D.2a Stellar spectra (introduction)

Stellar spectra

In the atomic physics topic we looked at how the electrons in the shells of atoms are able to be excited by the absorption of electromagnetic radiation with frequencies / wavelengths equivalent to the exact energy difference between two energy levels, and then re-emit that radiation in all directions when the electron drops to a lower energy level. If we observe the gas directly then we may see an emission spectrum of bright lines corresponding to the energy levels in the sample element. If we observe a broad spectrum light source shining through the gas then we will see an absorption spectrum with dark lines at those same wavelengths.

When we look at stars we see broad spectrum electromagnetic radiation (described as a black body spectrum) emitted from the surface of the star (the photosphere) with the absorption spectrum of the stars atmosphere superimposed. This combination as known as the stellar spectrum.