Astrophysics & Cosmology is a core part of A-Level Physics. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Light-yearThe distance light travels in one year (~9.5×10¹⁵ m).
Orbital speedv = 2πr ÷ T for a body in a circular orbit.
Main sequence starA stable star fusing hydrogen to helium, gravity balanced by radiation pressure.
Red giant & white dwarfLate stages of a low-mass star's life.
SupernovaThe explosive death of a massive star, leaving a neutron star or black hole.
Red-shiftLight from distant galaxies stretched to longer wavelengths — the universe is expanding.
Hubble's lawv = H₀ d for distant galaxies.
Hubble constantH₀ ≈ 70 km/s/Mpc (modern value).
Megaparsec (Mpc)10⁶ pc ≈ 3.09 × 10²² m.
Age of universe≈ 1 / H₀ assuming constant expansion.
Recession velocityFound from redshift: v = cz.
Standard candleKnown L source for distance, e.g. type Ia SN.
Big BangHot, dense early universe expanding outwards.
CMBCosmic microwave background, T ≈ 2.7 K.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
Does Hooke's law apply to a spring for any amount of stretching?✗ Yes — Hooke's law applies to any spring at any extension. ✓ No — it only holds up to the limit of proportionality; beyond that the relationship between force and extension is no longer linear.
Why do brighter-looking stars not necessarily have higher luminosity?✗ Brighter-looking stars are always intrinsically more luminous. ✓ Apparent brightness depends on both luminosity and distance — a closer dim star can look brighter than a distant luminous one.
When you push a wall, what does Newton's third law say about the wall?✗ The wall only pushes back if your force is strong enough to make it react. ✓ The wall exerts an equal and opposite force on you at the same instant.
Why do more massive stars have shorter main-sequence lifetimes than smaller ones?✗ All stars live the same time on the main sequence regardless of mass. ✓ They burn through their hydrogen fuel much faster because higher core temperature drives nuclear fusion at a vastly greater rate.
Do all stars end their lives as supernovae?✗ Yes — every star eventually explodes. ✓ No — only sufficiently massive stars (above about 8 solar masses) explode as supernovae; lower-mass stars become white dwarfs.
Practise Astrophysics & Cosmology — free games
Test yourself with these quick revision games for this topic:
See all 17 games in the Subjects Arcade →
More A-Level Physics topics
← All revision guides
Want to revise every topic this smart?
The Velvet Method teaches you to use AI to revise any subject — £25, lifetime access.
Explore the Course →