Astronomy 2020

Black holes

A black hole is an object that meets the conditions that even light cannot escape from it. The idea of a black hole arises from the concepts of Einstein's general relativity, particularly the curvature of space-time by the gravity of a massive object. The fact that light follows this curvature suggests the possibility of a curvature so extreme that light cannot escape the vicinity of the object. The gravity of a massive object interacts with photons to produce a gravitational red shift, representing a decrease in photon energy with increasing radial distance. If the photon frequency and energy are red-shifted to zero, this is another way of saying that the photon cannot escape. The red shift to zero is associated with a radius, the Schwarzschild radius. Using the existence of a Schwarzschild radius to characterize a black hole, it becomes evident that a black hole with larger mass will have a larger Schwarzschild radius so that black holes could conceivably exist with any mass provided that mass could be contained in a sufficiently small radius. The development that describes the gravitational red shift also points to gravitational time dilation, the fact that being closer to a gravitational mass will progressively slow down the progress of time. As an object approaches the Schwarzschild radius, the progress of time approaches zero so that time stands still. This radius is commonly called the "event horizon" since no information is available past this radius. If a massive object is contracting, there are certain processes which tend to halt the contraction, such as electron degeneracy which may stop the contraction to form a white dwarf. If the mass is too great for a white dwarf, neutron degeneracy may halt it to form a neutron star. But if the mass is greater than two to three solar masses there is no known mechanism to halt further collapse and it is presumed not only that a black hole will result from the collapse, but that the collapse will continue toward zero spatial extent, a singularity. One of the terms used with black holes is "photon sphere", the radius of the orbit of light around the black hole. For 3 solar masses this radius is 13.5 km, 3/2 x the event horizon radius. The event horizon radius is also called the Schwarzschild radius. Stellar Mass Black Holes Black holes with masses comparable to that of the sun Supermassive Black Holes Black holes with millions or billions of solar masses (solar mass = mass of the sun) Often in the centers of galaxies When the mass of a neutron star exceeds three solar masses, its core collapses on itself and a black hole is formed Only things that can be measured from the outside are mass, charge, and angular momentum Schwarzschild radius is the radius at which the escape speed is the speed of light Sphere with this radius centered on a collapsing star is the event horizon -- the point of no return The event horizon is the one-way filter in the black hole: anything can enter, but nothing can leave. A black hole is a very simple object: it has only three properties mass, spin and electrical charge. Because of the way in which black holes form, their electrical charge is probably zero, which makes them simpler yet As a black hole radiates via this virtual particle method it will lose mass and get smaller. Using Equation 4 we see that a smaller mass means a bigger temperature, so as the black hole radiates it gets smaller and hotter. If it gets hotter then the rate of radiation will increase and so you'll have a chain reaction with the black hole getting ever smaller and hotter until it gets to zero mass. Since black holes by their very definition cannot be directly observed, proving their existence is difficult. The strongest evidence for black holes comes from binary systems in which a visible star can be shown to be orbiting a massive but unseen companion. The indirect evidence for the black hole Cygnus X-1 is a good example of the search for black holes. Another excellent candidate in an object which was discovered in one of the Magellanic Clouds. Some astronomers think the binary system V404 Cygni is the strongest candidate yet. Neutron degeneracy pressure cannot support a neutron star having M > 3M. If during the supernova collapse the core exceeds this mass nothing known in physics can prevent gravity from collapsing the matter into some arbitrarily small volume. Gravity becomes so strong at the "surface" that not even light can escape: Black Hole.

Light Curves

Graphs that show the brightness of an object over a period of time Can be used to identify the object being observed A star's diameter is found from speed = (distance travelled)/(time it takes). The speed comes from the doppler shift and the time is the length of the eclipse. The distance travelled during the eclipse is equal to the diameter of the star = 2 × radius. The light curve---plot of brightness vs. time---is used to derive the star diameters

Gravitational Waves

Gravitational counterpart of an electromagnetic wave Results from changes in the strength of a gravitational field Objects most likely to produce these are binary systems containing black holes, neutron stars, and white dwarfs

evolution of high mass stars

High mass stars evolve much faster than low mass stars All evolutionary changes happen more quickly for higher mass stars because their larger mass and stronger gravity generate more heat After the star runs out of hydrogen, it leaves the main sequence Red giants move horizontally across the HR diagram after leaving the main sequence Helium burning happens smoothly and stably (no helium flash) High mass stars can fuse heavier elements and so the inner core continues to contract and the central temperature continues to rise The rate of burning accelerates As each element is burned to depletion, the core contracts, heats up, and fusion starts again A new inner core forms, contracts again, heats again, etc. The star's radius increases as its surface temperature drops and so the star swells to become a red giant Star is destined to die in a supernova When the star first runs out of hydrogen to fuse in its core it will behave similarly to lower mass red giants. It will first begin fusion of hydrogen in a shell around the core and the core will heat and fuse helium into carbon. There will also be carbon and helium fusion into oxygen. The star's envelope also bloats out to very large sizes (Supergiants). The star has enough mass that when the helium supply in the core is exhausted it will shrink further and heat sufficiently to begin fusion of carbon and oxygen into higher elements. Another shell of hydrogen burning will form, and beneath it a shell of helium burning will form. The supergiant's core will fuse very heavy elements from carbon and oxygen all the way up to Iron. The star takes on an "onion" like structure with shells of different elements fusing into heavier elements. This all happens over a few million years tops

Interstellar Medium

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space

Period-Luminosity Relationship

Used to infer the luminosity of a variable star Cepheids that have long periods have large luminosities and Cepheids with short periods have small luminosities Once distances are known to variable stars, can use relationship between luminosity and distance to calculate luminosity This works for the nearby galaxies Mv = - [2.76 (log10(P) - 1.0)] - 4.16, M is absolute magnitude, P is period

Peculiar Galaxies

Usually a spiral or elliptical with a weird shape

Death of a low-mass star

Before the carbon core of the low-mass star can attain the temperatures needed for carbon ignition, its density reaches a point beyond which it cannot be compressed further At this density, the electrons in the core become degenerate, the contraction of the core ceases, and the temperatures stop rising Some oxygen is formed via reactions between carbon and helium, but these reactions are not frequent or violent enough to create heavier elements Inner carbon core no longer generates energy and outer core continues to burn hydrogen and helium As more of the inner core reaches its final, high-density state, the nuclear burning increases in intensity The envelope continues to expand and cool, reaching maximum radius of about 300 times that of the sun The burning now becomes unstable The helium-burning shell is subject to a series of explosive helium shell flashes These are caused by the enormous pressure in the helium burning shell and the extreme sensitivity of the triple-alpha burning rate to small changes in temperature These flashes produce large fluctuations in the intensity of the radiation reaching the star's outermost layers, causing these layers to pulsate violently as the envelope repeatedly heats, expands, cools, and contracts The amplitude of these pulsations grows as the core's temperature continues to increase and the nuclear burning increases in surrounding shells Surface layers are increasingly unstable Around the peak of every pulsation, the surface temperature drops below the point at which electrons can recombine with nuclei to form atoms Each recombination produces additional photons, giving the gas a little outward push and allowing some of it to escape As a result of increasingly intense radiation from within and instabilities in the core and outer layers, pretty much all of the star's envelope is ejected into space in less than a few million years The "star" now consists of a well defined core of mostly carbon ash and an expanding cloud of dust and cool gas (the ejected envelope of the giant) The core is hot, dense, and very luminous, and only the outer layers still fuse helium into carbon and oxygen As the core exhausts the last of its remaining fuel, it contracts and heats up and moves to the left of the HR diagram Eventually, it becomes so hot that its ultraviolet radiation ionizes the inner parts of the surrounding cloud, creating a planetary nebula (planetary nebulas have no association with planets, were named back in the 18th century when viewed at poor resolution) The central star cools and the gas cloud disperses into space During the final stages of the current red giant's life, nuclear reactions between carbon and unburned helium in the core produce oxygen and heavier elements such as neon and magnesium Some of these reactions release neutrons which can interact with existing nuclei to form heavier elements as they have no electrostatic barrier to overcome The carbon core continues to evolve and becomes visible as the envelope recedes This core is about the size of Earth, has a mass about that of the sun, and has a white-hot surface The core is now a white dwarf Helium dwarf is when a star is so small that helium fusion never occurs and eventually all of its hydrogen is converted to helium (theory predicts this) In some stars more massive than the sun, temperatures in the core might become high enough for the reaction 16O + 4He —> 20Ne + energy to occur, leading to the formation of a neon-oxygen white dwarf Most white dwarfs are composed of carbon and oxygen Once a star becomes a white dwarf, its evolution is over The white dwarf continues to cool and dim, eventually becoming a black dwarf

