (see Elements, Orbital ) that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations. [S92]
Hubble's Classification
Elliptical, ranging from E0 (spherical) to E7 (greatest eccentricity); S0 (nuclei surrounded by disklike structure without arms); spiral, ranging from Sa (arms tightly wound around the nucleus) to Sc (arms widely spread out from the nucleus): barred spirals ranging from SBa (arms tightly wound) to SBc (arms widely spaced out): Irregular (Ir). [H76]
Jupiter
Fifth planet from the Sun. Mass 1.90 × 1030 g = 318 times Earth's. It is more massive than all other planets and satellites combined; if it were about 80 times more massive, it would become self-luminous. Equatorial radius 7.135 × 109 cm = 11 times Earth's; polar radius 6.7 × 109 cm; oblateness (from Pioneer 10) 0.065. Surface gravity 2.7 that of Earth; Vesc 61 km s-1; mean density 1.33 g cm-3. Rotational period 9h50m at equator; 9h55m at polar regions (see Systems I and II longitude). Semi-major axis 5.203AU, e = 0.048, i = 1°18'18". Obliquity 3°.1. Orbital period 11.86 years, mean orbital velocity 13.06 km s-1. Synodic period 398.9 days. Albedo 0.51. Surface temperature about 120 K. Current estimates of Jupiter's central temperature and density are 5 × 104 K and about 35 megabars (the heat flux to the surface is mainly convective). For both Jupiter and Saturn it is necessary to invoke a substantial source of internal heating (presumably gravitational contraction) to account for the surface temperature (Jupiter radiates about 2 1/2 times as much heat as it receives from the Sun). Jupiter's core is probably metallic hydrogen in a pressure-ionized liquid phase. Jupiter's surface shows pronounced horizontal striations: the light layers (zones) are at a slightly higher altitude and about 15° cooler than the dark layers (belts). Atmosphere primarily H2 (85%) and He (14%), with traces of methane, ammonia, etc. Pioneer 10 established that Jupiter has a magnetic field of about 4 gauss (the magnetic axis is inclined 15° to the rotational axis and is offset about 0.1 Jupiter radius from the center of the planet) and also that it is surrounded by a partial torus of atomic H in the orbit of Io. Thirteen satellites, the four outermost of which have retrograde motion, high eccentricity, and high inclination. (Jupiter XIII, discovered in 1974, has a period of 239 days; i = 26°.7, e = 0.147.)
H-R Diagram
Hertzsprung-Russell diagram. A diagram of stars arranged according to their luminosity (measured on the y axis) and temperature (on the x axis). In the early part of the twentieth century, the Danish astronomer E. Hertzsprung and the American astronomer H. Russell plotted known stars in such a diagram and found a definite correlation between luminosity and temperature. (See main sequence.) [LB90]
Lagrangian Points
Five points in the orbital plane of two massive particles in circular orbits around a common center of gravity, where a third particle of negligible mass can remain in equilibrium. Three of the points are on the line passing through the centers of mass of the two bodies - L2 beyond the most massive body, L1 (the point through which mass transfer occurs) between the two bodies, and L3 beyond the less massive body. All three of these points are in unstable equilibrium. The other two (L4 and L5) are stable, and are located at the two points in the orbit of the less massive component which are equidistant from the two main components. see Trojans [H76]
Meridional Flow
Flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator: a negative value, flow toward the equator. [H76]
Isothermal Fluctuations
Fluctuations in the matter density, without any associated perturbation of the radiation density. The radiation temperature therefore remains uniform. Prior to the Decoupling Era, isothermal fluctuations were frozen, neither growing nor decaying after decoupling, isothermal fluctuations became gravitationally unstable, if greater than the Jeans mass, about 106 M. [Silk90]
FU
Flux Unit [LLM96]
Mean Free Time
For gas atoms or molecules in a container, or electrons and impurity atoms in a semiconductor, the average time between particle collisions. see also Mean Free Path [DC99]
Laplacian Plane
For planets see Invariable Plane; for a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component. [S92]
FTS
Fourier Transform Spectrometer [LLM96]
FT
Fourier Transform [LLM96]
FWHM
Full Width at Half Maximum The full width of a profile (e.g. the seeing profile or a filter transmission curve) between the two points where the value is 50% of the peak value. [McL97]
GIS
Gas Imaging Spectrometer (ASCA X-ray satellite).
Methane
Gaseous hydrogen compound, one of the alkanes, in which every carbon atom is surrounded by four hydrogen atoms. [A84]
High-Velocity Object
Generally a celestial object in the galactic halo whose orbital velocity around the galactic center is less than that of the Sun, and that thus, relative to the Sun, has a high space motion. A "high-velocity" object usually travels around the galactic center in an eccentric orbit, often of large inclination to the galactic plane. [H76]
GRG
Giant Radio Galaxies, represent the biggest single objects in the Universe, having linear sizes larger than 1 Mpc. GRGs are rare among the entire population of radio galaxies, and their physical evolution is not well understood. GRGs are of special interest for several reasons: The lobes of radio sources can compress cold gas clumps and trigger star or even dwarf galaxy formation, they can also transport gas from a host galaxy to large distances and seed the IGM with magnetic fields. Since GRGs are larger in size than normal radio galaxies by anywhere from 10 to 100 times, their influence on the ambient medium is correspondingly wider and is pronounced on scales comparable to those of clusters of galaxies or larger. GRGs could therefore play an important role in the process of large-scale structure formation in the Universe. [BFM2004]
Jovian
Giant planets that have a gaseous surface; the sun's known Jovian planets are Jupiter, Saturn, Uranus, and Neptune. [F88]
GHz
Giga Hertz. [LLM96]
G$
Giga US Dollars. [LLM96]
GPS Radio Source
Gigahertz Peaked Spectrum A class of powerful compact extragalactic radio sources characterized by an overall turnover in their radio spectra at about 1 GHz in frequency.
GPS
Global Positioning System. [LLM96]
HB or Horizontal Branch Stars
Globular cluster stars defined by their position in the color-magnitude diagram. They are located on both sides of the RR Lyr gap and one speaks therefore of blue or red HB stars. [JJ95]
GSFC
Goddard Space Flight Center
GUT
Grand Unified Theory. [F88]
GUI
Graphical user interface. [McL97]
GSD
Greenwich Sidereal Date The number of sidereal days elapsed at Greenwich since the beginning of the Greenwich sidereal day that was in progress at Julian date 0.0. [S92]
Hirayama Families
Groups of minor planets with similar orbital elements. The members of a given family are widely believed to have resulted from collisions between larger parent bodies. [H76]
Negative Hydrogen Ion
H- ion. [H76]
Formic Acid
H2C02 -- A simple organic acid, the first to be detected in interstellar space (in 1970 at 18.3 cm). Formic acid is the "sting" of an insect. [H76]
Formaldehyde
H2CO -- An organic molecule, the first polyatomic molecule to be discovered in interstellar space (in 1969). In 1973 It was discovered in two external galaxies. [H76]
Formamide
HCONH2 -- A molecule discovered in interstellar space in 1971 at 6.5 cm. [H76]
HPBW
Half Power Beam Width. The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system. [H76]
Metal-Poor
Having a low metallicity with respect to Solar. [C95]
F-Type
Having a spectral type of F, that is, yellow-white, like Canopus and Procyon. [C95]
M-Type
Having a spectral type of M, that is, red like Betelgeuse and Antares. [C95]
Heliocentric
Having the Sun at the center. [A84]
Heterotic E-String Theory
Heterotic E8 × E8 string theory One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-O string theory. [G99]
HBL
High energy peaked BL Lac object
HEAO
High-Energy Astronomical Observatory. [LLM96]
Giant Stars
High-luminosity stars that lie above the main sequence on the Hertzsprung-Russell diagram. [F88]
HPQ
Highly Polarized Quasar
HTML
Hyper-Text Mark-up language.
