The subjects of atomic physics and astronomy have developed together over centuries. The understanding of stars, galaxies, nebulae, planets, and so on relies on detailed spectroscopic analysis of the atoms they contain. There is healthy feedback between the two disciplines, with astronomical observations prompting new research in atomic physics, and developments in atomic physics and spectroscopy leading to new understanding of astrophysical processes.
It is not possible to do justice to such a broad subject in a single chapter such as this. The purpose here is to highlight some of the ways the principles developed in the book apply in the astrophysical context, and to point out where interesting differences are observed compared to lab-based experiments. The reader is referred to specialist books for a more comprehensive treatment of the subject.
The atoms in astrophysical sources are frequently found in extreme environments that are very different to those in Earth-based laboratories. The conditions inside an atomic discharge tube are usually benign compared to those found in stars. At the same time, the atom densities can be orders of magnitudes higher than those found in nebulae. All of this has an effect on the spectra that are observed. The underlying principles of atomic physics are the same, but the spectra can appear very different due to the change of the environment of the atoms. The two main differences that have to be considered relate to the temperature and the density.
The gas in an atomic discharge tube or an oven might reach a temperature of a few hundred degrees celsius. By contrast, the surface temperature of the sun is 5800 K, with the corona reaching 106 K. Other stars can be hotter. At such high temperatures, the thermal energy kBT is more than sufficient to dissociate molecules. A hydrogen cylinder on Earth will contain molecular hydrogen H2, but the temperatures in stars are sufficient to break the molecular bond and dissociate H2 into atomic hydrogen. Hence the spectra of stars are dominated by atomic hydrogen, whereas a standard hydrogen lamp will emit the spectrum of molecular hydrogen.