Active Galactic Nuclei

Many galaxies have very bright nuclei, so bright that the central region can be more luminous than the remaining galaxy light. These nuclei are called active galactic nuclei, or AGN for short. Much of the energy output of AGNs is of a non-thermal (non-stellar) type of emission, with many AGN being strong emitters of X-rays, radio and ultraviolet radiation, as well as optical radiation. AGN can vary in luminosity on short (hours or days) timescales. This means that the light or energy emitting source must be of order light hours or light days (respectively) in size, and gives clues as to the energy mechanism. Carl Seyfert discovered the first class of AGN, that are now named after him. The nuclei of Seyfert galaxies display emission lines. Type 1 Seyfert galaxies have both narrow and broadened optical spectral emission lines. The broad lines imply gas velocities of 1000 - 5000 km/s very close to the nucleus. Seyfert type 2 galaxies have narrow emission lines only (but still wider than emission lines in normal galaxies) implying gas velocities of ~ 500-1000 km/s. These narrow lines are due to low density gas clouds at larger distances (than the broad line clouds) from the nucleus. Later, Seyfert type 1 galaxies that showed intermediate properties were divided into subclasses. For example, a Seyfert 1.9 is a Seyfert 1 in which only broad Halpha (653 nm) emission lines are seen, and a Seyfert 1.5 has broad and narrow Hbeta (486 nm) emission line components that are similar. Seyfert galaxies comprise ~ 10% of all galaxies. As well as Seyferts, other galaxies are also classified as AGN. These include radio galaxies, quasars, blazars and LINERs. Radio galaxies, as their name implies, are strong emitters of radio emission. These are elliptical galaxies with nuclear radio emission, often accompanied by single or twin radio lobes (straddling the galaxy) that can be Mpc-sized. The radio emission is non-thermal, due to fast moving electrons that spiral in magnetic fields, producing synchrotron emission. Sometimes the radio lobes and nuclear radio emission are joined by narrow radio jets (see Cygnus A). Quasars are the most luminous AGN. The spectra of quasars are similar to Seyferts except that stellar absorption features are weak or absent, and the narrow emission lines are weaker relative to broad lines as seen in Seyferts. Blazars are a class of AGN that are radio sources and consist of both Optically Violent Variables (OVVs) and BL Lac objects. They are highly variable AGN that do not display emission lines in their spectra. BL Lac objects are named after BL Lacertae, the class prototype, a highly variable AGN. It was originally thought to be a variable star. Low Ionisation Nuclear Emission-line Region galaxies (LINERs) are very similar to Seyfert 2 galaxies, except as their name implies the low ionisation lines like [O I] and [N II] are quite strong. AGN are thought to be powered by centrally located, supermassive black holes. The central regions of all AGN are thought to be similar and are explained by the Unified Model of AGN. The variation in AGN properties is thought to be related to the line of sight we have into the central region of the AGN. In the Unified Model, AGN have a central supermassive black hole surrounded by a gaseous accretion disk of ~ a few light days across. Moving outwards from the centre of the AGN fast moving gas clouds exist at a distance ~ 100 light days, known as the 'broad line region' which produce the broad emission lines seen in some AGN spectra. Continuing outwards, at ~ 100 light years in diameter, a molecular doughnut or torus of colder gas exists. It is optically thick, and if viewed edge on will block out the accretion disc and broad line region from view. At a distance of ~ 1000 light years, the 'narrow line region' exists. It is comprised of small, low density gas clouds moving at lower velocities (than the broad line region). It is these clouds that are energised (usually close to the direction of the radio jets) and they produce the narrow emission lines seen in some AGN spectra. Radio (synchrotron) emission is produced in many AGN, collimated into jets and propagates in a direction that is perpendicular to the plane of the accretion disc. These radio jets are prominent in many radio galaxies and can be as large as several Mpc in size. Many jets end in broadened and diffuse radio lobes. Compared to ordinary galaxies, some galactic nuclei have extraordinary luminosities and are called active galaxies. The most dramatic examples are the quasars. Other active galactic nuclei have unusually bright, starlike nuclei, or have strong emission lines, or show high variability. Some have streams or jets coming from their cores. One class of active galaxies is the Seyfert galaxies. Because of the enormous energies coming from rather small volumes, it is often suggested that these nuclei have massive black holes at their centers as a source of energy for the extraordinary luminosity. To date, the active galaxy M87 seems to be the best candidate.

Gamma Ray Binaries

composed of a massive star and a compact object most of their emission is radiated in the 1-1000 MeV band Most gamma ray bursts occur in distant galaxies

2nd stage of star formation

A fragment of an interstellar cloud that will form a star like the sun is about 1 to 2 solar masses at this point and spans a few hundredths of a parsec across Central density is about 10^12 particles/m^3 Average temperature not that different from that of the initial star despite the fact that the fragment has shrunk a lot This is because the gas constantly radiates large amounts of energy into space and the material of the fragment is so thin that photons can easily escape without being reabsorbed, allowing virtually all of the energy released in the collapse to escape without increasing the temperature The temperature only rises significantly at the center because radiation must traverse the greatest amount of material to escape This process is eventually stopped by the increasing density within the shrinking cloud As the fragment continues to contract, it eventually becomes so dense that radiation cannot get out of the cloud easily The trapped radiation causes the temperature and pressure to rise, which causes the fragmentation to cease

Neutron Star

A remnant of the core of a star that experienced a type 2 supernova A dense ball of neutrons that remains at the core of a star after a supernova has destroyed the rest of the star Extremely small and very dense Composed of neutrons packed into a tight ball about 20 km across Solid Normally about 20 km across Supported by neutron degeneracy pressure Rotate extremely rapidly Have very strong magnetic fields The original field of the original star is amplified because contracting material squeezes electric field lines closer together If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn't form a white dwarf when it dies Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off. For a sufficiently massive star, an iron core is formed and still the gravitational collapse has enough energy to heat it up to a high enough temperature to either fuse or fission iron. Either in the aftermath of a supernova or in just a collapsing massive star, the energy gets high enough to break down the iron into alpha particles and other smaller units, and still the pressure continues to build. When it reaches the threshold of energy necessary to force the combining of electrons and protons to form neutrons, the electron degeneracy limit has been passed and the collapse continues until it is stopped by neutron degeneracy. At this point it appears that the collapse will stop for stars with mass less than two or three solar masses, and the resulting collection of neutrons is called a neutron star. The periodic emitters called pulsars are thought to be neutron stars. If the mass exceeds about three solar masses, then even neutron degeneracy will not stop the collapse, and the core shrinks toward the black hole condition. This neutron degeneracy radius is about 20 km for a solar mass, compared to about earth size for a solar mass white dwarf. The density is quoted as about a billion tons per teaspoonful compared to 5 tons per teaspoonful for the white dwarf. Pasachoff suggests that neutron stars may be crystalline with crusts on the order of 100 meters thick and an atmosphere a few centimeters thick. They may have 1011x the earth's gravity and a powerful magnetic field. A neutron star might have an atmosphere a few centimeters thick and mountain ranges poking up a few centimeters through the atmosphere. A neutron star is thought to be about 1/100,000 the diameter of the Sun, and a nucleus is on the order of 100,000 times smaller than an atom. Though interesting as an order-of-magnitude comparison, this does not imply that the atoms in the sun are packed in close contact. The neutron stars would generally be formed from stars considerably more massive than our Sun. The incredible density of neutron stars does come from the fact that from atomic size, the electrons are collapsed into the nucleus to combine with protons to form neutrons so that the entire body approaches nuclear density. Recent research suggests that the heaviest elements may be formed primarily in neutron star mergers rather than supernovae The core left behind in a Type II Supernova is composed mostly of neutrons and has the density of an atomic nucleus. It is held up by neutron degeneracy pressure. Up to 2-3M in a 10 km sphere! Incredible density. One cubic centimeter of neutron star material weighs as much as a mountain! Neutron Stars were predicted in the 1930s but not observed until 1967 as Pulsars (Jocelyn Bell): Extremely regular bursts of radio waves (e.g., P = 1.3373011 seconds). So far hundreds have been found in the plane of the Milky Way Galaxy: Pulsars are highly magnetized and very quickly spinning neutron stars. Lighthouse Model: Very strong magnetic field channels charged particles to the poles of the magnetic field. Where they emit radiation in a beam. The axis of the magnetic field is offset from the spin of the neutron star and the beam sweeps past our line of sight so we see pulses of radio (and other wavelengths) radiation.

