Astrophysics

Astrophysics applies the principles of physics to understand the universe — from individual stars to the large-scale structure and evolution of the cosmos. This optional topic (AQA Option D, OCR Module 5.2) covers astronomical distances, stellar classification and evolution, the Hertzsprung-Russell diagram, cosmology, and the evidence for the Big Bang. Astrophysics draws together ideas from almost every other area of the course — thermal physics, waves, nuclear physics, and gravity all play essential roles.

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Astronomical Distances

Distances in astronomy are so vast that everyday units are impractical. Three key units are used:

Key Definition: An astronomical unit (AU) is the mean distance from the Earth to the Sun: 1 AU ≈ 1.496 × 10¹¹ m. It is useful for distances within the solar system.

Key Definition: A light-year (ly) is the distance that light travels in one year: 1 ly ≈ 9.46 × 10¹⁵ m. It is useful for distances to nearby stars and within galaxies.

Key Definition: A parsec (pc) is the distance at which a star has a parallax angle of one arcsecond: 1 pc ≈ 3.086 × 10¹⁶ m ≈ 3.26 ly. It is the preferred unit in professional astronomy.