Intrinsic brightness of a star. The apparent visual magnitude the star would have if it were 10 pc away.
absorption line
A dark line in a spectrum. Produced by the absence of photons because they were absorbed by atoms or molecules.
absorption spectrum
A spectrum that contains absorption lines.
accretion disk
The whirling disk of gas that form around a compact object such as a white dwarf, neutron star or black hole as matter is drawn in.
active galactic nuclei (AGN)
The centers of active galaxies that are emitting large amounts of excess energy.
active galaxy
A galaxy whose center emits large amounts of excess energy, often in the from of radio emission. Active galaxies are suspected of having massive black holes in their centers into which matter is flowing.
albedo
The ratio of the light reflected from an object divided by the light that hits the object. Albedo equals 0 for perfectly black and 1 for perfectly white.
Angstrom
A unit of distance. 1 Angstrom = 10^-10 meters = 0.000,000,000,1 m. Commonly used to measure the wavelength of light. Visible light ranges from about 4000 Angstrom (blue) to to about 7000 Angstrom (red).
angular momentum
A measure of the tendency of a rotating body to continue rotating. Mathematically, the product of mass, velocity, and radius.
aphelion
The orbital point of greates distance from the Sun.
apogee
The point farthest from the Earth in a body circling the Earth.
apparent visusal magnitude
The brightness of a star as seen by human eyes.
astronomical unit (AU)
Average distance from Earth to the Sun; 150 million km, or 93 million miles.
atmospheric window
Wavelength region in which our atmosphere is transparent - at visual, infrared, and radio wavelengths.