Evolution of a sun-like star

After about 10 billion years of hydrogen burning, the star runs out of hydrogen As amount of hydrogen decreases, the amount of helium increases As the hydrogen becomes depleted, an inner core of non-burning helium starts to grow Without nuclear burning to maintain it, the outward-pushing gas pressure weakens in the helium inner core However, the inward pull of gravity does not weaken, so structural changes occur in the star As the hydrogen is consumed, the inner core begins to contract, once all of the hydrogen in the core is depleted, this process accelerates The helium at the center cannot burn yet because although the core is hot, it is still too cold to fuse helium into anything heavier The shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying burning areas This causes hydrogen nuclei to fuse even more rapidly than before Hydrogen is burning quickly in a shell surrounding the non-burning core of helium ash This is known as the hydrogen-shell-burning stage The hydrogen shell generates energy faster than did the original main-sequence star's hydrogen-burning core The shell's energy production continues to increase as the helium core continues to shrink Star's temperature and luminosity change Surface temperature drops, luminosity increases slightly, leaves main sequence Star's radius has increased to three times that of sun Star is now a subgiant The path from its main sequence location to its subgiant location on the main sequence is called the subgiant branch Star is now far from main sequence and no longer in equilibrium Helium core is unbalanced and shrinking, hydrogen is being fused into helium at an increasing rate Surface temperature has fallen Convection now carries star's energy output to the surface, resulting in a stable surface temperature Radius and luminosity increase Star is now a red giant Helium begins to burn Helium is transformed into carbon through the triple alpha process Electron degeneracy pressure occurs, meaning the electrons are incompressible This makes the core unable to respond to the changing conditions and the temperature changes rapidly in a helium flash The helium burns rapidly for a few hours and then the core expands, density drops, and equilibrium is restored Helium flash causes a reduction in the star's energy output Star is now stably burning helium in the core and fusing hydrogen in the shell surrounding it Helium is consumed and a new carbon-rich inner core is formed Helium becomes depleted at the center of the star and fusion ceases The non burning carbon core shrinks in size, its mass increases due to helium fusion, and it heats up as gravity pulls it inward This causes the hydrogen and helium burning rates to increase Star is now once again a red giant Radius and luminosity are even bigger then they were before Carbon core grows in mass and continues to shrink in radius

Irregular Galaxies

Appearances distinguish themselves from all other categories Tend to be rich in interstellar matter and young blue stars Lack any regular structure Irr I galaxies and Irr II galaxies Irr I look like misshapen spirals Irregular galaxies are smaller than spirals and larger than dwarf ellipticals Smallest of these are called dwarf irregulars Most irregulars are small and faint The dwarf irregulars may be the most common type of galaxy in the universe (or maybe the dwarf ellipticals are)

4th stage of star formation

As it evolves, the protostar shrinks, density grows, and temperature rises Luminosity much brighter than the sun Emit most radiation in infrared Nuclear reactions have not yet begun, luminosity solely due to release of gravitational energy Can now be plotted on HR diagram(plot of surface temperature and luminosity) Evolutionary track is a graph of a star's development on the HR diagram As the fragment contracts, it spins faster and flattens into a rotating protostellar disk Internal heat gradually diffuses, slowing the contraction After this stage, luminosity decreases and temperature increases, at this stage, violent surface activity is often present, resulting in extremely strong protostellar winds 2000 to 3000 K, hot enough to glow a dull red with most of its energy in the infrared The cocoon of gas and dust surrounding them blocks the visible light The surrounding dust warms up enough to produce copious amounts of infrared and the cooler dust further out glows with microwave energy This longer wavelength electromagnetic radiation can pass through the dust. The low-mass protostars (those up to about 5 solar masses) are initially much more luminous than the main sequence star they will become because of their large surface area As these low-mass protostars collapse, they decrease in luminosity while staying at roughly a constant surface temperature A star remains in the protostar stage for only a short time, so it is hard to catch many stars in that stage of their life More massive protostars collapse quicker than less massive ones Fusion starts in the core and the outward pressure from those reactions stops the core from collapsing any further But material from the surrounding cloud continues to fall onto the protostar Most of the energy produced by the protostar is from the gravitational collapse of the cloud material

Bulge

At the center of the Galaxy lies a spherical distribution of stars about 2 kpc in diameter (6,000 light-years). The stars here have a somewhat more random sense of stellar orbits. It's like a swarm of bees. In the center of the bulge is the nucleus where a supermassive black hole (~106M) has been discovered. The bulge is a round structure made primarily of old stars, gas, and dust. The outer parts of the bulge are difficult to distinguish from the halo. The bulge of the Milky Way is roughly 10,000 light years across.

Hubble's Law

Except for a few nearby systems, every galaxy takes part in a general motion away from us Galaxy clusters also have an overall recessional motion although their individual galaxies move randomly with respect to each other The greater the distance that the galaxy is from us, the greater the redshift (rate it moves away from us) is Redshift resulting from universal recession is called cosmological redshift recessional velocity(km/s) = H0 x distance (megaparsecs) H0 = 70 (km/s/Mpc)

X-ray binary system

Binary system (two stars that orbit around their common center of mass) that emit X-Rays Made up of a normal star and collapsed star (white dwarf, neutron star, or black hole) Produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star X-rays come from the area around the collapsed star where the material that is falling towards it is heated to very high temperatures

Blackbody radiation

Blackbody curve is the radiation distribution curve of a blackbody A blackbody is an object that absorbs all radiation falling on it Intensity as a function of frequency Intensity specifies amount or strength of radiation at a point in space No real object absorbs and radiates as a perfect blackbody The frequency at which emitted intensity is highest is indicative of the temperature of an object "Blackbody radiation" or "cavity radiation" refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident upon it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating. The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced, and that the number of modes went up proportional to the square of the frequency. Somewhere in the range 900K to 1000K, the blackbody spectrum encroaches enough in the visible to be seen as a dull red glow. Most of the radiated energy is in the infrared. Essentially all of the radiation from the human body and its ordinary surroundings is in the infrared portion of the electromagnetic spectrum, which ranges from about 1000 to 1,000,000 on this scale. So this says that the wavelength (color) where the curve peaks is inversely proportional to the Temperature. As the temperature increases the peak moves to shorter wavelengths of radiation. And so we see the color of the object change as it is heated or cooled. When you turn on the burner of an electric oven it starts out glowing a red color and as it heats up goes from red to orange to yellow. The burner is emitting radiation even before you first see it glow red. It starts out emitting IR radiation because it's temperature is initially around 300 K A couple of things can cause an electron to be in an excited state: collisions with other atoms or absorption of photons. Think again of the blackbody. It's an object that absorbs all radiation that falls on it. That means that no matter what elements make up the blackbody there are essentially no photons that don't have just the right energy to excite an electron somewhere in the blackbody. Now as the blackbody absorbs energy the atoms within jiggle about and collide with one another. This can cause electrons to jump up to excited states as well. It can also cause them to fall back down and hence emit a photon. Electrons may also spontaneously fall down and emit photons. So with so many electrons absorbing and emitting the spectrum of a blackbody appears continuous. All the individual lines have run together. Thus, when you look at the emission from a blackbody with a spectrograph you will see a continuous rainbow of colors. This is called a Continuous Spectrum.

