How do astronomers know what elements are stars made of?

Michel van Biezen, from iLectureOnline, explains how do astronomers know what elements are in the atmospheres of exoplanets or in the atmospheres of stars. It all has to do with emission spectra and absorption spectrum. Basically, a heated and ionized gas will output light in certain wavelengths and that is called the emission spectrum. The same gas, heated, will always have the same emission spectrum anywhere in this Universe.

Now, if between a source of light and you is a cosmic gas cloud, then that given gas will always absorb given light frequencies. That is called absorption spectrum. Interestingly enough, the same gas of the same chemical element will have the same emission and absorption spectrum. And this is how you now know how to calculate what elements can be found in a star or in the atmosphere of an exoplanet.

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