What stars are made of

In order to find out what a star is made of, pass its light through a prism, and examine the resulting rainbow, or spectrum.

Here’s an article that includes an app that will let you play with the effect of different chemicals on a spectrum.

Absorption/emission lines

And here’s the history, with names and dates, on how we figured out this particular trick.

Spectral Lines and the History of Spectroscopy