Spectroscopy is the technique we use to observe stars in order to figure out their chemical composition.

Noticing what happens when light comes through a little water droplet and it gets split up into rainbow colors, we do the same thing with a spectrograph mounted at a telescope. What we actually see is less than the rainbow, because there are certain colors of the rainbow missing. And the missing parts contain all the information that we want.

When photons escape from the core, they pass through this outer layer. In the outer layer, we have hydrogen and helium atoms, because that’s what the stars are mostly made of. There will be other atoms of iron, magnesium, carbon, and oxygen. Each element absorbs photons with their very specific energy or wavelength that is equivalent and lets some other photons out.

By measuring the spectra or the line strength, we know about the information of star formation. As the figure shows, the strength corresponds to the abundance of magnesium atoms in the outer atmosphere.

If there are only a little calcium, magnesium, and sodium actually present in the star, which means that the star must have formed at a really early time when the cycle of chemical enrichment had only gone around a few times. These stars must be metal-poor or among the oldest, resulting in spectra displaying very weak lines.