A stamp from Germany, showing the solar spectrum (the rainbow) and the dark absorption lines. These were studied by Joseph von Fraunhofer (6 March 1787 – 7 June 1826).
According to Wikipedia, "In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun. ... About 45 years later Kirchhoff and Bunsen noticed that several Fraunhofer lines coincide with characteristic emission lines identified in the spectra of heated elements. It was correctly deduced that dark lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere."
Essentially, when there is a ball of hot gas (the Sun) giving off light, and then an area with cool (relatively) gases (hydrogen, oxygen, etc.) the cool gases absorb certain wavelengths of light. This happens in the outer atmosphere in the Sun. Lab experiments can help identify these elements. For example, the thicker line in the red is associated with hydrogen, and the two lines very close together in the yellow are associated with sodium.
So we can conclude: these elements seen on earth are also in the Sun, and when these lines are seen in the spectra of other stars, we know these elements exist in other stars.
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