Very large absorption lines in stellar spectrum I was puzzled by the wide absorption lines in a stellar spectrum I found. The following is what I expect absorption lines to look like - thin, crisp lines:

However, I found this stellar spectrum, which as large gaps, most notably from 6600 to 7100 angstroms:

Why are the absorption lines so wide on the second spectrum? Are they even absorption lines at all, or another phenomenon altogether?
 A: These are spectra from the Sloan Digital Sky Survey (SDSS). The first spectrum is that of a star hotter than Sun. There are prominent, but not particularly strong, hydrogen lines. Note the sodium line, the G band, and the H and K calcium lines. The second spectrum is that of a star significantly cooler than Sun. The wide bands you see are molecular bands, probably TiO (titanium oxide) and similar metallic species. These molecular bands are the signature of cool, red stars. Note the very strong sodium line and the prominent lines of singly ionized calcium in the infrared.
TiO is a molecule, not an atom. Molecules have different spectra than atoms have. Molecules can vibrate and rotate, both of which create spectral lines. However, these vibrational and rotational spectral features are going while the atoms in the molecules have their own characteristic spectral lines. All this going on together causes the various spectral features to blend into bands rather than in tight, discrete lines.
If you go to skyserver.sdss.org you'll find many classroom activities, also suitable for personal use, on how to interpret these spectra, how we classify them, and how to analyze them and use them for real research.
