Absolute magnitude
Absorption band
Abundance of the chemical elements
Accelerating universe
Accretion disc
Active galactic nucleus
Active galaxy
Age of the universe
Albedo
Algol variable
Alternatives to general relativity
Andromeda Galaxy
Andromeda–Milky Way collision
Angular diameter
Angular resolution
Anisotropy
Anthropic principle
Apparent magnitude
Apsis
Astronomical naming conventions
Astronomical object
Astronomical radio source
Astronomical spectroscopy
Astronomical system of units
Astronomical unit
Astronomy
Asymptotic giant branch
Atomic spectral line