Astronomy

cosmic microwave background (CMB) radiation

Isotropic microwave radiation from every direction in the sky having a 2.73-kelvin (K) blackbody spectrum.

MACHOs

"massive compact halo object." MACHOs include brown dwarfs, white dwarfs, and black holes, which are candidates for being considered dark matter

recombination

1. The combining of ions and electrons to form neutral atoms. 2. An event early in the evolution of the universe in which hydrogen and helium nuclei combined with electrons to form neutral atoms.

life

A biochemical process in which living organisms can reproduce, evolve, and sustain themselves by drawing energy from their environment

supermassive black hole

A black hole of 1,000 solar masses (M☉) or more that resides in the center of a galaxy, and whose gravity powers active galactic nuclei.

multiverses

A collection of parallel universes that together comprise all that is.

cosmological constant

A constant, introduced into general relativity by Einstein, that characterizes an extra, repulsive force in the universe due to the vacuum of space itself.

ultrafaint dwarf galaxies

A dwarf galaxy of very low luminosity.

dark energy

A form of energy that permeates all of space (including the vacuum) producing a repulsive force that accelerates the expansion of the universe.

S0 galaxy

A galaxy with a bulge and a disk-like spiral, but smooth in appearance like ellipticals

giant galaxies

A galaxy with luminosity greater than about 1 billion solar luminosities

irregular galaxy

A galaxy without regular or symmetric appearance.

Galaxy

A gravitationally bound system that consists of stars and star clusters, gas, dust, and dark matter

active galactic nucleus (AGN)

A highly luminous, compact galactic nucleus whose luminosity may exceed that of the rest of the galaxy.

axion

A hypothetical elementary particle first proposed to explain certain properties of the neutron and now considered a candidate for cold dark matter.

superclusters

A large conglomeration of galaxy clusters and galaxy groups; typically, more than 100 million light-years

Galaxy cluster

A large, gravitationally bound collection of galaxies containing hundreds to thousands of members

open cluster

A loosely bound group of a few dozen to a few thousand stars that formed together in the disk of a spiral galaxy.

unified model of AGNs

A model in which many different types of activity in the nuclei of galaxies are all explained by accretion of matter around a supermassive black hole.

rotation curve

A plot showing how the orbital velocity of stars and gas in a galaxy changes with radial distance from the galaxy's center.

Drake equation

A prescription for estimating the number of intelligent civilizations existing elsewhere.

void

A region in space containing little or no matter.

distance ladder

A sequence of techniques for measuring cosmic distances; each method is calibrated using the results from other methods that have been applied to closer objects.

dwarf galaxies

A small galaxy with a luminosity ranging from 1 million to 1 billion solar luminosities

galaxy group

A small, gravitationally bound collection of galaxies containing from several to a hundred members;

globular cluster

A spherically symmetric, highly condensed group of stars, containing tens of thousands to a million members.

barred spiral

A spiral galaxy with a bulge having an elongated, barlike shape

spiral density wave

A stable, spiral-shaped change in the local gravity of a galactic disk that can be produced by periodic gravitational kicks from neighboring galaxies or from nonspherical bulges and bars in spiral galaxies.

theory of everything TOE

A theory that unifies all four fundamental forces of nature: strong nuclear, weak nuclear, electromagnetic, and gravitational forces

thermophiles

A type of extremophile that tolerates high temperatures

grand unified theory GUTs

A unified quantum theory that combines the strong nuclear, weak nuclear, and electromagnetic forces but does not include gravity.

cosmic ray

A very-fast-moving particle

luminous matter

Also called normal matter. Matter in galaxies—including stars, gas, and dust—that emits electromagnetic radiation.

antiparticle

An elementary particle of antimatter identical in mass but opposite in charge and all other properties to its corresponding ordinary matter particle.

photino

An elementary particle related to the photon. One of the leading candidates for cold dark matter.

Hubble time

An estimate of the age of the universe from the inverse of the Hubble constant

astrobiology

An interdisciplinary science combining astronomy, biology, chemistry, geology, and physics to study life in the cosmos.

unresolved

Images overlapping to the extent that they cannot be distinguished from one another.

mutation

In biology, an imperfect reproduction of self-replicating material.

