Stars, galaxies and the Big Bang · Grade 8
Stars are born in clouds of dust and gas, shine for billions of years by fusing hydrogen into helium, and end as white dwarfs, neutron stars or black holes depending on their mass. They gather in galaxies – ours is the Milky Way – and the light from distant galaxies is red-shifted, showing the universe is expanding from a hot, dense beginning: the Big Bang.
Key facts
- Life cycle (Sun-like star): nebula → protostar → main sequence → red giant → white dwarf.
- Massive star: … → red supergiant → supernova → neutron star or black hole.
- A main-sequence star is stable because outward pressure from fusion balances gravity.
- A light-year is a distance: about 9.46 × 10¹² km.
- Red shift z = Δλ ÷ λ, and v = zc. Hubble’s law v = H₀d; the age of the universe ≈ 1 ÷ H₀.
- Evidence for the Big Bang: red shift of galaxies and the cosmic microwave background (CMB). Wien’s law: λ_max = 2.9 × 10⁻³ ÷ T.
Worked examples
Example 1
A galaxy’s hydrogen line (656 nm) is observed at 669 nm. How fast is it receding? (c = 3.0 × 10⁵ km/s)
Answer: about 5,900 km/s
Example 2
Using H₀ = 70 km/s per Mpc, how far away is a galaxy receding at 14,000 km/s? Roughly how long ago did its light leave?
Answer: 200 Mpc; about 650 million years