Physics · 6.2 Stars & the Universe
Stars & Universe. Expand it.
Step a star through its life cycle, then explore redshift: light from distant galaxies is stretched to longer (redder) wavelengths. The further the galaxy, the faster it recedes — Hubble's law v = H₀d — evidence for the expanding Universe and the Big Bang.
0625 Topic 6.2.1 — Stellar life cycle
6.2.2 — Redshift & Hubble's law
The Big Bang
Current stage
Stage
Main sequence star
What's happening
Hydrogen fuses to helium in the core; outward radiation pressure balances gravity (stable).
📋 Stellar life cycle (Cambridge)
- Nebula → gravity pulls gas/dust together → protostar heats up.
- Main sequence — hydrogen fuses to helium; radiation pressure balances gravity (stable, long-lived).
- A Sun-like star → red giant → planetary nebula → white dwarf.
- A massive star → red supergiant → supernova → neutron star, or a black hole if massive enough.
- Supernovae scatter heavier elements into space, forming new nebulae.
📋 Redshift & the Big Bang
- Light from distant galaxies is shifted to longer wavelengths (redshift) — they are moving away from us.
- Hubble's law: recession speed ∝ distance, v = H₀d. More distant galaxies recede faster.
- This shows the Universe is expanding, supporting the Big Bang theory — everything began from a single point and has been expanding ever since.
- The Hubble constant H₀ links to the estimated age of the Universe.
🎯 Syllabus reference (0625)
- 6.2.1 — describe the role of gravity in star formation; describe the life cycle of a star (Sun-like and massive); recall nuclear fusion as the Sun's energy source.
- 6.2.2 — describe redshift of light from distant galaxies; recall and use Hubble's law v = H₀d; explain redshift as evidence for an expanding Universe and the Big Bang.