Radiation laws and spectral methods for studying the physical nature of celestial bodies
The radiation of hot bodies depends on temperature. For a black body Wien’s law gives the wavelength of maximum emission λ_max · T = 2.9 · 10⁶ nm · K, so hotter stars shine brightest at shorter wavelengths. The Stefan–Boltzmann law: the power radiated from each square metre of surface is E = σT⁴ (σ = 5.67 · 10⁻⁸ W/m²·K⁴), and the luminosity of a whole star is L = 4πR² σT⁴. Spectra are of three kinds: continuous (a hot dense body), emission-line (a hot rarefied gas) and absorption (a cooler gas in front of a continuous source) – Kirchhoff’s laws. The positions of the lines reveal chemical elements: helium was first found in the Sun’s spectrum in 1868, then on the Earth. If a source moves along the line of sight with speed v the lines are shifted: Δλ / λ = v / c (the Doppler effect); a receding source shifts to the red.
Shine the light of a lamp (not the Sun) on the surface of a CD and watch it split into a coloured spectrum; write down the order of red, green and violet. Do not look at the Sun.