- accretion, 1
- Adams, Douglas, 1
- aliens see extraterrestrials
- alpha decay, 1
- alpha particles, 1, 2
- triple-alpha process, 1, 2
- Alpher, Ralph, 1
- aluminium, 1
- Anaxagoras, 1
- Andromeda, 1
- angular momentum, conservation of, 1, 2
- anthropic principle, 1
- antimatter, 1, 2
- antiparticles, 1, 2
- Aspect, Alain, 1
- Aston, Francis, 1, 2
- Atkinson, Robert, 1
- atomic bombs see nuclear bombs
- atoms
- composition and structure, 1
- and compound formation, 1
- etymology, 1
- initial formation, 1
- introduction to, 1
- and light, 1
- nuclei, 1, 2, 3
- proving existence of, 1, 2