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LearnAQA7408First teaching 2015Paper 3B · Option: AstrophysicsClassification of stars

Paper 3B · Module

Classification of stars

2 subtopics≈ 80 min in total

Start the module: Stars and the HR diagram Open graphing calculator

Stars and the HR diagram
The Hertzsprung–Russell diagram plots luminosity against surface temperature (hot on the left). Most stars lie on the main sequence; red giants and supergiants are top right; white dwarfs bottom left. Nearby distances come from parallax: $d$ (parsecs) $=\frac1p$ (arcseconds). AQA also uses magnitudes: $m-M=5\log\frac d{10}$ .
≈ 45 min
Stellar radii
Stars radiate approximately as black bodies. Wien's displacement law: the peak wavelength is inversely proportional to the surface temperature, $\lambda_{\max}T=2.9\times10^{-3}$ m K, so hotter stars look bluer. The Stefan–Boltzmann law gives the luminosity: $L=4\pi\sigma r^2T^4$ , with $\sigma=5.67\times10^{-8}$ W m⁻² K⁻⁴. Knowing $T$ (from Wien) and $L$ (from a standard candle method), you can find a star's radius.
≈ 35 min