Accretion, accretion disk—Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk.

Active galaxy—Certain galaxies emit far more energy than can be accounted for by their stars alone. The central regions of these galaxies harbor a compact, solar-system-sized object capable of outshining the rest of the galaxy by a factor of 100. The ultimate energy source for active galaxies may be the accretion of matter onto a supermassive black hole. Active galaxies can emit strongly across the entire electromagnetic spectrum, from radio waves to gamma rays. See quasar.

Active optics—A technique to reduce the effects of slowly varying forces, such as gravitational deflections and temperature drifts, that can distort a mirror on time scales of minutes to hours, resulting in imperfect images.

Adaptive optics—A set of techniques to adjust the mirrors of telescopes on time scales of hundredths of a second to correct for distortions in astronomical images due to turbulence in Earth’s atmosphere.

Anisotropy—Dependence of the properties of a system on the orientation or the direction of observation. The distribution of galaxies in space is not uniform, whereas the intensity of the cosmic background radiation from the Big Bang is highly uniform in all directions—i.e., it is almost isotropic. Astronomers are using sensitive telescopes to study the small anisotropies in the cosmic background radiation that should be present given the non-uniform distribution of galaxies.

Arcminute—A unit of angle corresponding to 1/60th of a degree. The full moon is 30 arcminutes in diameter.

Arcsecond—A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a dime viewed from a distance of 1 mile.

Array—There are two examples of arrays in common use in astronomy: (1) A group, or array, of telescopes can be combined to simulate a single large telescope, kilometers or even thousands of kilometers across.

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