Ablation;Melting of the surface of a body resulting from friction with the atmosphere of the Earth.
Absolute magnitude;Measurement for the intrinsic brightness of a star.
Absolute temperature scale;Temperature scale where zero is the absolute zero temperature.
Absolute zero temperature;Temperature of 0 K, equivalent to minus 273.16° Celsius.
Accretion;Increase in mass due to gravitational forces that attract additional matter.
Albedo;Rate of reflectivity of a non-luminescent body. If all the light received is reflected, the Albedo is 1.
Altitude of stars;Angular distance of a star from the horizon. The maximum distance is 90°, when the star is at its zenith.
Antimatter;Elementary particles with mirrored electric charge.
Aperture;Diameter of the opening through which light enters an optical system.
Aphelion;The point of an orbit of a planet or comet at which it is farthest from the Sun.
Apollo missions;US space program including 6 manned lunar expeditions between 1969 and 1972.
Arc minute;1/60 of a degree or 1/21,600 of a full circle.
Arc second;1/3600 of a degree or less than a millionth of a full circle.
Asteroid;Rocky object smaller than a planet that orbits the Sun.
Asteroid belt;Area located between the orbits of Mars and Jupiter where numerous asteroids are found.
Asteroids;Most asteroids orbit the Sun in a belt between the orbits of Mars and Jupiter.
Astrology;Attempt to predict man's actions and events from the constellation of celestial bodies.
Astronomical Unit;Mean distance from the Earth to the Sun (approx. 150 million kilometers).
Astronomy;Science dealing with all celestial bodies outside the Earth's atmosphere.
Atmosphere;Mixture of gases surrounding a celestial body.
Atom;Smallest particle of a chemical element (consisting of nucleus and electron shell).
Atomic nucleus;Body at the center of an atom with positive charge, accounting for most of the atom's mass.
AU;Abbreviation for astronomical unit, defined as the mean distance between the Sun and the Earth.
Axis;Imaginary line around which bodies (e.g. celestial objects) rotate.
Azimuth;Number of degrees clockwise from due south to the object's vertical circle.
Background radiation;Cosmic low-temperature microwave radiation that cannot be traced back to a specific source.
Barred spiral galaxy;Spiral galaxy containing a bar of dense matter at its center.
Big Bang;Hypothetical event at the beginning of the universe.
Big Crunch;Hypothetical event ending the existence of the universe.
Binary star system;System of two stars orbiting a common center of gravity.
Binary stars;System of two stars orbiting a common center of gravity.
Black body;Object that absorbs all electromagnetic radiation including light.
Black dwarf;Former white dwarf that has ceased to emit energy.
Black hole;Extremely dense body that has compacted due to its own gravitational forces to such an extent that nothing, not even light, can escape from it.
Brightness;Total light or other radiation emitted per second by a star.
Brown dwarf;Small star-like object of little mass, where no hydrogen fusion takes place at its core.