3rd stage of star formation

By the beginning of this stage, the fragment has shrunk to roughly the size of our solar system Density in the inner regions becomes high enough so that the gas is opaque to the radiation it emits, causing the core of the fragment to heat up Temperature on outer parts still hasn't increased much Dense opaque region in the center is called a protostar Protostar's mass increases as more material rains down on it from the surrounding, shrinking fragment Protostar's radius continues to decrease because pressure is still unable to overcome gravity After this stage, the protostar has a surface called the photosphere

Galactic Interactions

Collisions and interactions between galaxies provide opportunities for gas to mix, which can set off rapid star formation When the gravitational tug from one galaxy is enough to disrupt the other one, they are interacting Major mergers and minor mergers Major mergers are mergers between two galaxies of similar mass Minor mergers are mergers where one galaxy is significantly more massive than the other one When two galaxies collide the stars will pass right on by each other without colliding The stars may be flung out from the colliding galaxies to form long arcs These collisions are not destructive at all. In fact, the distances between stars are so immense that direct stellar collisions almost never happen, except for perhaps in the much more dense galactic nuclei. The collisions of spiral galaxies with a lot of gas and dust often lead to increased star formation. The colliding gas shocks and collapses under gravity and in a very short time forms many, many more stars than normal. Such an event is called a starburst, and these galaxies are sometimes called starburst galaxies.

Dark Energy

Dark Energy is a hypothetical form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity. It has been hypothesised to account for the observational properties of distant type Ia supernovae, which show the universe going through an accelerated period of expansion. Like Dark Matter, Dark Energy is not directly observed, but rather inferred from observations of gravitational interactions between astronomical objects. Dark Energy makes up 72% of the total mass-energy density of the universe. The other dominant contributor is Dark Matter, and a small amount is due to atoms or baryonic matter.

1st stage of star formation

Dense interstellar cloud such as the core of a dark dust cloud or a molecular cloud Must become unstable, start to collapse under the weight of its own gravity, and break up into small pieces for star formation to start Most astronomers think that this happens when an external event pushes a cloud beyond the point where pressure can resist gravity's inward pull This whole process can take a few million years

Radio Galaxies

Emits huge amounts of radio energy The radio emission comes from the core AND from very large regions on either side of the optical part of the galaxy called ``radio lobes'' The radio lobes can extend for millions of light years from the center of the galaxy The radio emission from normal galaxies is thousands to millions of times less intense and is from the gas between the stars Most radio galaxies are elliptical galaxies

Spiral Galaxies

Flattened galactic disk Spiral arms Central galactic bulge with a dense nucleus Extended halo of old, faint stars Type Sa have largest bulges and more open spiral arms, type Sc have smallest bulges and loose, poorly defined spiral structures, Sb in the middle The tightness of the spiral pattern correlated with the size of the bulge Bulges and halos of these galaxies contain large numbers of reddish old stars and globular clusters Galactic disk populated by A-G type stars Flat disks rich in gas and dust Spiral arms contain many emission nebulae and newly formed O and B type stars they have more orderly, rotational motion than random motion (the rotation refers to the disk as a whole and means that the star orbits are closely confined to a narrow range of angles and are fairly circular) they have some or a lot of gas and dust between the stars this means they can have new star formation occurring in the disk, particularly in the spiral arms Disks are where most of the star formation is taking place (as indicated by the O stars and the emission nebulae). All of the regions between the spiral arms contain stars, not just the spiral arms themselves. Stars with high abundances of metals are found in the disk, indicating that they are at least second generation stars and likely third or more. The bulges contain little gas (except often near the nucleus) and relatively old stars. The halos contain few enough stars that they are not easily seen in images. The bulges and haloes contain stars with few metals which are likely first generation stars that are quite old. they have a spiral structure.

Type 1 supernova

Hydrogen poor Caused by sudden onset of nuclear burning Light curve similar to that of a nova Results from the detonation of a carbon white dwarf, has little hydrogen because these stars have little hydrogen Appearance of light curve results from radioactive decay of unstable elements produced in the explosion Only a small fraction of low mass stars evolve into white dwarfs that explode as type 1 supernovae Also known as carbon detonation supernovae Novae eject matter from a white dwarf's surface, but they don't necessarily expel or burn all material that has accumulated since the last outburst, so white dwarfs have a tendency to increase mass with each new nova cycle The maximum mass of a white dwarf is 1.4 solar masses (Chandrasekhar Mass), when the mass exceeds that, the star starts to collapse because the degeneracy pressure of electrons that holds up the white dwarf is unable to withstand the force of gravity, causing the star to collapse This causes the internal temperature to rise, which causes carbon fusion, which causes the star to explode One model for how a Type Ia supernova is produced involves the accretion of material to a white dwarf from an evolving star as a binary partner. If the accreted mass causes the white dwarf mass to exceed the Chandrasekhar limit of 1.44 solar masses, it will catastrophically collapse to produce the supernova. Another model envisions a binary system with a white dwarf and another white dwarf or a neutron star, a so-called "doubly degenerate" model. As one of the partners accretes mass, it follows what Perlmutter calls a "slow, relentless approach to a cataclysmic conclusion" at 1.44 solar masses. A white dwarf involves electron degeneracy and a neutron star involves neutron degeneracy. Return to the white dwarf in a binary system accreting mass from its companion. Or consider a binary system with two white dwarfs that merge. If the mass of a white dwarf should exceed 1.4 solar masses. (Chandrasekhar Limit), then the star will become so hot as to begin nuclear fusion again, but this time it will not be a controlled reaction. The entire star will begin fusion all at once (no envelope above to keep it in check), a runaway nuclear chain reaction occurs and a huge amount of energy is released all at once. The star EXPLODES and is completely obliterated. Probably takes out its companion as well as any planets left in the system from its earlier life. Heavy elements are synthesized during the chain reactions from Carbon, Oxygen, and He (but there is little hydrogen in this star -- hence no hydrogen in the spectrum) and are flung out into space. Binary star system where a white dwarf is gaining matter from a companion White dwarfs have burnt up all of the hydrogen, so no hydrogen lines in the spectra Type I supernovae happen in close binary systems and do not show strong hydrogen emission lines. Type I (especially Ia) supernovae create most of the iron and nickel found in the interstellar medium. Type Ia supernovae are several times more luminous than Type Ib, Ic, and Type II supernovae, leave no core remnant behind, and result from when a low-mass star's core remnant (a white dwarf) detonates.

Barred Spiral Galaxies

Like spiral galaxies but with elongated bar of stellar and interstellar matter passing through the center and extending beyond the bulge into the disk

Type 2 supernova

Lots of hydrogen Caused by enormously energetic shock wave Plateau in light curve a few months after the maximum Caused by the explosion of the core of a massive star Light curve caused by expansion and cooling of the star's outer envelope as it is blown into space by shock waves Expanding envelope consists mainly of unburned gas such as hydrogen and helium All high mass stars become type 2 supernovae Also known as core collapse supernovae Elements in the core of a star are burnt to depletion within the star's lifetime, once the star starts to burn iron, gravity overwhelms the pressure of the star's hot gas and the star implodes Type II supernovae are modeled as implosion-explosion events of a massive star. They show a characteristic plateau in their light curves a few months after initiation. This plateau is reproduced by computer models which assume that the energy comes from the expansion and cooling of the star's outer envelope as it is blown away into space. This model is corroborated by the observation of strong hydrogen and helium spectra for the Type II supernovae, in contrast to the Type I. There should be a lot of these gases in the extreme outer regions of the massive star involved. Type II supernovae are not observed to occur in elliptical galaxies, and are thought to occur in Population I type stars in the spiral arms of galaxies. Type Ia supernovae occur in all kinds of galaxies, whereas Type Ib and Type Ic have been seen only in spiral galaxies near sites of recent star formation (H II regions). This suggests that Types Ib and Ic are associated with short-lived massive stars, but Type Ia is significantly different. . Fuel is exhausted so it collapses This happens because iron lies at the dividing line between elements that engage in fusion and elements that engage in fission, and so it engages in neither, causing it to dampen the inferno in the star's core Once the core of a supergiant becomes mostly Iron the star's last hopes of fight off gravity are gone. Iron is the most tightly bound of all atomic nuclei. Trying to fuse it to heavier elements will cost the star energy. (Endothermic reaction). The star cannot do this and so there will be no more nuclear reactions in the core. It continues to shrink and becomes degenerate (like a white dwarf star) The mass of the core continues to increase as matter from the fusion shells above it falls down. When the mass of the core reaches 1.4 solar masses, the core will collapse. Not even electron degeneracy pressure can withstand gravity's pull now. Have retained their hydrogen and helium layers, so strong hydrogen emission lines Type II supernovae happen in single star systems (or at least far enough away from any companion star to retain their hydrogen outer layers) and have strong hydrogen emission lines. Type II create most of the oxygen found in the interstellar medium.

