Astrophysics

Astrophysics applies the laws of physics to the study of celestial objects and the universe as a whole. This optional topic covers stellar classification, the life cycle of stars, cosmological models, and observational techniques used by astronomers.

Topics Covered

Stellar Classification

• Luminosity and brightness — apparent magnitude vs. absolute magnitude; the inverse square law $b = \frac{L}{4\pi d^2}$
• Stellar spectra — absorption spectra, spectral classes (OBAFGKM); surface temperature classification
• The Hertzsprung-Russell diagram — main sequence, red giants, white dwarfs, supergiants; luminosity vs. temperature
• Stellar parallax — measuring distance using $d = \frac{1}{p}$ (parsecs); limitations for distant stars

Stellar Evolution

• Protostars — gravitational collapse of nebulae; the Jeans criterion
• Main sequence — hydrogen fusion, hydrostatic equilibrium; lifetime $\propto \frac{M}{L}$
• Post-main-sequence — red giants, planetary nebulae, white dwarfs, neutron stars, black holes; dependence on initial mass
• Supernovae — Type Ia (white dwarf in binary system) and Type II (massive star core collapse); standard candles

Cosmology

• Olbers’ paradox — why the night sky is dark; evidence for a non-static, finite-age universe
• Hubble’s law — $v = H_0 d$; expansion of the universe; estimating the age $t \approx \frac{1}{H_0}$
• Cosmic microwave background radiation — evidence for the Big Bang; black-body spectrum at 2.7 K
• Dark matter and dark energy — observational evidence; galaxy rotation curves, accelerating expansion
• The fate of the universe — open, closed, and flat models; critical density $\rho_c = \frac{3H_0^2}{8\pi G}$

Observational Astronomy

• Telescopes — refracting vs. reflecting; resolving power and collecting power
• Detection across the electromagnetic spectrum — radio, infrared, X-ray, gamma-ray astronomy
• Satellite observatories — advantages of space-based telescopes (no atmospheric distortion)

Study Tips

1. Learn the HR diagram cold — be able to sketch it, label the regions, and explain why stars occupy different regions based on their properties.
2. Trace stellar evolution — for a given initial mass, describe the full lifecycle from protostar to end state. Know the mass thresholds that determine the path.
3. Understand the evidence — for every cosmological claim, know the observational evidence: CMB for the Big Bang, redshift for expansion, rotation curves for dark matter.
4. Practise luminosity-distance calculations — the inverse square law and Hubble’s law calculations appear frequently.
5. Link to other physics topics — gravitational fields (orbital mechanics), thermal physics (black-body radiation), nuclear physics (fusion processes in stars).

How to Use These Notes

Follow the sidebar order. Each page provides key physics principles, derivations, worked examples, and exam-style problems.