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The compositions of stars are determined through spectroscopy. Spectroscopy is the study of something using spectra. Recall from the Electromagnetic Radiation chapter that a spectrum is what results when you spread starlight out into its individual colors. By noting what absorption lines (or sometimes, emission lines) are present and their strengths, you can find out a tremendous amount of information. Stars have absorption lines patterns similar to the Sun. This means that they are composed mostly of hydrogen and helium with traces of other elements.
From these absorption lines you learn some important things besides the star's composition:
Structure of stars: From the simple fact that you see absorption lines in most stellar spectra, you know that the stars must have a hot dense part that produces a continuous spectrum and an outer layer, or atmosphere, made of cooler, low density gas. The general trend is density and temperature of stars decreases as the distance from the star's center increases. The hot dense part is also gaseous because of the extreme temperatures. Stars have no molten rock in them like the interiors of some of the planets.
The next two things have already been noted elsewhere but they are important enough to state again.
Universality of physical laws: The same pattern of hydrogen lines are seen in the in spectra of the Sun, stars, distant galaxies, and quasars (active galaxies at very great distances from us). This is a sensitive test of whether or not the laws of physics used in the structure of atoms works everywhere in the universe. Even slight differences in the rules of quantum mechanics that govern the interactions of the protons, electrons, and neutrons or differences in the strengths of the fundamental forces of natures from that observed on the Earth would produce noticeable changes in the spacing and strength of the spectral lines. If the subatomic particles had different amount of charge or mass, the pattern of lines would be different than what you see on the Earth.
Because the same patterns are seen in the spectra, regardless of where the light comes from, the physics used on Earth must work everywhere else in the universe! All of the absorption lines seen in celestial objects can be seen in laboratories on Earth. The charge and mass of the electron and proton are the same everywhere you look. Physical laws are the same everywhere!
Permanence of physical laws: Since light has a finite speed and the distances are vast, the light received from very distant galaxies and quasars has been travelling for billions of years. The light from those remote regions tells us about the physical laws way back then. The spectra seen can be explained with the same physical laws in operation here on Earth at the present time. Physical laws are the same throughout time!
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last updated: June 6, 2019