HTTP
Hyper-Text Transfer Protocol.
Nebular Hypothesis
Hypothesis, maintained in the nineteenth and early twentieth century, that the spiral nebulae are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare Island Universe Theory [F88]
Little Bangs
Hypothetical explosions of supermassive stars shortly after the big bang, with the release of processed elements into the interstellar medium, postulated by Wagoner to account for the anomalously high abundance of Helium in the Universe, and for the fact that even the oldest stars have some metals. [H76]
Intergalactic Matter
Hypothetical material within a cluster of galaxies, whose gravitational effect is to maintain the equilibrium of the cluster. Theoretically comprising 10-30 times the mass of the galaxies themselves (in order to have the observed effect), it has yet to be detected in any form - although the most likely form is as hydrogen. [A84]
Intergalactic Medium
Hypothetical matter (in the form of gas) in the regions between galaxies. It has not yet been detected (but see Magellanic Stream), but Oort (1970) has argued that as much as a factor of 16 or more matter may be present in uncondensed form. [H76]
Impurity and Conduction
IBC A form of infrared array detector which replaces the photoconductor and provides higher performance. [McL97]
Initial Mass Function
IMF A quantity that determines the number of stars per unit time evolving from the main sequence. [H76]
Froissart Bound
If in a hadron-hadron collision the absorption is complete, then the interaction radius cannot increase faster than the logarithm of the energy. [H76]
IRAF
Image Reduction and Analysis Facility. An extensive suite of programs developed for astronomy applications and supported by the US National Optical Astronomy Observatories. [McL97]
L Galaxy
In Morgan's classification, an elongated galaxy of low surface brightness. [H76]
Lyot Division
In Saturn's rings, the gap between rings B and C. [H76]
Mini Black Holes
In a chaotic early Universe, black holes may form at eras as early as the Planck time. The characteristic size of these mini black holes is 10-6 gram, the minimum mass of a collapsing inhomogeneity at that time. Larger mini black holes may form at later eras. Since conventional theories of stellar evolution show that only very massive stars can form black holes, the possible formation of mini black holes is a unique characteristic of the very early Universe. [SILK90]
K correction
In extragalactic studies, the observed luminosities and colors of objects participating in the expansion of the Universe are known to require procedural corrections for two physically distinct, but redshift-related, reasons: (1) the stretching of the restframe spectral energy distribution across the observer's (fixed) bandpass, and (2) the redshift/displacement of the restframe wavelength across and into the observer's filter. The first effect is purely a function of the redshift, z, where the effective filter width decreases in proportion to 1 / (1 + z). The second effect is primarily a function of the spectral energy distribution of the source, rising and falling with the relative continuum (or emission-line) strength being sampled for that filter and redshift combination. Failure to appreciate these "technical effects" associated with the redshift led to a very early claim of rapid evolution of nearby elliptical galaxies, the so-called Stebbins-Whitford Effect. (also called K term) [BFM99]
Fission
In nuclear physics, the splitting of the atomic nucleus of a heavy element, resulting in the emission of nuclear energy and possibly causing a chain reaction (with similar results) within a mass of the element. [A84]
Lobes
In radio astronomy, regions of sensitivity in an antenna pattern. Lobes are analogous to fringes in optical astronomy. [H76]
Massless Black Hole
In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially massive black hole has no remaining mass - it is massless. In this state, it no longer manifests such usual black hole properties as an event horizon. [G99]
Hierarchy Problem
In the context of grand unified theories, the hierarchy problem is our inability to understand theoretically why the energy scale at which the unification becomes apparent, about 1016 GeV (billion electron volts), is so much higher than other energy scales of relevance to particle physics, such as the mass/energy of a proton, which is only 1 GeV. [G97]
Mirror Symmetry
In the context of string theory, a symmetry showing that two different Calabi-Yau shapes, known as a mirror pair, give rise to identical physics when chosen for the curled-up dimensions of string theory. [G99]
Galactic Nucleus
In the innermost region of a galaxy, there is often a concentration of stars and gas, sometimes extending over thousands of light-years from the center of the galaxy. [Silk90]
Mean Anomaly
In undisturbed elliptic motion, the product of the mean motion of an orbiting body and the interval of time since the body passed pericenter. Thus the mean anomaly is the angle from pericenter of a hypothetical body moving with a constant angular speed that is equal to the mean motion. see True Anomaly; Eccentric Anomaly [S92]
Limb Brightening
Increase in the intensity of radio or X-ray brightness of the Sun or other star from its center to its limb. [H76]
Line Broadening
Increase in the range of wavelengths in which some characteristic emission or absorption occurs, due to a number of causes (e.g., Doppler broadening, the effects of perturbers, etc.). [H76]
InSb
Indium Antimonide A compound semiconductor used as an infrared photoelectric detector. [McL97]
IRAS
Infrared Astronomy Satellite. [LLM96]
ISO
Infrared Space Observatory. [LLM96]
IRS
Infrared source. [C95]
IR
Infrared. [LLM96]
gravitational field
Instead of describing gravity as an action-at-a-distance force, modern physicists describe it in terms of a gravitational field. At each point of space, the field is defined as the force that would be experienced by a standard mass, if the mass were positioned at that point. While Newton's law of gravity can be expressed equally well in terms of an action-at-a-distance or a field, Einstein's theory of general relativity, which is now the accepted description of gravity, can be formulated only in terms of fields. [G97]
IRAM
Institut de RadioAstronomie Millimetrique. [LLM96]
Interstellar Matter
Interstellar gas (99%) and dust (1%). The observed density of the interstellar medium is about 1-5 atoms per cm3. The two other components of the interstellar medium are magnetic fields and cosmic-ray electrons. [H76]
Lorentz Invariant
Invariant with respect to Lorentz transformations. [H76]
Hyperdimensional
Involving more than the customary four dimensions (three of space plus one of time) of relativistic space-time. [F88]
H II
Ionized hydrogen - that is, hydrogen with its electron missing. [C95]
Jitter
Irregular random variations in a radio signal. [H76]
Law of Universal Attraction
Isaac Newton's formulation of the law of gravity. [A84]
Helium Shell Flash
It has been shown that helium shell burning outside a degenerate core is unstable; the helium-burning shell does not generate energy at a constant rate but instead produces energy primarily during short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection. The resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning. [H76]
Jacobi Ellipsoid
Jacobi discovered that homogeneous, self-gravitating masses rotating uniformly and sufficiently rapidly can have the shape of triaxial ellipsoids. These are the Jacobi ellipsoids. [H76]
JPL
Jet Propulsion Laboratory, Pasadena, California. Funded by NASA and operated by the California Institute of Technology (Caltech). [McL97]
JD
Julian Day -- A unit of time within the Julian Dating System where the number of ephemeris days that have elapsed since 12h ephemeris time on January 1, 4713 B.C. JD for 1970 January 1 is 2440588.
