Not all the great stuff astronomers get the from sky is in the form of pretty pictures. Granted, gorgeous astrophotos are addictive, but they don’t tell the whole story of the universe. Astronomers also study data in the form of spectra. The figure above is a good example of a spectrum. Basically it tells astronomers that a star called G271-162 has a certain amount of an element called “lithium” — which is relatively rare in the cosmos compared to other elements. This star is what is known as a “metal-poor” star — one that formed in the earliest times of the universe. “Metal-rich” stars are those formed from interstellar gas and dust that has probably been “recycled” through at least one star and enriched with metals. So, if we study older stars like G271-162 and figure out how much they have of certain elements, that will tell us a lot about what elements were most plentiful in the early, early universe. Astronomers want to understand how much lithium was produced in the birth of the universe — the Big Bang — some 12 to 14 billion years ago. The amount of lithium older star will help them understand it.
You can’t take a picture of lithium, but you can study the light coming from a star — and break it up into a spectrum. If lithium is present in the star, it will show as a “dip” in the spectral line — which is exactly what you see in this graph.