- absolute zero, 1
- Adams, Walter, 1
- Alpha Centauri, 1
- Alpher, Ralph, 1, 2, 3, 4, 5, 6
- Alvarez, Luis, 1
- American Astronomical Society, 1, 2
- American Physical Society, 1
- amplifiers, 1, 2
- Andromeda: Great Nebula (M31), 1
- Antarctica, 1, 2
- antennas, 1
- Arctic, 1
- Associated Press, 1
- atmosphere: thickness, 1
- atoms
- composition, 1
- formation, 1
- nuclei, 1
- splitting, 1
- axis of evil, 1