Io
Jupiter I The innermost Galilean satellite of Jupiter, similar in size and density to the Moon (R 1850 km from Pioneer 10, period 1.77 days; e = 0.01, i = 0.03). Jupiter's decametric radiation has been linked at least partially to Io. Mean density (from Pioneer 10) 3.48 g (the highest of any of the Galilean satellites). Pioneer 10 also detected the presence of an ionosphere, and Na D emission. Albedo 0.91 (?), the highest in the solar system. [H76]
KAO
Kuiper Airborne Observatory. [LLM96]
Luminous Blue Variables
LBV -- A variable-star designation for the high-luminosity early type objects. Also called S Dor variables or Hubble-Sandage Variables. [JJ95]
Light-Emitting Diode
LED -- A semiconductor diode, made from certain materials (e.g. gallium arsenide), in which light is emitted in response to the forward-bias current. The light results from the recombination of electrons and positive holes, with a transition to a lower energy state. see also Diode [DC99]
Long-Period Variables
LPV -- Pulsating disk-population red giants or supergiants with periods of 100 to 1000 days (Population I typically have periods greater than 200 days; Population II, periods less than 200 days). Typical is Mira (o Ceti), which has a period of 331 days. Long-period variables may vary by as much as 9 magnitudes in the visible, but in the integrated spectrum (most of their radiation is in the infrared) they vary by only 2 or 3 magnitudes. They are usually of spectral type M, R, or N. (sometimes called Red Variables) [H76]
LISA
Large Interferometric Space Antenna
Hot Big Bang
Later, but fundamental, concept within the big-bang theory, that the primordial explosion occurred in terms of almost unimaginable heat. The concept, formulated by George Gamow, led to considerable study of thermonuclear reactions and the search for background radiation. [A84]
Newton's Laws of Motion
Laws describing the motion of bodies based on the conception of an absolute and immutable space and time; these laws held sway until Einstein's discovery of special relativity. [G99]
LSB
Least Significant Bit see also DN [McL97]
Johnson Noise
Low-frequency electromagnetic radiation associated with thermal fluctuations, which is emitted by all bodies whose temperature is above 0 K. [H76]
Interstellar Molecules
Molecules in interstellar space. As of late 1974, at least 33 molecular species had been identified with reasonable certainty: methylidyne CH, ionized methylidyne CH+, the cyanogen radical CN, the hydroxyl radical OH, ammonia NH3, water vapor H2O, formaldehyde H2CO, carbon monoxide CO, hydrogen cyanide HCN, hydrogen isocyanide HNC, molecular hydrogen H2, X-ogen, cyanoacetylene HC3N, methyl alcohol CH3OH, formic acid HCOOH, carbon monosulfide CS, carbonyl sulfide 0CS, formamide NH2CHO, silicon monoxide SiO, methyl cyanide CH3CN, isocyanic acid HNCO, methyl formate HCOOCH3, methyl acetylene CH3C2H, acetaldehyde CH3CHO, thioformaldehyde H2CS, hydrogen sulfide H2S, methanimine H2CNH, ethynyl, sulfur monoxide SO dimethyl ether (CH3)2O, methyl amine CH3NH2, silicon monosulfide SiS, and ethyl alcohol C2H5OH. [H76]
MCAO
Multi-Conjugate Active Optics -- An active optics techniquefor increasing the resolution-emhance field of view in a terrestrial telescope, by compensating fo atmospheric turbulence simultaneously along several adjacent lines of sight. [BFM2002]
MIDAS
Munich Image Data Analysis System -- A suite of programs and a software environment developed at the European Southern Observatory for astronomy applications. [McL97]
NRAO
National Radioastronomy Observatory (USA). [LLM96]
NSF
National Science Foundation (USA). [LLM96]
Moon
Natural satellite of Earth. Mass 7.35 × 1025 g = 0.0123 Earth's; mean radius 1738 km; mean density 3.34 g cm-3; mean distance from Earth 384,404.377 ± 0.001 km (1.28 lt-sec); Vesc 2.38 km s-1; surface gravity 162.2 cm s-2 = 0.165 Earth's. Sidereal period 27d7h43m11s, e = 0.0549, inclination of orbital plane to ecliptic 5°8'43". Obliquity 6°41'. Synodic period 29d12h44m2s.9. Vorb = 1.02 km s-1. Albedo 0.07. The Moon's center of mass is displaced about 2 km in the direction of Earth. Studies of lunar rocks have shown that melting and separation must have begun at least 4.5 × 109 years ago, so the crust of the Moon was beginning to form a very short time after the Solar System itself. Thickness of crust, 60 km; of mantle, 1000 km. Temperature of core, 1500 K. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. The Moon's orbit is always concave toward the Sun. [H76]
NICMOS
Near Infrared Camera for Multi-Object Spectrography [HST] [LLM96]
Isobars
Nuclei with the same A number but different Z numbers. Also, lines connecting equal atmospheric pressures. [H76]
Number Density
Number of particles per cm3 (cf. column density). [H76]
Halo Population
Old stars typical of those found in the halo of the Galaxy; also called Population II. [H76]
MeV
One million (106) electron volts. [F88]
Mintaka
One of the three stars in Orion's belt, and the star along whose line of sight interstellar gas was first spectroscopically detected. [C95]
Flat Field
Or flat-fielding. An extensive, very uniformly illuminated source of light used to determine the relative sensitivity of an imaging detector composed of many picture elements (pixels) each with a slightly different response to light. The correction process is called flat-fielding. [McL97]
High-Energy Particles
Particles of electromagnetic radiation that contain high energies, measured in terms of electron volts. The energy in gamma radiation is of the order of 8 x 107 to 8 x 105 electron volts and in X-rays of 8 x 103 to 8 × 101 electron volts. [A84]
Linear Electron Collider
Particles traveling in a curved path continuously radiate p hotons that carry away some of the particles' energy. The fraction of energy radiated increases with the energy of the particle, and the radiation happens with greater probability for lighter particles than for heavier ones. For electrons this loss of energy is a large effect at the circular CERN LEP collide r, and it would be worse at a higher-energy collider, so it is unlikely that any future electron collider will be circular. The next electron collider built is expected to be a linear one, where the radiated energy loss is greatly decreased (NLC, for Next Linear Collider), modeled on the first linear collider, the SLC at SLAC. [K2000]
Helium Problem
Poses the question: what physical process caused the current abundance of helium in the Universe? [C97]
Nuclear Fusion
Process by which the Sun (and other stars) radiates energy. The nucleus of an atom fuses with the nuclei of other atoms to form new, heavier atoms at the same time releasing large amounts of energy. In the Sun, hydrogen atoms are converted into helium by this process, with carbon and nitrogen as intermediates. Cooler stars undergo the proton-proton cycle with a similar result. [A84]
Forbidden
Processes can be naively imagined that might occur, but should not occur according to the predictions of the Standard Model. Whether they occur is then a test of the Standard Model. If they occurred at the same rate as other processes the Standard Model would be wrong; if they occur at much smaller rates, or do not occur at all, they provide a clue as to how to extend the Standard Model. None of the processes forbidden by the Standard Model have been observed. [K2000]
Inertia
Property of a moving body to continue moving at the same speed in the same direction - or of a static body to remain static - unless and until acted upon by some force for change. The inertial mass of a body is reckoned as equal to the body's gravitational mass. [A84]
Non-Thermal Radiation
Radiation emitted by energetic particles for reasons other than high temperature of the source. The spectrum of nonthermal radiation is different from that predicted by Planck's law for a blackbody. [H76]
Gaussian Noise
Random fluctuations in an otherwise smooth distribution of something. [LB90]
Image Spectrometers
Refers to a class of instruments which preserve the image field while also determining the spectrum. Integral Field Unit (IFU). Usually implies some kind of image slicing either with facets or fiber optics. [McL97]
Macroscopic
Refers to scales typically encountered in the everyday world and larger; roughly the opposite of microscopic. [G99]
H I Region
Region of neutral (atomic) hydrogen in interstellar space. The temperature is about 125 K (the spin temperature of neutral hydrogen - far too low for electrons to emit radiation in the optical part of the spectrum (see 21-cm radiation). At least 95 percent of interstellar H is H I. (Density is about 10 atoms per cm3, about the same as in H I regions.) [H76]
Hugoniot Relations
Relations expressing conservation of baryon number, momentum, and energy across a shock front. [H76]
Greenhouse Effect
Retention and escalation of temperature beneath a mantle of clouds or denser atmosphere. [A84]
de Vaucouleurs-Sandage Classification
SA (ordinary spirals), SB (barred spirals): then in parentheses a lower case s (for S-shaped spirals) or r (for the ringed type). Finally, several transitional stages have been added between the SA or SB spirals and the Magellanic irregulars Im. In this classification the Andromeda Galaxy is SA(s)b. [H76]
International System of Units
SI Units A practical system of units of measurement adopted in 1969 by the 11th International General Conference of Weights and Measures (CGPM). The seven base units are the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. [H76]
Holomorphic
Same as complex analytic. [LB90]
Gravitational Energy
Same as gravitational potential energy. [G97]
Homogeneity Problem
Same as horizon problem. [LB90]
Inverse Plasmon Scattering
Scattering of electrostatic plasma waves by a flux of relativistic electrons. [H76]
Mie Scattering
Scattering of light (without regard to wavelength) by larger particles, such as those of dust or fog in Earth's atmosphere (see also Rayleigh Scattering). [H76]
Non-Conservative Scattering
Scattering that occurs in the presence of absorption. [H76]
Heliocentric Cosmology
School of models in which the sun was portrayed as standing at the center of the universe. [F88]
H-R Diagram
See Hertzsprung-Russell Diagram. [C95]
Helmholtz Contraction
See Kelvin-Helmholtz contraction. [H76]
Great Red Spot
See Red Spot. [H76]
head (of comet)
See coma. [H76]
Gamma-Ray Bursts
See cosmic gamma-ray bursts. [H76]
Grating
See diffraction grating. [A84]
Galactic Light
See diffuse galactic light. [H76]
Hydromagnetics
See magnetohydrodynamics. [H76]
Hartle-Hawking Proposal
See no-boundary proposal. [LB90]
Imperfect Scattering
See nonconservative scattering. [H76]
Galactic Cluster
See open cluster. [H76]
Leonid Meteor Shower
Shower of meteors emanating from an apparent point in Leo every 33 years; the most recent one having occurred in 1999.
