Stellar Atmosphere Handout

Stellar Spectra
Each dark line in a stellar spectrum is due to the presence of a particular chemical element in the atmosphere of the star observed. You might guess, therefore, that stellar spectra differ from one another because of differences in the chemical makeup of stars. Detailed analysis shows, however, that most stars have very similar compositions. The primary cause of the differences in stellar spectra is that stars have very different temperatures in their outer layers.

Absorption Lines
Hydrogen is by far the most abundant element in the outer layers of all stars, except probably in those at advanced stages of evolution. In the atmospheres of the very hottest stars, however, hydrogen is completely ionized. Since the electron and proton are separated, ionized hydrogen can produce no absorption lines. In the atmospheres of the coolest stars, hydrogen is neutral and can produce absorption lines. At these low temperatures, the photons absorbed produce absorption lines which lie in the ultraviolet part of the spectrum and are unobservable from the ground. In a stellar atmosphere of about 10,000K, some hydrogen atoms are excited to a higher energy level, and produce absorption lines in the optical part of the spectrum. Absorption lines due to hydrogen are strongest in the spectra of the stars whose atmospheres are near 10,000K, and they are less noticeable in the spectra of both hotter and cooler stars, even though these stars have about the same amount of hydrogen. Similarly, every other chemical element, in each of its possible stages of ionization, has a characteristic temperature at which it is most effective in producing absorption lines. Looking at the absorption lines enables us to determine a star's temperature.

Emission Lines
Sometimes material is ejected from a star. If enough material is ejected, and if the star has radiation of high enough energy to excite or ionize the gas, emission lines are produced. The material ejected ionizes the local interstellar cloud. As the electrons decrease in energy, back to their original states, they emit light. We "see" this light in the form of emission lines.

What do we learn from stellar spectra?
The following can be discovered by studying the spectra of a star: temperature, pressure in the star's atmosphere, chemical composition, rotation of the star, turbulence in the atmosphere, magnetic fields, and ejected gases (determined by emission lines).