Suppose you want to know the chemical composition of a distant star. which piece of information is most useful to you?

The electromagnetic radiation spectrum, including visible light that radiates from stars is a very useful element if we want to know the chemical composition of stars.

It should be noted that the spectrum of a chemical element is obtained when the light it emits is decomposed in the colors (wavelengths) that makes it up, passing it through a scattering element (a prism or a diffraction grating, for example).

In this sense, the spectra of the stars are like their fingerprints, comparing them with the spectra of the known chemical elements we can determine their composition.

Another important point is that the spectrum of the stars is of thermal origin and, in most of them, it is constituted by a bright background,  on which absorption lines (dark lines) are superimposed and, as well as, emission lines (bright lines), which are emitted by the elements and chemical compounds that constitute the emitting matter, that is, the stellar atmosphere.

Then, we have two types of spectra:

-The emission spectrum, which is also called the atomic emission spectrum and is a set of frequencies of electromagnetic waves emitted by atoms of that element in a gaseous state.

-The absorption spectrum, which shows the fraction of the incident electromagnetic radiation that a material absorbs within a range of frequencies and it is the opposite of the emission spectrum.

Therefore, each chemical element has absorption lines at some wavelengths, as well as, emission lines. When two stars have similar spectra they also present common physical properties. This makes easier the development of a classification system based on the appearance of the spectra.