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absolute brightness The apparent brightness a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absolute zero The lowest point on any temperature scale, the temperature at which all (non-quantum mechanical) motion ceases; hence absolute zero occurs at zero degrees in the Kelvin scale, -273 degrees on the centigrade (Celsius) scale and -459.7 degrees on the Fahrenheit scale.
absorption line Dark line in an otherwise continuous bright spectrum, where light within one narrow frequency range has been moved. A region of the electromagnetic spectrum, limited in frequency and wavelength, from which electromagnetic radiation has been removed, so that the intensity of the radiation falls below that of the neighboring spectral regions.
absorption, X-ray X-ray photons - tiny high-energy packets of electromagnetic radiation - are absorbed by encounters with individual atoms in the Earth's atmosphere, and to a lesser extent, in interstellar space, or around the source of X-rays. [More Info: Field Guide]
abundances of the elements The amounts of each chemical element relative to hydrogen in the universe, or a star or in some particular object.
acceleration The rate of change of velocity of a moving object.
accretion disk A disk of gas and dust that can accumulate around a center of gravitational attraction, such as a normal star, a white dwarf, neutron star, or black hole. As the gas spirals in due to friction, it becomes hot and emits radiation.
Active Galactic Nucleus (AGN) The central region of a galaxy that shows unusual energetic activity. [More Info: Field Guide]
active galaxy A galaxy which shows explosive activity and can emit large amounts of energy, especially from its central regions.
Advanced X-ray Astrophysics Facility (AXAF) Renamed the Chandra X-ray Observatory and launched on July 23, 1999, the largest and most sophisticated X-ray observatory to date. [More Info]
angular momentum The momentum of rotation about a fixed axis.
angular resolution The ability of a telescope to distinguish two adjacent objects on the sky, or to study the fine details on the surface of some object; often synonymous with "clarity" or "sharpness."
angular size The apparent size of an object, measured by the angle the object subtends, in degrees of arc, minutes of arc, or seconds of arc. [More Info: Photo Album]
antimatter Matter made of antiparticles, i.e., particles with identical mass and spin as those of ordinary particles, but with opposite charge and magnetic properties. When a particle and its antiparticle meet they can mutually annihilate and produce energy.
apparent brightness The brightness of an object as it naturally appears in the sky.
arc degree A unit of angular measure of which there are 360 in a full circle.
astronomy Branch of science dedicated to the study of everything in the universe that lies above Earth's atmosphere.
atmosphere A layer of gas confined close to a planet's surface by the force of gravity.
atom Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.