For astronomers interested in the formation and evolution of galaxies, chemistry usually means measuring spectroscopically the abundances of elements relative to hydrogen in the stars and in the gas of galaxies. Although a few of the lightest elements (He, D, Li, Be, B) were produced in the Big Bang itself, most of the heavier elements are produced on different timescales by nuclear processes in different kinds of stars, so we can use element abundances to work out what happened as the galaxies were built up. We derive the element abundances in stars by using stellar spectroscopy to measure the strengths of the absorption lines in the spectra of starlight.
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