Starburst Galaxies

Luminous galaxies, luminosities more than 10^10 times the solar value Non Stellar emission, meaning that most energy is emitted as invisible wavelengths Previously normal galaxies now characterized by widespread episodes of star formation most likely caused by violent events such as interactions with a neighbor galaxies that are observed to be forming stars at an unusually fast rate (about 103 times greater than in a normal galaxy) At these high levels of star formation it is estimated that the supply of gas and dust within the galaxy would be exhausted within about 108 years, meaning that these episodes of intense star formation must have started relatively recently and will end in the not too distant future Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus It was initially detected in 2010 using the Sunyaev-Zel'dovich effect Although hidden from us at optical wavelengths by the enshrouding dust, massive stars are formed out of the available gas. They emit copious amounts of ultraviolet wavelengths which is absorbed by the surrounding dust and reemitted at infrared wavelengths, making starburst galaxies among the most luminous infrared objects in the Universe.

Spectroscopic Parallax

Measurement of apparent brightness of a light source, combined with some knowledge of its luminosity, can yield an estimate of its distance Steps to use spectroscopic parallax Measure the star's apparent brightness and spectral type without knowing how far away it is Use the spectral type to estimate the star's luminosity Use the inverse square law to determine the distance to the star (apparent brightness is directly related to luminosity/(distance2)) Spectral type is letter classification (OBAFGKM — corresponds to luminosity (O is most luminous, M is least luminous)) followed by a number between 0-9 that corresponds to how hot a star is (0 is hottest)

Galactic Evolution

Most ellipticals formed very early. Some ellipticals formed from the merging of 2 or more spirals. Some (many?) galaxies formed from the agglomeration of numerous small subunits of stars There used to be far more small, blue, irregular galaxies. Some probably merged together. Perhaps others faded and are now faint. Most spirals used to look peculiar ("train wrecks")

Elliptical Galaxies

No spiral arms, often no obvious galactic disk Stellar density increases in nucleus E0 are most circular, E7 are most elongated, anything else is in between Giant ellipticals and dwarf ellipticals Dwarf ellipticals much more common Little to no cool gas or dust Generally no evidence of young stars or star formation Mostly made up of old, reddish, low mass stars Lots of hot interstellar gas in the interiors they have much more random star motion than orderly rotational motion (star orbits are aligned in a wide range of angles and have a wide range of eccentricities) they have very little dust and gas left between the stars this means that they have no new star formation occurring now and no hot, bright, massive stars in them (those stars are too short-lived) they have no spiral structure. The elliptical galaxy's mass = k × (velocity dispersion)2 × (the distance the stars are from the galaxy center)/G, where k is a factor that depends on the shape of the galaxy and the angle the galaxy is from Earth. Elliptical galaxies compared to spirals contain very little gas and dust show little evidence of star formation show little evidence for rotation. contain mostly old stars and are therefore usually redder in color than spiral galaxies. The biggest ellipticals have masses of about M = 1013 solar masses

Stellar Populations

Old stars with few heavy elements are referred to as population II stars and are found in the halo and in globular clusters, 10-13 billion years old, masses less than or equal to 0.8 solar masses, metal poor Population I stars contain more heavy elements than globular cluster and halo stars, are typically younger and found in the disk, and are especially concentrated in the spiral arms Population III consisting of the very first stars with little to no metal content, as they did not exist near the beginning of the universe. They did not last very long, but helped the metals to form for the later populations. Stars may be classified by their heavy element abundance, which correlates with their age and the type of galaxy in which they are found. Population I stars include the sun and tend to be luminous, hot and young, concentrated in the disks of spiral galaxies. They are particularly found in the spiral arms. With the model of heavy element formation in supernovae, this suggests that the gas from which they formed had been seeded with the heavy elements formed from previous giant stars. About 2% of the total belong to Population I. Population II stars tend to be found in globular clusters and the nucleus of a galaxy. They tend to be older, less luminous and cooler than Population I stars. They have fewer heavy elements, either by being older or being in regions where no heavy-element producing predecessors would be found. Astronomers often describe this situation by saying that they are "metal poor", and the "metallicity" is used as an indication of age.

Metallicity

One might expect the metallicity of a star to be related to when it formed. Indeed, very old stars formed before there had been many generations of stars, and hence before many SNe exploded, and so would have been formed from gas containing mostly hydrogen and helium. But stars forming now, will contract from gas that has already been polluted by SNe, and hence will be more metal rich. Within the MW, this seems to be born-out, at least to some extent. Stars in old GCs, for example, typically have [Fe/H]~1, so quite a bit below the solar value. And most of the old stars in the halo also have low metallicity. These are called population II stars. In contrast, stars in the disk usually have higher metallicity, and are called population I. There is no strict divide between those, some disk stars also have low Z for example. Recently, there has been interest in the very first generation of stars that formed after the Big Bang. Those would have Z=0! They are called population III. The halo star with the lowest metallicity currently known, has [Fe/H] ~-5, and might well be one of the first stars to have formed in the MW. Astronomers call all elements more massive than Helium`metals', and denote them by Z (X and Y being the hydrogen and helium abundance by mass, respectively). These are produced in stars. For some elements, like e.g. Carbon, we don't really know which stars are the dominant source.

5th stage of star formation

Protostar approaches the main sequence It has shrunk to about 10 times the mass of the sun, surface temperature is about 4000 K, luminosity is about 10 times the solar value, central temperature is 5,000,000 K Gas is completely ionized but the protons do not have enough energy to overcome the electromagnetic repulsion Core is still too cool for nuclear fusion to start

6th stage of star formation

Protostar becomes a true star Radius is about 1,000,000 km, central temperature is about 10,000,000 K, surface temperature is about 4,500 K Luminosity is ⅔ of the solar value

Pulsar

Pulsars are astronomical objects emitting radio radiation in the form of pulses Each one has its one pulse period and duration Pulsars are compact spinning neutron stars that periodically flash radiation towards earth Radiation caused by charged particles flowing along magnetic field lines Two hotspots on the neutron star that emit radiation Tend to have high speeds All pulsars are neutron stars but not all neutron stars are pulsars This is because the factors that make a pulsar pulse -- a strong magnetic field and rapid rotation -- decrease with time so that the pulses weaken and become less frequent any of a class of cosmic objects that emit extremely regular pulses of radio waves; a few such objects are known to give off short rhythmic bursts of visible light, X rays, and gamma radiation as well Sometimes, pulsars aren't observed as pulsars from earth because the pulsar beam isn't oriented the right way

Quasars

Quasars tend to be found at great distances from us; there are no nearby quasars When we look at quasars, we see them as they were billions of years ago The number of them increases at greater distances, so that must mean they were more common long ago The number of quasars peaks at a time when the universe was about 20% of its current age Back then the galaxies were closer together and collisions were more common than today Also, the galaxies had more gas that had not been incorporated into stars yet The number of quasars was hundreds of times greater than the time closer to the present At very great distances the number of quasars drops off The light from the most distant quasars are from a time in the universe before most of the galaxies had formed, so fewer quasars could be created A quasar is an extremely luminous active galactic nucleus It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of our Sun In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk Quasars have large red shifts, indicative of great distance from the earth, but have variability with periods of weeks or months which indicate that they are small. Their size is on the order of light weeks, but are brighter than our galaxy which is about 100,000 light years across. The observed quasar red shifts correspond to a speed range of .15 c to 0.91 c. Using a Hubble constant of 55 km/s per megaparsec gives distances of 2.6 to 16 billion light years for these quasars. The evidence on quasars suggests greater luminosity than our entire galaxy of 200 billion stars. The turbulent velocities in the quasars are up to a few 10s of thousands of m/s, so are constantly occurring explosions, i.e., that kind of turbulent velocity in a chemical reaction would make a potent bomb. Some of the quasars are a few light-days across, as evidenced by their periods of variability, and yet much brighter than our entire galaxy which is 100,000 light years across. This makes them about solar system size. The suggested energy source is a black hole with several billion solar masses. There are examples of multiple images of quasars caused by gravitational lens effects

Galactic clusters

Some clusters have only a handful of galaxies and are called poor clusters Other clusters with hundreds to thousands of galaxies are called rich clusters The low mass of a poor cluster prevents the cluster from holding onto its members tightly The poor cluster tends to be a bit more irregular in shape than a rich cluster Galaxy clusters attract each other to produce superclusters of tens to hundreds of clusters. Their mutual gravity binds them together into long filaments (thin, stringlike structures) 300 to 900 million light years long, 150 to 300 million light years wide, and 15 to 30 million light years thick on average. The discovery of these huge structures was made recently from years of taking doppler shifts of thousands of galaxies.