Greenwich
Site now in London of the first Royal Greenwich Observatory, designed and built by Christopher Wren in 1675. [A84]
Mural Arc
Sixteenth- to nineteenth-century astronomical apparatus comprising a carefully oriented wall on which a calibrated device was fixed, by which the altitudes of celestial objects could be measured. [A84]
Messenger Particle
Smallest bundle of a force field; microscopic conveyer of a force. [G99]
Interstellar Space
Space between the stars of a galaxy. It is generally not, however, a void vacuum, and is the subject of considerable spectral research. [A84]
Interstellar Gas
Sparse, cool gas (mainly hydrogen) in interstellar space. Dust absorbs and scatters radiation; gas does not interact directly with radiation but is coupled to the dust by collisions. Interstellar gas affects only light of certain wavelengths. Temperature 10-100 K. [H76]
Mulliken Bands
Spectral bands of the C2 radical. [H76]
G
Spectral type for yellow stars, such as the Sun, Alpha Centauri A, and Capella. [C95]
Massive Halos
Spherical distributions of dark matter surrounding galaxies. see Dark Matter [LB90]
Jet
Spray of particles produced from the vacuum by the passage of a high momentum quark or gluon. The direction of the jet indicates the direction of the said quark or gluon. [D89]
Magnetic Stars
Stars (usually of spectral type a) with strong integrated magnetic fields ranging up to 30,000 gauss. [H76]
Fundamental Stars
Stars for which coordinates have been determined to a very high degree of accuracy. [H76]
Metal-Rich Stars
Stars having metal-to-hydrogen ratios greater than those of the Hyades. [H76]
G star
Stars of spectral type G are yellowish stars with surface temperatures of about 5000 to 6000 K, in which the H and K lines of Ca II have become dominant and in which a tremendous profusion of spectral lines of both neutral and ionized metals, particularly iron, begins to show. The Balmer lines of hydrogen are still recognizable. Examples are the Sun and Capella. [H76]
K star
Stars of spectral type K are cool, orange to red stars with surface temperatures of about 3600-5000 K. Their spectra resemble those of sunspots, in which the Hydrogen lines have been greatly weakened. The H and K lines (q.v.) reach their greatest intensity. Strongest lines are Ca I (4227 Å) and the G-band (4303 Å). Examples are Arcturus and Aldebaran. [H76]
Leptonic Era
The era following the Hadronic Era, when the Universe consisted mainly of leptons and photons. It began when the temperature dropped below 1012 K some 10-4 seconds after the Big Bang, and it lasted until the temperature fell below 1010 K, at an era of about 1 second. At this stage, the characteristic photon energy fell below the rest mass energy of an electron, and the abundance of electron-positron pairs fell by many orders of magnitude. Only one electron survived for every 108 photons. The Universe was subsequently radiation-dominated (substantial numbers of neutrinos were also present, but they did not interact directly with the matter or the radiation). [Silk90]
Lepton Era
The era following the hadron era, when the temperature had dropped to about 1012 K and when the Universe consisted mainly of leptons and photons. It started about 10-4 s after the big bang and lasted until about 10 s after the big bang; it was followed by the radiation era. [H76]
Matter Era
The era following the radiation era. The matter era started when the temperature of the primeval fireball had dropped to 3000 K, at which time the recombination of hydrogen became possible. [H76]
Night-Sky Light
The faint, diffuse glow of the night sky. It comes from four main sources : airglow, diffuse Galactic light, Zodiacal light, and the light from these sources scattered by the troposphere. [H76]
Larmor Frequency
The frequency of precession of a charged particle orbiting in a uniform magnetic field. It is equal to eH / 4 me, where e is the electron charge, me is the electron mass, and H is the magnetic field strength. [H76]
Geometric Position
The geocentric position of an object on the celestial sphere referred to the true equator and equinox, but without the displacement due to planetary aberration. (See apparent place; mean place; aberration, planetary.) [S92]
Janus
The innermost satellite of Saturn, just outside Saturn's rings. P = 0.75 days; R = 175 (?) km; i 0; e 0. It was discovered by Dollfus in 1966 and was named Janus for the first and the last. [H76]
Hadronic Era
The interval lasting until some 10-4 seconds after the Big Bang when the universe was matter-dominated, containing many hadrons in equilibrium with the radiation field. The Hadronic Era ended when the characteristic photon energy fell below the rest mass of a pion or -meson (270 electron masses), and very few hadrons remained (about one hadron for every 108 photons). [Silk90]
kelvin (K)
The kelvin is the fundamental unit of temperature in the SI system. It is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The definition was adopted by the 16th CGPM (1968) when it was decided to change the name of the unit from the degree kelvin (°K) to the kelvin (K). It is named after Lord Kelvin (1824-1907), one of the outstanding physicists of the nineteenth century. [KM92]
Geocentric Coordinates
The latitude and longitude of a point on the Earth's surface relative to the center of the Earth; also celestial coordinates given with respect to the center of the Earth. (See zenith; latitude, terrestrial; longitude, terrestrial.) [S92]
Local Sidereal Time
The local hour angle of a catalog equinox. [S92]
Mt. Wilson
The location, in California, of the 100-inch diameter telescope used by Edwin Hubble and others. [LB90]
H-magnitude
The magnitude derived from infrared observations at 1.6 microns. [H76]
L-magnitude
The magnitude derived from observations at an infrared wavelength of 3.5 microns. [H76]
M magnitude
The magnitude derived from observations at an infrared wavelength of 5 microns. [H76]
J magnitude
The magnitude derived from the observations at an infrared wavelength of 1.3 microns. [H76]
Luminous Mass
The mass contributed by luminous matter in galaxies (see Missing Mass). Luminous mass density, 5 × 10-32 g cm-3 for H0 = 50 km s-1 Mpc-1. [H76]
Mass of the Galaxy
The mass has been assessed at various galactocentric distances: At R = 9 kpc, M = 1.0 × 1011 Solar masses. At R = 35 kpc, M = 4.0 × 1011 Solar masses. And, at R = 230 kpc, M = 13.0 × 1011 Solar masses. [BFM2002]
Inertial Mass
The mass of an object as measured by the property of inertia. It is equal to the ratio force/acceleration when the object is accelerated by a constant force. In a uniform gravitational field, it is equal to gravitational mass, since all objects have the same gravitational acceleration at the same place. [DC99]
Geometry
The mathematics of lines drawn through space. In euclidean geometry space is postulated to be "flat", i.e., to be the three-dimensional analog of a plane. In noneuclidean geometry space is "curved", i.e., is the three-dimensional analog of a sphere or a hyperbola. [F88]
Infrared Photometry
The measurement of light intensities using infrared light instead of optical (visible to the human eye) light. Infrared light has longer wavelengths than optical light. (See photometry.) [LB90]
Micrometry
The measurement of the apparent sizes and separations of astronomical objects by use of knife blades or crosshairs in the eyepiece of a telescope. If the distance of an object is known, its size can be determined through micrometry. [F88]
Interstellar Medium
The medium of gas and dust that fills the space between the stars. [LB90]
Mile
The mile employed in this book is the statute mile, equal to 5,280 feet. [F88]
Nyquist Sampling
The minimum number of resolution elements required to properly describe or sample a signal, such as a star image, without causing erroneous effects known as aliasing. For electronic imaging, this number is generally taken as 2 pixels across the seeing disk diameter at the half intensity points (FWHM). [McL97]
Iapetus
The ninth satellite of Saturn, about 850±100 km in radius; period 79d7h55m, e = 0.028, inclination to Saturn's orbital plane 14°.7. It has the most extreme variation in albedo of any satellite in the solar system (0.04 for the leading side, 0.28 for the trailing side). Discovered by Cassini in 1671. [H76]
