Stellar Atmosphere Handout
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.
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.
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).