Binary systems

Stars stay on the opposite side of the center of mass from each other. The massive star moves slower than the low-mass star. The center of mass is also the point where mass1 × velocity1 = mass2 × velocity2 A binary star system consists of two stars which orbit around a common point, called the center of mass following Kepler's Laws. Visual binaries are systems in which the individual stars can be seen through a telescope. Spectroscopic binaries are systems in which the stars are so close together that they appear as a single star even in a telescope. The binary nature of the system is deduced from the periodic doppler shifts of the wavelengths of lines seen in the spectrum, as the stars move through their orbits around the center of mass. In some instances, the spectrum shows the lines from both stars; this case is called a double-lined spectroscopic binary. In other cases, only one set of lines is seen, the other star being too faint, and we call the system a single -lined spectroscopic binary. Eclipsing binaries are systems in which the orbital plane is oriented exactly edgewise to the plane of the sky so that the one star passes directly in front of the other, blocking out its light during the eclipse. Eclipsing binaries may also be visual or spectroscopic binaries. The variation in the brightness of the star is called its light curve. Five to ten percent of the stars visible to us are visual binary stars. Careful spectroscopic studies of nearby solar-type stars show that about two thirds of them have stellar companions. We estimate that roughly half of all stars in the sky are indeed members of binaries. Most stars are in binary systems and Mass exchange can occur. Two stars in orbit about each other have a "sphere of influence" about them in which their own gravity dominates, but outside that sphere the gravity of the other star becomes important. In essence stars have Roche limits just like those we talked about with planets and the formation of rings. We call these spheres of influence, Roche Lobes. When one star bloats up into a Red Giant it can fill its Roche Lobe and the matter on the surface can be more gravitationally attracted to the companion star. Matter Flows on to the companion which in turn can now evolve faster due to the increased mass. If the smaller companion star is a white dwarf (a long since dead star) and the other stars begins to fill its Roche Lobe (i.e., become a Red Giant), matter will flow onto the White Dwarf's surface. It doesn't fall directly onto the surface but misses and spirals down onto it through an accretion disk. The material falling onto the surface is heated considerably by friction in the disk and the collision with the surface (very dense). This gives off much luminosity. The matter on the surface can actually undergo nuclear fusion in a great explosion. This explosion lifts some of the white dwarf surface away and brightens the star considerably for a short time. We witness a Nova (Latin for "new star"). The star brightens by a factor of 100 - 106 in a few days or weeks; then fades over months or years. The process can recur.

Halo

Surrounding the disk is a much less dense population of stars in a spherical distribution about 100 kpc in diameter (~300,000 light years). These stars are on fairly random and highly elliptical orbits about the center of the Galaxy. The globular clusters are all distributed in the halo. The stars in the halo seem to have much less heavy elements in them and are very old. The halo primarily contains individual old stars and clusters of old stars ("globular clusters"). The halo also contains "dark matter," which is the material that we cannot see but whose gravitational force can be measured. The Milky Way's halo may be over 130,000 light years across.

Background radiation

The blackbody radiation is seen as a remnant of the transparency point at which the expanding universe dropped below about 3000K so that radiation could escape. A uniform background radiation in the microwave region of the spectrum is observed in all directions in the sky. Currently it is commonly called the Cosmic Microwave Background or just CMB, alluding to its Wien peak in the microwave region. It shows the wavelength dependence of a "blackbody" radiator at about 3 Kelvins temperature. It is considered to be the remnant of the radiation emitted at the time the expanding universe became transparent at about 3000 K temperature. The discovery of the 3K microwave background radiation was one of the crucial steps leading to the calculation of the standard "Big Bang" model of cosmology, its role being that of providing estimates of relative populations of particles and photons. Research using the Far Infrared Absolute Spectrophotometer (FIRAS) onboard the COBE satellite have given a temperature of 2.725 +/- 0.002 K. Previous experiments had shown some anisotropy of the background radiation due to the motion of the solar system, but COBE collected data showing fluctuations in the background. Some fluctuations in the background are necessary in big bang cosmology to give enough non-uniformity for galaxies to form. The apparent uniformity of the background radiation is the basis for the "galaxy formation problem" in big bang cosmology. The more recent WMAP mission gave a much higher resolution picture of the anisotropies in the cosmic background radiation.The precision of the mapping of the CMB was improved with the Planck satellite, giving the best current values for the descriptive parameters. The data for the round figure of 109 photons per nuclear particle is the "most important quantitative conclusion to be drawn from the measurements of the microwave radiation background ..."(Weinberg p66-70). This allowed the conclusion that galaxies and stars could not have started forming until the temperature dropped below 3000K. Then atoms could form and remove the opacity of the expanding universe; light could get out and relieve the radiation pressure. Star and galaxy formation could not occur until the gravitational attraction could overcome the outward radiation pressure, and at 109photons/baryon a critical "Jean's mass" of about a million times that of a large galaxy would be required. With atom formation and a transparent universe, the Jeans mass dropped to about 10-6 the mass of a galaxy, allowing gravitational clumping. The 3K background provides foundational evidence for cosmological models. The 3K background implies about 5.5 x 105 photons/liter. This is based on the radiation energy density and the average energy per photon at this temperature. The range of estimates for baryon density is from twice critical density at 6 x 10-3/liter to the low end estimate of the visible galaxy, 3 x 10-5/liter. This gives a range of 1 x 108 to 2 x 1010 photons/baryon. It is this estimate of the number of photons per baryon which was crucial in calculations of the big bang. In the modeling of nucleosynthesis in the big bang, including the hydrogen/helium ratio, the relative population of baryons and photons agreed with observations. When the trace quantities of D, 3He, and 7Li are examined and made a part of the big bang model, the ratio of baryons to photons is constrained more tightly. The Particle Data Group gives the baryon/photon ratio η as 2.6 x 10-10 < η < 6.3 x 10-10 baryons/photon Since the conservation of baryon number is a strong conservation principle, it is inferred that the ratio of photons to baryons is constant throughout the process of expansion. No known process in nature changes the number of baryons. An anisotropy of about 0.1% exists in the cosmic microwave background radiation which is attributed to a Doppler shift caused by the motion of the solar system through the radiation. The Particle Data Group reports the asymmetry as mostly dipole in nature with a magnitude of 1.23 x 10-3. This value is used to calculate a velocity of about 600 km/s for the Earth compared to an observer keeping track with the general expansion. The COBE satellite has discovered fluctuations in the cosmic microwave background radiation with the use of a differential microwave radiometer. The size of the fluctuations are ΔT/T = 6x10-6. This is just above the level at which the big bang cosmological calculations would have been in trouble. The scale of the fluctuations is larger than the horizon at the time the background radiation was emitted, indicating that the fluctuations are primordial, dating from a time before the separation of radiation and matter, the transparency point. The "horizon" is the distance within which there can be causal connections, i.e., within light transit time of each other. In 1965 Arno A. Penzias and Robert W. Wilson of Bell Laboratories were testing a sensitive horn antenna which was designed for detecting low levels of microwave radiation. They discovered a low level of microwave background "noise", like the low level of electrical noise which might produce "snow" on a television screen. After unsuccessful attempts to eliminate it, they pointed their antenna to another part of the sky to check whether the "noise" was coming from space, and got the same kind of signal. Being persuaded that the noise was in their instrument, they took other, more sophisticated steps to eliminate the noise, such as cooling their detector to low temperatures. Finding no explanations for the origin of the noise, they finally concluded that it was indeed coming from space, but that it was the same from all directions. It was a distribution of microwave radiation which matched a blackbody curve for a radiator at about 2.7 Kelvins. After all their efforts to eliminate the "noise" signal, they found that a group at Princeton had predicted that there would be a residual microwave background radiation left over from the Big Bang and were planning an experiment to try to detect it. Penzias and Wilson were awarded the Nobel Prize in 1978 for their discovery. The cosmic microwave background (CMB) is an almost-uniform background of radio waves that fill the universe. The CMB is, in effect, the leftover heat of the Big Bang itself - it was released when the universe became cool enough to become transparent to light and other electromagnetic radiation, 100,000 years after its birth. At this time, the universe was filled with a hot, ionized gas. This gas was almost completely uniform, but did have slight deviations - spots that were slightly (1 part in 100,000) more or less dense. The slight changes in the intensity of the CMB across the sky (deviations of only than 1 part in 100,000) give us a map of the early universe. When the CMB was initially emitted it was not in the form of microwaves at all, but mostly visible and ultraviolet light. Over the past few billion years, the expansion of the universe has redshifted this radiation toward longer and longer wavelengths, until today it appears in the microwave band. In its early days, the universe was extremely smooth and homogenous... but not quite perfectly so. At the time the CMB was released, for example, its density was constant to about 1 part in 100,000. It is believed this smoothness comes about because of inflation, a time of extremely rapid expansion in the first 10-34 seconds of so of the universe's existence. This rapid expansion smoothed out any lumpiness the universe may have initially had, but quantum mechanical fluctuations introduced new ones - tiny fluctuations of density at all length scales. These tiny fluctuations have grown with time due to gravity (slightly denser regions attract more stuff to become denser yet), eventually providing the seeds for the galaxies and galaxy clusters we see today. This lumpiness affects the CMB largely because of gravitational redshifting. Radiation emitted from a dense spot in the sky has to fight against a bit of extra gravity as it heads toward our detectors. When it leaves that gravity well, the radiation will be a little less energetic than radiation emitted from a less-dense region, so that spot of the sky will appear to be a little colder. A map of the apparent temperature of the CMB across the sky thus gives you a map of the density of matter in the early universe. Most of the cosmological information we get from the CMB is found by studying its power spectrum, a plot of the amount of fluctuation in the CMB temperature spectrum at different angular scales on the sky. The upper plot at right shows measurements of the power spectrum as of 2003 - large angular scales are at the left of the plot, while smaller sky features contribute to the right of the plot. The shape of this power spectrum is determined by oscillations in the hot gas of the early universe, and the resonant frequencies and amplitudes of these oscillations (which "notes" the universe likes to play!) are determined by its composition. Since we know the physics of hot gases very well, we can compute the properties of the oscillating gas by studying the positions and relative sizes of these peaks. The position of the first peak, for example, tells us about the curvature of the universe (and hence how much total stuff there is in it), while the ratio of heights between the first and second peaks tells us how much of the matter is baryonic (ordinary matter). In practice, there are many variables that affect all parts of the power spectrum, and detailed computer simulations (the red curve in the plot) are used to sort it all out.