WIMP

Literally, "weakly interacting massive particle." A hypothetical massive particle that interacts through gravity but not with electromagnetic radiation and is a candidate for dark matter.

dark matter

Matter in galaxies that does not emit or absorb electromagnetic radiation. Dark matter is thought to constitute most of the mass in the universe.

normal matter

Matter in galaxies—including stars, gas, and dust—that emits electromagnetic radiation

Extremophiles

Microbes that are adapted to tolerate extreme conditions, such as high or low temperatures.

recession

Moving away, receding.

large-scale structure

Observable aggregates on the largest scales in the universe, including galaxy groups, clusters, and superclusters.

Cold dark matter

Particles of dark matter that move slowly enough to be gravitationally bound even in the smallest galaxies.

Hot dark matter

Particles of dark matter that move so fast that gravity cannot confine them to the volume occupied by a galaxy's normal luminous matter.

hierarchical clustering

The "bottom-up" process of forming large-scale structure. Small-scale structure first produces groups of galaxies, which in turn form clusters, which then form superclusters.

SETI

The Search for Extraterrestrial Intelligence project, which uses advanced technology combined with radio telescopes to search for evidence of intelligent life elsewhere in the universe.

quark

The building block of protons and neutrons.

dark matter halo

The centrally condensed, greatly extended dark matter component of a galaxy that contains up to 95 percent of the galaxy's mass.

Hubble constant

The constant of proportionality relating the recession velocities of galaxies to their distances.

pair production

The creation of a particle-antiparticle pair from a source of electromagnetic energy.

habitable zone

The distance from its star at which a planet must be located in order to have a temperature suitable for life

Planck era

The early time, just after the Big Bang, when the universe as a whole must be described with quantum mechanics.

Big Bang

The event that occurred 13.8 billion years ago that marks the beginning of time and the universe.

weak nuclear force

The force underlying some forms of radioactivity and certain interactions between subatomic particles.

electromagnetic force

The force, including both electric and magnetic forces, that acts on electrically charged particles.

Big Bang nucleosynthesis

The formation of low-mass nuclei (H, He, Li, Be, B) during the first few minutes after the Big Bang.

Hubble's law

The law stating that the speed at which a galaxy is moving away from us is proportional to the distance of that galaxy.

observable universe

The part of the universe that can be observed, because light has had time to travel to Earth.

natural selection

The process by which forms of structure—ranging from molecules to whole organisms—that are best adapted to their environment become more common than less-well-adapted forms.

heredity

The process by which one generation passes on its characteristics to future generations

reionization

The process that reionized matter in the early universe after the Big Bang.

electroweak theory

The quantum theory that combines descriptions of both the electromagnetic force and the weak nuclear force.

standard model

The theory of particle physics that combines electroweak theory with quantum chromodynamics to describe the structure of known forms of matter.

string theory

The theory that conceives of particles as strings in 10 dimensions of space and time; the current contender for a theory of everything.

look-back time

The time that it has taken the light from an astronomical object to reach Earth.

tuning fork diagram

The two-pronged diagram showing Hubble's classification of galaxies into ellipticals, S0s, spirals, and barred spirals.

critical density

The value of the mass density of the universe that, ignoring any cosmological constant, is just barely capable of halting expansion of the universe.

inflation

brief phase of ultrarapid expansion of the very early universe. After inflation, the standard Big Bang models of expansion apply

homogeneous

describing a universe in which observers in any location would observe the same properties

isotropic

describing a universe whose properties observers find to be the same in all directions.

flatness problem

it is surprising the universe is so close to being exactly flat.

horizon problem

observation that the cosmic background radiation is so uniform in all directions, despite the fact that widely separated regions should have been "over the horizon" from each other in the early universe.

quasar

short for quasi-stellar radio source. The most luminous of the active galactic nuclei, seen only at great distances from our galaxy.

elliptical galaxy

with a circular to elliptical outline on the sky containing almost no disk and a population of old stars.

Spiral galaxy

with a discernible disk in which large spiral patterns exist