Accretion. Accumulation of dust and gas onto larger bodies such as stars, planets, and moons.

Accretion Disk. A relatively flat sheet of dust and gas surrounding a newborn star, a black hole, or any massive object growing in size by attracting material.

Active Galactic Nuclei (AGN). A core region in certain galaxies that, like a powerful engine, spews large amounts of energy from its center. Believed to be powered by the accretion of matter onto black holes.

Baryon. Any subatomic particle of half-integral spin that interacts via the strong nuclear force. (Most commonly, these are protons and neutrons.) The term "hadron" includes the lighter integer spin mesons as well the half-integral spin baryons.

Big Bang. A theory of cosmology in which the expansion of the Universe is presumed to have begun with a primeval explosion.

Big Bang Observer. A Beyond Einstein gravitational wave detector vision mission.

Brane. An object or subspace in string theory that can have various spatial dimensions. A 1-brane is a string; a 2-brane is a surface or membrane; a p-brane has lengths in p dimensions.

Brane World. A four-dimensional surface (brane) in a higher-dimensional spacetime.

Black Hole. An object whose gravity is so strong that not even light can escape from it.

Black Hole Imager. A Beyond Einstein X-ray interferometer vision mission.

Coded aperture mask. A plate with both opaque and transparent elements placed in front of a position-sensitive gamma-ray detector. Sources at different places in the sky cast shadows of the grid at different positions on the detector, allowing an image of the entire sky to be reconstructed, with an angular resolution set by the size of the mask elements.

Cosmic Background Radiation. Radiation of the cosmos left over from the Big Bang.

Cosmological Constant. A term Einstein added to his equations of the general theory of relativity, to account for an apparently non-expanding Universe, but later rejected when Hubble's observations seemed to indicate it was not needed. Can be interpreted as a special form of Dark Energy.

Cosmology. The astrophysical study of the history, structure, and dynamics of the Universe.

Dark Energy. The residual energy in empty space which is causing the expansion of the Universe to accelerate. Einstein's Cosmological Constant was a special form of dark energy.

Dark Matter. Mass whose existence is deduced from the analysis of galaxy rotation curves and other indirect evidence but which has so far escaped direct detection.

Doppler Effect. An observer receives sound and light from bodies moving away from her with lower frequency and longer wavelength than emitted (see Redshift) and from bodies moving toward her with higher frequency and shorter wavelength. The shift in frequency increases as the speed of the body increases.

eV. Electron Volt. The energy an electron has after being accelerated by a 1 volt potential. Quanta of visible light (photons) have energies of a few eV.

Event Horizon. The boundary of the region around a black hole from which nothing can escape once crossed.

General Relativity. The theory of gravitation developed by Albert Einstein incorporating and extending the theory of special relativity and introducing the principle that gravitational and inertial forces are equivalent.

Graviton. The quantum particle, associated with gravitational waves, which carries the gravitational force.

Inflation. Postulated period in the very early Universe of extremely rapid expansion, inflating what is now the observable Universe from an atomic size to its cosmological size in a fraction of a second. This process makes the Universe very smooth and flat, as observed.

Interferometry. The use of interference phenomena of light waves to measure distances and angles between objects.

Jets. Beams of energetic particles, usually coming from an active galactic nucleus or a pulsar.

keV. kilo electron Volt. A unit of energy equal to one thousand eV. X-ray photons have energies of 0.1-100 keV.

L2. The second of five Lagrangian equilibrium points, approximately 1.5 million kilometers beyond Earth, where the gravitational forces of Earth and Sun balance to keep a satellite at a nearly fixed position relative to Earth. (Illustration)

Light Year. The distance light travels in a year (9.5 million million kilometers, or 5.9 million million miles).

MeV. Mega electron Volt. A unit of energy equal to one million eV. Gamma-ray photons are those with energies greater than 0.1 MeV, equal to 100 keV.

Neutron Star. The imploded core of a massive star remaining after a supernova explosion. Contains about the mass of the Sun in less than a trillionth of the Sun's volume.

Parsec. The distance to an object that has a parallax of one arcsecond (equivalent to 3.26 light years).

Polarization. A property of light in which the planes of vibration of the (electric field of the) light are at least partially aligned.

Pulsar. A rotating neutron star which generates regular pulses of radiation.

Quantum Mechanics. The well-tested theory of the behavior of matter on the microscopic scales of atoms and computer chips, where the constituents of matter behave simultaneously like waves and particles.

Quasar. Enormously bright objects at the edge of our Universe that emit massive amounts of energy and are likely powered by black holes.

Redshift. An apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by motion of the emitting object away from the observer.

Singularity. A place where spacetime becomes so strongly curved that the laws of Einstein's general relativity break down and quantum gravity must take over. Found inside black holes and perhaps at the beginning of the Big Bang.

Spectroscopy. The study of spectral lines (light given off at a specific frequency by an atom or molecule) from different atoms or molecules that can indicate the chemical composition of stars, gas, or dust.

String Theory. A theory that what we perceive as particles are actually vibrations on strings or membranes in a 10- or 11-dimensional space, respectively. These theories resolve the incompatibility between general relativity and quantum mechanics and unify them.

z. The symbol for the amount of redshift. The ratio of the observed change in wavelength of light emitted by a moving object to the rest wavelength of the emitted light. The most distant known galaxies and quasars have values of z = 6 or greater.