By looking at the spectra of an object we can determine what it is made up of chemically. In the image of Betelgeuse's spectrum below, many of the dark lines come from elements in its atmosphere. (some are from our atmosphere). Certain lines indicate specific elements.
@evilscientistca did you DIY that with a DVD?
@rook no was an eschelle spectrograph at the Rothney Astrophysical Observatory.
@evilscientistca kinda thought I might be wrong because of the spacing, but I've never seen a proper one before and it looks like there's pitting in the grating... which you'd expect with a DVD.
@rook the dark lines are caused by elemental lines in the star and our atmosphere.