Kepler's Laws

The orbital paths of the planets are elliptical with the sun at the focus An imaginary line connecting the sun to any planet sweeps out equal areas of the ellipse in equal intervals of time The square of a planet's orbital period is proportional to the cube of its semimajor axis (half the length of the long axis of the ellipse)

Gravitational Lensing

The phenomenon at the root of gravitational lensing is the deflection of light by gravitational fields predicted by Einstein's general relativity, in the weak-field limit. The deflection has well-known observable effects, such as multiple images, magnification of images, and time delays for propagation of light along the paths forming different images. Because the effect is independent of wavelength, multiple lensed images share a single spectrum, which helps us to identify lensed images. We currently distinguish three regimes: strong (or macro) lensing, weak lensing and microlensing. In strong lensing, the light from a distant source such as a quasar or galaxy is deflected by an intervening galaxy or cluster of galaxies, forming multiple images of the source. In weak lensing, the light from a distant source is distorted but not multiply imaged. The strength and spatial distribution of the effect are determined by measuring many distorted images and deriving a statistical measure of the lensing. In microlensing, the light from a distant source is already macro-lensed, but components such as stars in the lens add their own deflections. The effect appears as additional, measurable magnification. A few examples of the application of gravitational lenses are: estimates of the amount of dark matter in galaxies and clusters of galaxies, measurements of the evolution of galaxies with cosmic time, the age of the universe and estimates of constraints on dark energy. Because the physics of gravitational lensing is well-established, these estimates are robust and provide unique constraints, complementary to those from other astrophysical techniques.

Tully-Fisher Relationship

The relationship between rotational speeds and luminosities of spiral galaxies within a few tens of megaparsecs of the milky way Published by Tully and Fisher We can calculate rotational speed because radiation from the side of the galaxy whose matter is approaching us and radiation from the side of the galaxy that is receding from us is redshifted by a similar amount Can use this relationship to measure distances up to 200 Mpc L* is 10 billion solar luminosities The Tully-Fisher relation for the infrared luminosity is: circular velocity = 220 × (L/L*)0.22

Spiral Arms

The spiral arms are curved extensions that begin at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.

7th stage of star formation

The star contracts a little more over the next 30 million years, causing the central density to rise up to about 10^32 particles/m^3 Central temperature increases to 15,000,000 K and surface temperature reaches 6000 K Star finally reaches the main sequence (band on the HR diagram where most stars are found that runs from the top left to bottom right of the diagram) Pressure and gravity are finally balanced, rate at which nuclear energy is generated in the core matches the rate at which energy is radiated from the surface The star settles down to spend about 90% of its life as a main sequence star. It is fusing hydrogen to form helium in the core A star remains at a given spectral type during the entire main sequence stage---the main sequence is not an evolutionary sequence

Warm-Hot Intergalactic Medium

The warm-hot intergalactic medium (WHIM) refers to a sparse, warm-to-hot (105 to 107 K) plasma that cosmologists believe to exist in the spaces between galaxies and to contain 40-50% of the baryons (that is, 'normal matter' which exists as plasma or as atoms and molecules, in contrast to dark matter) in the universe at the current epoch. It can be described as a web of hot, diffuse gas. Much of what is known about the warm-hot intergalactic medium comes from computer simulations of the cosmos. The WHIM is expected to form a filamentary structure of tenuous, highly ionized baryons with a density of 1−10 particles per cubic meter. Within the WHIM, gas shocks are created as a result of active galactic nuclei, along with the gravitationally-driven processes of merging and accretion. Part of the gravitational energy supplied by these effects is converted into thermal emissions of the matter by collisionless shock heating. Because of the high temperature of the medium, the expectation is that it is most easily observed from the absorption or emission of ultraviolet and low energy X-ray radiation. To locate the WHIM, researchers examined X-ray observations of a rapidly growing super massive black hole known as an active galactic nucleus, or AGN. Oxygen atoms in the WHIM were seen to absorb X-rays passing through the medium. In May 2010 a giant reservoir of WHIM was detected by the Chandra X-ray Observatory lying along the wall-shaped structure of galaxies (Sculptor Wall) some 400 million light-years from Earth

Disk

There is a disk of stars all rotating with the same sense of rotation. The disk is about 30 kpc in diameter (100,000 light-years) and 500 pc thick (1,500 light-years)(approximately the same ratio as a CD). Our Sun lies about 2/3 of the way out in the disk from the center of the Galaxy (~ 8.5 kpc). From this position it takes the Sun about 250 million years to orbit the Galaxy. So the Sun has completed about 18 orbits since its birth (4.5 billion/ 250 million). The disk exhibits a beautiful spiral structure. The disk is a flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The Milky Way's disk is 100,000 light years across and 1,000 light years thick. It contains mostly young stars, gas and dust, which are concentrated in spiral arms. Some old stars are also present.

SB0 galaxies

Thin disk and flattened bulge No gas No spiral arms Bar

S0 galaxies

Thin disk and flattened bulge No gas No spiral arms No bar Aka lenticular galaxies these are intermediate between ellipticals and spirals - they have featureless disks and bulges, but no spiral arms and little gas and dust, like ellipticals.