Helium Flash
The onset of runaway helium burning under degenerate conditions. The helium flash occurs in the hydrogen-exhausted core of a star in the red-giant phase of evolution. When gravitational pressure has brought the degenerate core to a temperature of about 108 K, the helium nuclei can start to undergo thermonuclear reactions. Once the helium burning has started, the temperature builds up rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached where the thermal pressure expands the core and thus removes the degeneracy and limits the flash. The helium flash can only occur when the helium core is less than the 1.4 M Chandrasekhar mass limit and thus it is restricted to low-mass stars. [H76]
L Component
The part of the Solar corona whose spectrum consists of emission lines. [H76]
Higgs Particle
The particle or particles associated with the bundles of energy in the Higgs field. Such particles are analogous to the photons that are associated with the electromagnetic field. The standard model of particle physics predicts one electrically neutral Higgs particle which has not yet been found, but which will be sought in upcoming particle accelerator experiments. The grand unified theories predict many Higgs particles, but they are too massive to be accessible at existing or foreseeable accelerators. [G97]
Null Geodesic
The path of a light ray in curved spacetime. It is characterized by the fact that its tangent U at any point is of zero length: Uµ Uµ = 0. (same as null line) [H76]
Null Line
The path, in space-time, of a light ray or other massless object. Space-time distances measured along a null line are zero. (same as null geodesic) [P88]
Gaussian Year
The period associated with Kepler's third law with a = 1. [H76]
Normal
The perpendicular to a reflecting or refracting surface at the point of incidence of the ray concerned. Angles of incidence, reflection, and refraction are measured between the normal and the incident ray, reflected ray, and refracted ray respectively. A normal ray is one incident perpendicularly on a surface - the angle of incidence is zero. [DC99]
Invariant Plane
The plane defined by the total angular momentum of the solar system. It is within about 1°.5 of the ecliptic. [H76]
Galactic Plane
The plane that contains the disk of the Milky Way. By definition, one direction perpendicular to this plane is called "above" or "north", and the opposite direction, also perpendicular to the Galactic plane, is called "below" or "south". From Earth, due Galactic north is marked by the north Galactic pole, which lies near the bright star Arcturus, and due Galactic south is marked by the south Galactic pole, which lies in the faint constellation Sculptor. [C95]
Invariable Plane
The plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system. [S92]
Galactic Disk
The plate-shaped component of a spiral galaxy, in which the spiral arms are found. [F88]
North Point
The point at which the meridian intersects the horizon below the North Celestial Pole. [H76]
Galactic Anticenter
The point in the Galactic plane that lies directly opposite the Galactic center. Here we gaze toward the edge of the Galactic disk. The nearest bright star to the anticenter is El Nath, in the constellation Taurus. [C95]
Main-Sequence Turn-Off
The point on the HR diagram of a star cluster where main-sequence stars are beginning to leave the main sequence. The main-sequence turnoff measures age: all other things being equal, the older a star cluster, the fainter the main-sequence turnoff. [C95]
Lense-Thirring Effect
The precession of the plane of the geodesic orbit of a test particle around a rotating mass in general relativity. It arises from the coupling of the rotation of the central mass with the orbital angular momentum of the test particle. This precession is described as resulting from the dragging of inertial frames. [H76]
Interstellar Hydrogen
The presence of hydrogen gas between the stars of a galaxy, thus "filling out" the shape of the galaxy in a way that can be detected by spectral analysis and radio monitoring. [A84]
Magnetic Pressure
The pressure exerted by a magnetic field on the material that contains the field. in gaussian units it is given by pm = b2 / 8, where b is the magnetic field strength. [H76]
Hierarchical Clustering
The process by which a system of self-gravitating particles will gradually aggregate into larger and larger gravitationally bound groups and clusters. Small clusters merge into larger clusters, which retain little trace of the subunits from which they formed. Elliptical galaxies may have formed in this way from mergers of globular-cluster-sized star clusters; clusters of galaxies may have formed by a similar process. [Silk90]
Gravitational Instability
The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation. In the expanding universe, the growth is slowed by the expansion, but the unstable fluctuations eventually collapse into stable, gravitationally bound systems. [Silk90]
Internal Symmetry
The properties of different elementary particles can be related to each other by mathematical transformations that look very much like the more familiar symmetry properties of our own physical space. Physicists have therefore hypothesized an abstract internal space in which these internal symmetries are defined. With the help of these internal symmetries, the elementary particles can be gathered into families. The relationship between space-time and these internal symmetries remains to be fully explained. [P88]
Hamiltonian Function (H)
The quantity in classical mechanics corresponding to the total energy of a system, expressed in terms of momenta and positional coordinates. [H76]
Holmberg Radius
The radius of an external galaxy at which the surface brightness is 26.6 mag arcsec-2. This criterion was developed by Holmberg in 1958 to estimate the actual dimensions of the major and minor axes of a galaxy without regard to its orientation in space. [H76]
Larmor Radius
The radius of the circular orbit that a charged particle describes transverse to a magnetic field. [H76]
Gravitational Radius
The radius which an object should have in order that light emitted from its surface just ceases to escape from its surface. [H76]
Neutron Drip
The rapid increase in the abundance of free neutrons that occurs when physical conditions are such that the neutron becomes the stable nucleon with respect to electron capture (as will happen in a degenerate assembly of electrons with sufficiently high Fermi threshold energy). [H76]
Matter-to-Antimatter Ratio
The ratio of mass in particles to mass in antiparticles. For every type of particle, there is an antiparticle counterpart. The positron, for example, is the antiparticle of the electron and is identical to the electron except for having opposite electrical charge. The abundances of particles and antiparticles do not have to be equal. It appears that our Universe is made up almost entirely of particles, rather than antiparticles, although there is no fundamental difference between the two kinds of matter. [LB90]
Focal Ratio
The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number; f/# = F/D. Also defines the cone angle of the beam. Small focal ratios e.g. f/# = F/D = 1 are said to be "fast" and represent a very large cone angle. Large focal ratios e.g. f/# = 35 are said to be "slow" and indicate a very small cone angle. [McL97]
Knudsen number
The ratio of the mean free path length of the molecules in a fluid to a characteristic length of the structure in the fluid stream. [H76]
Index of Refraction (n)
The ratio of the speed of light in a vacuum to that in a given medium. [H76]
Neutrino Bremsstrahlung
The reaction in which an electron scatters from a nucleus, emitting a neutrino-antineutrino pair. [H76]
Interstellar Extinction
The reddening of starlight passing through interstellar dust, caused by the fact that dust scatters blue light more than red. [H76]
Interstellar Reddening
The reddening of starlight passing through interstellar dust, caused by the fact that dust scatters blue light more than red. [H76]
Life Zone
The region around a star where a planet can have liquid water and so may support life. [C95]
Magnetopause
