Ablation;0; 
Absolute magnitude;101; 
Absolute temperature scale;173; 
Absolute zero temperature;264; 
Accretion;348; 
Albedo;436; 
Altitude of stars;549; 
Antimatter;674; 
Aperture;738; 
Aphelion;818; 
Apollo missions;909; 
Arc minute;1003; 
Arc second;1062; 
Asteroid belt;1202; 
Asteroid;1136; 
Asteroids;1305; 
Astrology;1395; 
Astronomical Unit;1494; 
Astronomy;1587; 
Atmosphere;1672; 
Atom;1731; 
Atomic nucleus;1821; 
AU;1929; 
Axis;2029; 
Azimuth;2103; 
Background radiation;2188; 
Barred spiral galaxy;2302; 
Big Bang;2386; 
Big Crunch;2449; 
Binary star system;2518; 
Binary stars;2595; 
Black body;2666; 
Black dwarf;2745; 
Black hole;2809; 
Brightness;2965; 
Brown dwarf;3038;