A ring One of three Saturnian rings visible from Earth. The A ring is farthest from the planet and is separated from the B ring by the Cassini division.

absolute brightness The apparent brightness a star would have if it were placed at a standard distance of 10 parsecs from Earth.

absolute magnitude The apparent magnitude a star would have if it were placed at a standard distance of 10 parsecs from Earth.

absorption line Dark line in an otherwise continuous bright spectrum, where light within one narrow frequency range has been removed.

acceleration The rate of change of velocity of a moving object.

accretion Gradual growth of bodies, such as stars or planets, by the accumulation of gas or other, smaller, bodies.

accretion disk Flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary system.

active galaxies The most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way.

active optics Collection of techniques now being used to increase the resolution of ground-based telescopes. Minute modifications are made to the overall configuration of an instrument as its temperature and orientation change, to maintain the best possible focus at all times.

active region Region of the photosphere of the Sun surrounding a sunspot group, which can erupt violently and unpredictably. During sunspot maximum, the number of active regions is also a maximum.

active Sun The unpredictable aspects of the Sun's behavior, such as sudden explosive outbursts of radiation in the form of prominences and flares.

adaptive optics Technique used to increase the resolution of a telescope by deforming the shape of the mirror's surface under computer control while a measurement is being taken, to undo the effects of atmospheric turbulence.

amino acids Organic molecules which form the basis for building the proteins that direct metabolism in living creatures.

amplitude The maximum deviation of a wave above or below the zero point.

angular momentum problem The fact that the Sun, which contains nearly all of the mass of the solar system, accounts for just 0.3 percent of the total angular momentum of the solar system. This is an aspect of the solar system that any acceptable formation theory must address.

angular resolution The ability of a telescope to distinguish between adjacent objects in the sky.

annular eclipse annular eclipse Solar eclipse occurring at a time when the Moon is far enough away from the Earth that it fails to cover the disk of the Sun completely, leaving a ring of sunlight visible around its edge.

aphelion The point on the elliptical path of an object in orbit about the Sun that is most distant from the Sun

Apollo asteroid See Earth-crossing asteroid.

apparent brightness The brightness that a star appears to have, as measured by an observer on Earth.

apparent magnitude The apparent brightness of a star, expressed using the magnitude scale.

arc degree Unit of angular measure. There are 360 arc degrees in one complete circle.

association Small grouping of (typically 100 or less) stars, spanning up to a few tens of parsecs across, usually rich in very young stars.

asteroid One of thousands of very small members of the solar system orbiting the Sun between the orbits of Mars and Jupiter. Asteroids are often referred to as "minor planets."

asteroid belt Region of the solar system, between the orbits of Mars and Jupiter, in which most asteroids are found.

asthenosphere Layer of Earth's interior, just below the lithosphere, over which the surface plates slide.

astronomical unit (A.U.) The average distance of Earth from the Sun. Precise radar measurements yield a value for the A.U. of 149,603,500 km.

astronomy Branch of science dedicated to the study of everything in the universe that lies above Earth's atmosphere.

asymptotic giant branch Path on the H-R diagram corresponding to the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time.

atmosphere 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.

aurora Event which occurs when atmospheric molecules are excited by incoming charged particles from the solar wind, then emit energy as they fall back to their ground states. Aurorae generally occur at high latitudes, near the north and south magnetic poles.

autumnal equinox Date on which the Sun crosses the celestial equator moving southward, occurring on or near September 22.