Expert: Philip A. Stahl - 11/15/2006

Question
How are the hydrogen absorption lines formed in a star like the Sun?

Answer
Differences in stellar spectra are actually due mostly to the widely varying temperatures in the outer layers of various stars. Hydrogen, for instance, is by far the most abundant element in all stars except the most advanced in stellar evolution.

In the atmospheres of the hottest stars, hydrogen is almost completely ionized so can produce no absorption lines. In the coolest stars, by contrast, hydrogen is neutral so can produce absorption lines.  Since practically all the hydrogen exists in the lowest energy state (e.g. n=1 or unexcited) they can absorb only those photons that can lift them to the first energy level to higher ones.

:Photons so absorbed in this way produce the Lyman series of absorption lines. These lie almost entirely in the ultraviolet part of the spectrum.

In a stellar atmosphere with a temperature of only about 10,000 K many H –atoms are not ionized, but a significant number can be excited to the second energy level (n = 2) – from which they can absorb additional photons and rise to still higher levels of excitation (n= 3, 4, 5 etc.)

These photons correspond to the wavelengths of the Balmer series, which lies in the part of the spectrum readily observable.

Absorption lines due to hydrogen, therefore, are stongest in the spectra of stars whose temperature ~  10,000 K. They’re less conspicuous in the spectra of both much hotter and cooler stars. (Even though hydrogen is equally abundant in all the stars)

Hope this helps!