The region in earth's ionosphere where the magnetosphere meets the Solar Wind. essentially, it is the place where earth's magnetic field stops; the region above the magnetopause is no longer part of earth's atmosphere, but is part of interplanetary space. [H76]
Local Bubble
The region of the Galaxy near the Sun which has little neutral hydrogen gas. It extends about a hundred light-years in most directions but up to a thousand in some. The local bubble may have been produced by supernovae. [C95]
Focal Plane Scale
The relationship between angles on the sky, in seconds of arc, and millimeters of size at the focus of the telescope; i.e. the number of arcsecs per millimeter. [McL97]
Inverse -decay
The relatively rare process p + vbar -> n + e+. Free-electron capture (e + p -> n + v) is sometimes called inverse -decay in astrophysics. [H76]
Hubble Program
The research program carried out in the 1920s and 1930s by Edwin Hubble to measure the recessional speeds and distances of a large number of galaxies and to attempt to measure the deceleration parameter. This last parameter can in principle be determined by measuring the apparent brightness and redshift of a large number of objects of identical intrinsic luminosity. (See deceleration parameter; Sandage program; standard candle.) [LB90]
Galactic Rotation
The revolving of a galaxy round its central nucleus even as it continues its proper motion. Such rotation, however, is not uniform but differential. One revolution of the Sun within our own Galaxy takes about 225 million years, or 1 cosmic year. [A84]
Mimas
The second innermost satellite of Saturn, discovered by Herschel in 1789. P = 0d.94, R 250 km. Albedo 0.49. It is the perturbations of Mimas and Janus that produce the divisions in Saturn's rings. [H76]
Leo II
The second most distant galaxy that orbits the Milky Way, lying 720,000 light-years from the Galactic center. Like Leo I, Leo II is a dwarf galaxy that was discovered in 1950.[C95]
Native Oxide
The silicon dioxide layer which grows in air on the exposed backside surface of a thinned CCD. [McL97]
Newtonian Cosmology
The simplest cosmological models, including the standard Big Bang models, can be derived in the framework of Newtons classical theory of gravitation, although Birkhoff's theorem from the theory of general relativity is needed to justify the use of Newtonian theory in an infinite medium. [Silk90]
Molecules
The smallest units of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their electrons. [F88]
Intercloud Medium
The sparsely populated (about 0.5 atoms per cm3) regions of space between the interstellar concentrations of gas and dust. [H76]
Maxwell-Boltzmann Statistics
The statistical rules for studying systems of identical particles in classical physics. It was tacitly assumed that particles, although identical, could be distinguished in principle. It can be shown that, for low concentrations of particles, especially at high temperatures, the classical statistics gives results similar to the more exact Fermi-Dirac and Bose-Einstein statistics. It was the failure of classical statistics to predict results in agreement with experiment in certain cases which led to the development of quantum theory. See Maxwell distribution, ultraviolet catastrophe. [DC99]
Galactic Astronomy
The study of the Milky Way. [C95]
Mechanics
The study, in physics, of the influence of forces. [F88]
Gravitational Collapse
The sudden collapse of a massive star when the radiation pressure outward is no longer sufficient to balance the gravitational pressure inward. In gravitational collapse there is a sudden, catastrophic release of great quantities of gravitational potential energy, and this release has been postulated as the cause of supernovae, neutron stars, and black holes. [H76]
General Precession
The sum of the lunisolar and the planetary precession(q.v.). It causes the ecliptic longitude to increase at a constant rate (50".27 per year) but has no effect on ecliptic latitude. [H76]
Lightest Superpartner
The superpartner with the least mass. The LSP may have seve ral important roles. In particular, it may be the cold dark matter of the universe, and its properties are crucial for identifying the events of superpartner production at colliders, because all of the heavier superpartners decay into the lightest one. [K2000]
LHe
The symbol for Liquid Helium. The temperature of liquid helium is normally 4 K, that is, four degrees above absolute zero. [McL97]
LN2
The symbol for Liquid Nitrogen. The temperature of liquid nitrogen is normally 77 K, that is, 77 degrees above absolute zero. [McL97]
Gaseous Fragmentation
The systematic breakup of a gas cloud into smaller and smaller subunits as the gas cools and continues to collapse. The gravitational forces continually overtake the opposing pressure gradients as long as the cloud is able to radiate freely; consequently, the Jeans mass decreases, and fragments divide into smaller subfragments. The process stops only when opacity intervenes to inhibit the cooling and radiation. [Silk90]
Malmquist Bias
The systematic distortion in a standard candle's effective range due to failure in detecting the fainter examples of the standard candle at large distances. [C97]
Inversion
The term used with CCDs to indicate that the applied voltage has not only driven away the majority carriers but has actually attracted the minority carriers of the opposite sign. [McL97]
Friedmann-Lemaître Universes
The three standard Big Bang models that were formulated by Friedmann and Lemaître. [Silk90]
Kelvin timescale
The time it takes a star to contract gravitationally from infinite radius down to its present radius by radiating its thermal energy (for the Sun, about 2-3 × 107 years). The Kelvin time is roughly equal to (gravitational binding energy) / luminosity. (Compare nuclear time scale). [H76]
Hold-Time
The time taken to use up all the liquid cryogens, like LN2, in a cooled CCD cryostat. [McL97]
Moonrise, Moonset
The times at which the apparent upper limb of the Moon is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - , where s is the Moon's semidiameter, is the horizontal parallax, and 34' is the adopted value of horizontal refraction. [S92]
Main Lines (of an OH source)
The transitions that emit radiation at 1665 and 1667 mhz. [H76]
kilometer per second
The unit of speed in astronomy. One kilometer per second is 2237 miles per hour - five times the speed of an airplane. [C95]
Intensity Interferometry
The use of two telescopes linked by computer to study the intensity of light received from a star. Analysis of the combined results has enabled measurement of the diameters of stars as apparently small as 2 × 10-4 seconds of arc. [A84]
Marching Subpulse
The weaker component of a pulsar pulse when its period is more than half that of the main pulse, so that the subpulse occurs at progressively later intervals between successive main pulses. [H76]
General Theory of Relativity
Theory formulated by Albert Einstein after his own Special Theory of Relativity, founded upon an understanding of gravitation as a curvature in four-dimensional space-time (rather than as a force existing between two masses). In proposing this new definition, Einstein had also to derive the four-dimensional structure of space-time. The theory is the basis of most modern research, although alternatives - such as twistor theory - have been proposed since. [A84]
Newton's Universal Theory of Gravity
Theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity. [G99]
kinematic relativity
Theory proposed by Edward Milne as a viable alternative to Einstein's general theory of relativity, and based generally on kinematics (the science of pure motion, without reference to matter or force), from which Milne successfully derived new systems of dynamics and electrodynamics. [A84]
Grism
This is a right-angled glass prism with a transmission diffraction grating deposited on the hypotenuse surface. The spectrum produced by the grating is deflected by the prism to remain on the optical axis and the apex angle of the prism is chosen to get a certain wavelength in the center of the detector. Grisms can be placed in a filter wheel. [McL97]
Non-Linear Optics
This is concerned with the optical properties of matter in intense radiation fields. The induced electromagnetic polarization does not depend linearly on the radiation strength but is severely distorted by the strong field. Optical harmonics, frequency mixing and intensity dependent refractive indices are all produced by this nonlinear response. [D89]