HR Diagram

This law tells us that if we approximate the surface of a star as a blackbody then we can see that the luminosity of a star is related to its radius and temperature. So if two stars have the same temperature but different luminosities then they must have different radii. Stars with higher luminosity must therefore be bigger than stars with lower luminosities but the same temperature. Hence, we call the stars in the upper right Red Giants and the stars in the lower left White Dwarfs. Further analysis also finds that stars tend to have larger radii on the hotter (blue) end of the main sequence and smaller radii on the cooler (red) end of the main sequence. When we combine the information about mass that we have learned from the careful study of binary stars, we find that there is another important trend in the H-R diagram. The more luminous main sequence stars are also more massive. Thus the O and B main sequence stars are more massive than our Sun (a G star) which is in turn more massive than a K main sequence star. This relationship is only true for Main Sequence stars. There are also trends with the red giants and white dwarfs but they are not as simple. Generally, more luminous Red Giants are more massive.

Globular Clusters

Tightly bound star clusters Spherical clusters found away from the Milky Way plane that contain hundreds of thousands of stars spread over about 50 pc Found on halos of galaxies Lack upper main sequence stars Biggest stars are 0.8 times the sun's mass Spectra show few heavy elements which implies that they formed in the past before heavy elements were as abundant as they are now Cluster color-magnitude diagrams change with age. More massive stars evolve quicker than low-mass stars. The hot, luminous main sequence stars will die before the cool, dim main sequence stars. This means that an old cluster will have only the low-mass stars still on the main sequence, but a young cluster will have both high and low-mass stars on the main sequence. The most massive star still on the main sequence tells us the age of the cluster. That point on the main sequence is called the main sequence turnoff. All stars in a cluster are assumed to have formed at about the same time (observations of current star formation do show that stars form in batches) For the common lower mass stars (< 10 solar masses), age of the cluster = (1010)/(MST mass3) years. Use solar masses! For the rare massive stars (> 30 solar masses), use age of the cluster = (1010)/(MST mass2) years. Densely packed. Contain about 104 - 106 stars. Span about 30 parsecs (~100 light-years). Located in a roughly spherical Halo about Milky Way. ~150 known globular clusters. All contain old stars. The H-R diagram of a globular cluster has a small main sequence and a large population of Red Giants and white dwarfs. Thus these clusters are very old (~12 Billion years).

Stellar Parallax

To determine the parallax, observe it from some baseline and measure the angle through which the line of sight to the object shifts Distance in parsecs = 1/parallax in arcseconds Parallax is defined as half of its apparent angle shift relevant to the background To measure parallax, observe the star at different points in the year to get a larger parallactic shift and thus get a more accurate parallax calculation

ULXs

Ultraluminous X-ray sources millions of times brighter than the Sun At least some powered by neutron stars, not black holes

Cepheids

Variable stars, meaning that their luminosities change with time Pulsation periods vary greatly Pulsate due to changes in opacity of interior Opacity is the degree to which gas hinders the passage of light through it Rising opacity causes radiation to become trapped, which causes the pressure to increase, causing the star to puff up If the opacity falls, radiation can escape more easily, causing the star to shrink These conditions occur because of the star's temperature and luminosity placing it on the instability strip of the H-R diagram, when the star is on the instability strip, it is internally unstable Both the temperature and radius vary in a regular way, causing the pulsations High mass stars become Cepheids Pulsating variable stars are normal stars passing through a million year phase of instability Period related to luminosity Type I: classical Cepheids are from young high-metallicity stars and are about 4 times more luminous than Type II Cepheids Type II: W Virginis Cepheids are from older low-metallicity stars and are about 4 times less luminous than Type I Cepheids have pulsation periods of 1 to 50 days

RR Lyrae Stars

Variable stars, meaning that their luminosities change with time Shorter periods and lower luminosities than cepheids RR Lyrae stars all pulsate similarly with only small differences in the period between them RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have previously shed mass during the Red-giant branch phase, and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses All RR Lyrae stars in a cluster have the same average apparent magnitude All RR Lyrae stars have the same average absolute magnitude of 0.6 All of them have approximately the same luminosity Pulsate due to changes in opacity of interior Opacity is the degree to which gas hinders the passage of light through it Rising opacity causes radiation to become trapped, which causes the pressure to increase, causing the star to puff up If the opacity falls, radiation can escape more easily, causing the star to shrink These conditions occur because of the star's temperature and luminosity placing it on the instability strip of the H-R diagram, when the star is on the instability strip, it is internally unstable Both the temperature and radius vary in a regular way, causing the pulsations Low mass horizontal branch stars that lie in the lower part of the instability strip become RR Lyrae stars Pulsating variable stars are normal stars passing through a million-year phase of instability This type of low-mass star has consumed the hydrogen at its core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium at its core, and the star has entered an evolutionary stage called the horizontal branch They pulsate with a period between 5 and 15 hours (Cepheid pulsation periods are greater than 24 hours) Low-mass stars will go through a RR Lyrae pulsation stage while the high-mass stars will go through a Cepheid stage Because low-mass stars live longer than high-mass stars, the Cepheid stars as a group are younger than the RR Lyrae stars.

Seyfert Galaxies

a spiral galaxy with a compact, very bright nucleus that produces a non-thermal continuous spectrum with broad (fat) emission lines on top Some of the emission lines are produced by atoms that have several electrons removed from them Such highly ionized atoms are found only in regions of intense energy Many Seyfert nuclei are in disks with distorted spiral arms and a companion galaxy nearby that is probably gravitationally interacting with the galaxy The key to classifying a galaxy as a Seyfert Galaxy is the presence of broad emission lines from the bright, star-like nucleus. They are often strong radio and infrared sources. The first such active galaxies were discovered in 1943 by Carl Seyfert. Kaufmann cites the example of NGC 4151 which has 28% of its light concentrated in its emission lines. The emission lines include spectra of iron with 9 and 13 electrons stripped away, so the process involves very hot gases. The Seyfert galaxies exhibit variability and some have luminosities approaching the fainter quasars. Approximately 10% of the brightest galaxies in the sky are Seyferts. They are divided into Type 1 and Type 2 Seyferts.

Stellar Classification

classified using the letters O, B, A, F, G, K, and M, where O is hottest and M is coolest Numbers after letters indicate the tenths of the range between the two star classes Roman numerals often also used afterwards to indicate luminosity (see bottom picture) Class "W" stars are very hot stars known as "Wolf-Rayet" Stars. "R", "N", "S" stars are cool stars with particular types of molecular bands. "L" stars are possibly not truly stars at all, in the sense that they may not have nuclear reactions at their cores. Most stars are currently classified under the Morgan-Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars. In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd (or VI) for sub-dwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K. The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.[16] This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity (whilst the Harvard classification is based on just surface temperature). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan-Keenan classification, or MK,[17] and this system remains in use. During the 1860s and 1870s, pioneering stellar spectroscopist Angelo Secchi created the Secchi classes in order to classify observed spectra. In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue. The catalogue used a scheme in which the previously used Secchi classes (I to IV) were subdivided into more specific classes, given letters from A to N. Also, the letters O, P, and Q were used - O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class. No green or purple stars because of the way humans perceive light A green star is radiating right in the center of the visible light spectrum, which means it is emitting some light in all the possible colors. The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white. Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light. Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light.

Dark Matter

material that does not produce detectable amounts of light but it does have a noticeable gravitational effect Just as is the case with the Milky Way Galaxy, rotation curves of other galaxies are also found to be flat and do not drop off at the extent of the visible matter. So all galaxies seem to have large quantities of dark matter in their haloes. Clusters of galaxies also seem to have large quantities of dark matter. The clusters have the appearance of being gravitationally bound systems. However, the typical velocities of individual galaxies appear to be much larger than the velocity required to escape the system, if the required "escape velocity" (v2 = 2GM/R) is calculated assuming a cluster mass, M, traced only by the visible light from the galaxies. Thus the only way the clusters can actually be bound is if there is much more mass in "dark" invisible matter. It is common to find that 90% of a cluster's mass is in the form of "dark matter".

Color index

the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature The smaller the color index, the bluer the object is Conversely, the larger the color index, the redder the object is