Astrophysics and Cosmology: Stars, Galaxies and the Evolving Universe
Light carries evidence across space. By combining spectra, physical laws and distance measurements, we reconstruct the lives of stars and the history of the universe.
Learning goals
- extract temperature, composition and motion from starlight;
- connect luminosity, flux and distance;
- interpret the Hertzsprung-Russell diagram and mass-dependent stellar evolution;
- use redshift and Hubble's law;
- evaluate the main evidence for an evolving universe.
1. Reading light from distant objects
A spectrum separates radiation by wavelength. A hot dense object produces an approximately continuous thermal spectrum; cooler gas in front of it adds absorption lines, while excited thin gas produces emission lines. Each element has a characteristic line pattern because electron energy levels are quantized. Line strengths and shapes also reveal temperature, density and magnetic conditions.
The peak wavelength of an approximate blackbody spectrum follows Wien's law, $\lambda_{\max}T=b$: a shorter peak wavelength means a higher surface temperature. Motion along the line of sight shifts spectral lines. For small recession speeds, redshift $z=(\lambda_{\mathrm{obs}}-\lambda_{\mathrm{emit}})/\lambda_{\mathrm{emit}}\approx v/c$. A redshift is a fractional wavelength change, not simply the colour seen by the eye.