Nuclear Time Scale
Time required for a star to evolve a significant distance off the main sequence; the time it takes a star to convert all its available hydrogen into helium. For the Sun, it is 1010 years. (Cf. Kelvin time scale.) [H76]
Isochrones
Time-constant loci. [H76]
Homogeneous Expansion
To a good approximation, our universe appears to be undergoing homogeneous expansion, which means that successive snapshots of a given region would each look like a photographic blowup of the first snapshot. Homogeneous expansion is also called Hubble expansion, since it implies Hubble's law. See Figure 2.1 on page 21. [G97]
Mean Molecular Weight
Total atomic (or molecular) weight divided by the total number of particles. For instance, the mean molecular weight of a plasma of pure ionized 4He would be 4 (the atomic mass number) divided by 3, the total number of particles (1 nucleus plus 2 electrons). So µ would equal 4/3. [H76]
Leo Systems
Two dwarf elliptical galaxies (about 220-250 kpc distant) belonging to the Local Group. Leo I (dE3), Mv - 11, diameter 1.8 kpc; Leo II, Mv = -9.5, diameter 1.3 kpc. [H76]
Maffei 1 and 2
Two galaxies discovered on infrared plates in 1968 and identified in 1970 as members (probably temporary) of the local group. the two galaxies lie in the Zone of Avoidance. Maffei 2 has since been classified as a medium-sized, average luminosity SBc II spiral at a distance of 5 ± 2 Mpc - too far away to belong to the local group - but Maffei 1 (a giant elliptical) is probably only 1 Mpc distant, marginally within the local group. [H76]
N Lines
Two green forbidden lines of doubly ionized oxygen [O III]. N1 is at 5007 Å; N2 at 4959 Å. [H76]
Gamma
Unit of magnetic field intensity equal to 10-5 gauss. [H76]
Natural Units
Units of length, time, mass, etc. in which the fundamental constants c (the speed of light), (Planck's constant) and kB (Boltzmann's constant) are equal to unity. That is, c, and kB have the numerical value 1. (For example, if we measure length in light-years and time in years, then c = 1 light-year per year.) The use of natural units allows these constants to be omitted from mathematical equations, leading to less-cluttered calculations. In natural units, E = mc2 becomes E = m and E = kBT becomes E = T, so that both mass and temperature can be expressed in units of energy. (Of course, the correct factors of c, and kB must be inserted at the end of a calculation to obtain measurable quantities.) [CD99]
Hestia
Unofficial name for Jupiter VI. Discovered by Perrine in 1904. P = 250 days, e = 0.16, i = 29°. [H76]
Hera
Unofficial name for Jupiter VII. P = 259.65 days, e = 0.21, i = 28°. Discovered by Perrine in 1905. [H76]
Massive Black Hole
Utilized in a theoretical model for quasars and active galactic nuclei, according to which the energy source is due to infall (and resultant heating) of gas and stars onto a supermassive central black hole. [SILK90]
J-value
Value of the total angular momentum (orbital plus spin). J is the rotational quantum number which specifies the rotational level of a molecule. [H76]
Librations
Variations in the orientation of the Moon's surface with respect to an observer on the Earth. Physical librations are due to variations in the rate at which the Moon rotates on its axis. The much larger optical librations are due to variations in the rate of the Moon's orbital motion, the obliquity of the Moon's equator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface. [S92]
Five-Minute Oscillations
Vertical oscillations of the Solar atmosphere with a well-defined period of 5 minutes. [H76]
Interference Pattern
Wave pattern that emerges from the overlap and the intermingling of waves emitted from different locations. [G99]
Neutral Current Reactions
Weak-interaction reactions in which no electric charge is exchanged between the colliding particles. Observation of such reactions in 1973 provided important support for the then-developing gauge theory of the weak interactions. We now know that these reactions are mediated by the exchange of a massive, neutral gauge boson - the Z0. [CD99]
Metal-Weak Stars
Weak-line stars. [JJ95]
gf-values
Weighted oscillator strengths. f = oscillator strength of the transition: g = statistical weight of the lower level. [H76]
Hall Effect
When an electric current is passed through a conductor and a magnetic field is applied at right angles, a potential difference is produced between two opposite surfaces of the conductor. The direction of the potential gradient is perpendicular to both the current direction and the field direction. It is caused by deflection of the moving charge carriers in the magnetic field. The size and direction of the potential difference gives information on the number and type of charge carriers. [DC99]
Killing vector
When it exists, the Killing vector describes symmetry properties of spacetime. If spacetime admits a timelike Killing vector, then it is stationary; similarly, axial symmetry is derived from a spacelike Killing vector. [H76]
Inertial Frame
a frame of reference in which force-free bodies move along straight lines; postulates of special relativity are said to be valid in an inertial frame. [D89]
Geodesic
a path or line of shortest distance joining two points in space (or space-time). A geodesic is a straight line if the space is flat. Another familiar geodesic is a great circle on the surface of a sphere. In the general theory of relativity, freely falling particles follow geodesic paths in space-time. [D89]
Markovian process
a random process in which the probability of performing a transition to a certain state at a given time depends solely on the state in which the system is found at this time. [D89]
Gravitational Sigularity
a region where the gravitational field has become so strong that the curvature of space-time is infinite. The occurrence of such a situation signals the breakdown of the theory and is a central feature of the classical theory of relativity. [D89]
New Inflationary Universe
a revised form of the Inflationary Universe model that provides a mechanism to avoid the gross inhomogeneities which result from the theory as originally proposed. [D89]
Inversion Layer
a very thin layer of electrons trapped on an interface between a semiconductor and an insulator, or between two different semiconductors. [D89]
Infrared Astronomy
astronomy carried out at wavelengths between about 1 µm and 300 µm. Ground-based observations are possible in a number of astronomical `windows' in the 1-5 µm, 8-13 µm, 18-22 µm and 30*m wavebands from high, dry observing sites. Observations in the wavelength ranges 5-8 µm, 13-l8µm and 30-300µm can only be carried out from above the Earth's atmosphere because of atmospheric absorption. The wavelength region 100 µm to 1 mm is often referred to as the submillimeter waveband. [D89]
Gamma-Ray Astronomy
astronomy carried out in the waveband of photon energies > 100 MeV. Except at the very highest energies at which cosmic gamma rays can be detected by the electron-photon cascades which they initiate in the atmosphere, these studies have to be conducted from above the Earth's atmosphere. Many different types of process have characteristic gamma-ray properties including gamma rays produced by 0 decays following collisions between cosmic rays and nuclei of the interstellar gas, line emission such as the electron-positron annihilation line at 512 keV, inverse Compton scattering gamma rays and other high energy astrophysical processes. [D89]
Minkowski Metric
form of the metric that is valid in an inertial frame; underlying geometry of special relativity. [D89]
Fractal Geometry
generalisation of Euclidean geometry suitable for describing irregular and fragmented patterns. A noninteger `fractal dimension' can frequently (but not always) be associated with such patterns. [D89]
Heterotic O-String Theory
heterotic O(32) String Theory One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-E string theory. [G99]
Multiphoton Process
in very intense radiation fields atoms or molecules can absorb several photons simultaneously in a multiphoton process. [D89]
Lyapounov Exponent
