absolute magnitude The apparent magnitude that a star would have if it were located at a distance of 10 parsecs (32.6 light-years).

absolute zero The temperature at which all random molecular motion ceases, and hence the lowest possible temperature: OK (-273° C).

absorption line A dark spectral line corresponding to radiation of one particular wavelength that is absorbed when electrons jump up from a lower to a higher energy level within atoms or ions of a particular element.

absorption line spectrum Dark (absorption) lines superimposed on a continuous spectrum as a consequence of the absorption of those particular wavelengths by atoms, ions, or molecules.

acceleration The rate of change of velocity. Acceleration may take the form of a change in speed, a change in direction, or a change in both.

accretion The gradual accumulation of matter in a particular location under the action of gravity. The colliding and sticking together of small particles to make progressively larger ones.

accretion disc A disc of gas around a star or a compact massive object such as a white dwarf, neutron star, or black hole that has been accreted from a neighboring star or from its surroundings.

achromatic lens A lens consisting of two components, made of different types of glass, that reduces the effects of chromatic aberration.

active galactic nucleus (AGN) The compact core of an active galaxy. An AGN is normally highly luminous and in many cases varies markedly in brightness.