Types of Stars Astronomers classify stars by their color, which reveals the temperature at the star’s photosphere (the visible “surface”). They pass starlight through a spectroscope, an instrument that splits light into its component wavelengths (creating a spectrum). Each element that exists in the star leaves distinctive fingerprints in that spectrum in the form of dark dropouts called absorption lines. Those lines show that the temperature of the star is just right for that element to exist there. Stellar spectra are incredible tools. They tell us: How fast a star is rotating What chemical
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