measures the rate of exponential separation of initially nearby states of a dynamical system. In a system undergoing chaotic dynamics there is at least one positive Lyapounov exponent. [D89]
Hidden Variables Theory
one of a class of physical theories which deny that the quantum state of a physical system is a complete specification. The hidden variables are those components of the hypothetical complete state which are not contained in the quantum state. [D89]
Loop Nebula
see 30 Doradus [H76]
Least Action
see Action [P88]
Nightglow
see Airglow [H76]
Minor Planet
see Asteroid [H76]
Mass Number
see Atomic Mass Number. [C95]
j-j Coupling
see LS coupling [H76]
Libration Orbits
see Lagrangian Points [H76]
Light Pressure
see Radiation Pressure [H76]
Magnetic Bremsstrahlung
see Synchrotron Radiation [H76]
First Point of Aries
see Vernal Equinox. [H76]
Newton's Universal Gravitational Constant
the constant that sets the scale of gravitational forces. Its value is independent of the actual constitution of the matter producing the gravitational field.[D89]
Matter Fields
the fields whose quanta describe the elementary particles making up the material content of the Universe (as opposed to the gravitons and their supersymmetric partners).[D89]
Indefiniteness
the suspension of an eventuality between truth and falsity, or of a physical variable among its possible definite values, which occurs, according to quantum mechanics, in certain states of a system. This suspension is not a matter of ignorance on the part of the observer but is rather an objective fact. [D89]
Microwave Background Radiation
thermal radiation with a temperature of about 3 K that is apparently uniformly distributed in the Universe; the radiation, discovered by A.A. Penzias and R.W. Wilson in 1964, is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the big bang. [D89]
Four-Wave Mixing
this is the combination of three optical waves to generate a fourth. It occurs in many optical media provided frequencies and intensities are carefully chosen. It is widely used in phase-conjugation. [D89]
Gauge Invariance
this would be better called `local phase-angle independence'. It is the property of electrically charged particles in quantum theory that says that the phase-angle has no physical significance, and can be chosen at will at each point of space, QCD possesses a generalized form of gauge invariance. [D89]
Fine-Structure Constant ()
"(a) A ""coupling constant,"" e2 / c, approximately 1/137, that measures the strength of an interaction between a charged particle and the electromagnetic field. It gives a rough measure of the relative importance of relativistic and spin effects in the spectra of atoms. [H76]
Faculae
"(a) Bright areas on the face of the Sun, commonly in the vicinity of sunspots. Named by Johannes Hevelius, they are thought to be caused by luminous hydrogen clouds close to the photosphere. They last on average about 15 Earth-days. [A84]
Fine Structure
"(a) Closely spaced lines seen at high resolution in a spectral line or band. Fine structure may be caused by vibrational motion of the molecules or by electron spin. Hyperfine structure, seen at very high resolution, is caused by the atomic nucleus affecting the possible energy levels of the atom. [DC99]
Feynman Diagram
"(a) Schematic representation of an interaction between particles. [F88]
Fiber Optics
"(a) The use of fine transparent fibers to transmit light. The light passes along the fibers by a series of internal reflections. Optical fibers of this type can be used to view inaccessible objects and to carry laser signals in telecommunications. [DC99]
Feed Horn
A device used on a radio telescope. It is located at the focal point and acts as a receiver of radio waves which the antenna collects and focuses on it. It couples the energy into the lines to go into the amplifier. [H76]
Fermi Gas
A gas of electrons (or, more generally, fermions) in which all the lowest quantum states are occupied. For such a gas, the pressure in the nonrelativistic limit is proportional to the 5/3 power of the density. [H76]
Field Lens
A lens placed in or near the focal plane of a telescope to create an image of the primary mirror inside the instrument. [McL97]
Field Theory
A theory in which forces are communicated between two particles by a "field," which fills up the space between the two particles. In a field theory, any particle, such as an electron, is surrounded by a field. The field continuously creates and destroys intermediary particles, which transmit the force of the electron to other particles. In fact, particles themselves are considered to be concentrations of energy in the field. An axiomatic field theory is a field theory that begins with certain assumptions or axioms, as in Euclid's geometry, and deduces the necessary consequences of these assumptions. [LB90]
Faber-Jackson Relation
An empirically observed correlation between the speeds of stars in the center of a galaxy and the intrinsic luminosity of the galaxy - the higher the random speeds, the more luminous the galaxy. Since the speeds of stars can be directly measured by the Doppler shift in their colors, the Faber-Jackson relation permits an estimate of the intrinsic luminosity of a galaxy. By comparison of this with the observed brightness, the distance to the galaxy may be inferred. [LB90]
Fictitious Mean Sun
An imaginary body introduced to define Mean Solar Time; essentially the name of a mathematical formula that defined Mean Solar Time. This concept is no longer used in high precision work. [S92]
Filar Micrometer
An instrument used with a telescope for visually measuring small angular separations (as between binary stars). [H76]
Field Equations
Equations which relate to one of the fundamental fields of force. General relativity is called a field theory because it describes the gravitational field, and Einstein's equations of general relativity are called field equations. Maxwell's field equations describe the electromagnetic field. (also called Cold Emission) [H76]
Field Effect Transistor
FET A solid-state electronic device with three terminals that, like the junction transistor, is used in amplifiers. It controls the current between two terminals, the source and the drain, by the voltage at a third, the gate. An n-type FET consists of a single piece of n-type semiconductor, which has the source at one end and the drain at the other. A heavily-doped p-type region in the middle forms the gate. If the gate voltage is more negative than the source, electrons move from the n-type region to the p-type region leaving an area round the gate with fewer electrons to carry current. This has the effect of narrowing the conducting channel through which the source-drain current flows by an amount that depends on the source-gate voltage difference. In the FET, only one type of charge carrier - electrons in the n-type FET and holes in the p-type FET - determines the current and it is therefore known as a unipolar transistor. In the bipolar junction transistor, both positive and negative charge carriers contribute to the other current. [DC99]
Field Horizontal Branch Stars
High-velocity metal-weak stars of either B or A spectral type. [JJ95]
Figure of Merit
The extent to which an optical system falls short of perfection. [H76]
F component
The outer part of the Solar corona (see K Component) which emits a continuous spectrum in which absorption lines can be seen. The F corona is caused by radiation from the photosphere scattered by interplanetary dust, and it decreases slowly with distance from the Sun. (F stands for Fraunhofer) [H76]
Field of View
The patch of sky or of any image scene which can be seen by an optical system or by one picture element (pixel) of a detector system. Usually expressed in angular measure. [McL97]
False Color
The use of colors, instead of shades of grey, on a computer image display screen to represent different brightness levels and highlight very small differences in a dramatic way. For example, in an ordinary black-and-white image, objects which differ only slightly in brightness appear as almost the same shade of grey and are hard to distinguish. If instead the numerical brightness values are assigned carefully chosen colors then two objects of almost equal brightness will be strongly distinguished when the image is displayed. [McL97]