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absorption line spectrum A spectrum showing dark lines at some narrow color regions (wavelengths). The lines are formed by atoms absorbing light, which lifts their electrons to higher orbits.

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

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

active galactic nuclei (AGN) A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output.

angular momentum A quantity obtained by multiplying the mass of an orbiting body by its velocity and the radius of its orbit. According to the conservation laws of physics, the angular momentum of any orbiting body must remain constant at all points in the orbit, i.e., it cannot be created or destroyed. If the orbit is elliptical the radius will vary. Since the mass is constant, the velocity changes. Thus planets in elliptical orbits travel faster at perihelion and more slowly at aphelion. A spinning body also possesses spin angular momentum.

apastron The point of greatest separation between two stars which are in orbit around each other. See binary stars. Opposite of periastron.

aphelion The point in its orbit where a planet is farthest from the Sun. Opposite of perihelion.

apoapsis The point in an orbit when the two objects are farthest apart. Special names are given to this orbital point for commonly used systems: see apastron, aphelion, and apogee.

apogee The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee.

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Imagine the Universe! Dictionary - NASA