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How can absorption spectra reveal molecules between Earth and a distant star?

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Absorption spectra allow astronomers to identify molecules located in the space between Earth and a distant star by analyzing the light that reaches us from that star.12

When light from a background star passes through the interstellar medium—which contains gas and dust—the atoms and molecules present in that medium absorb specific wavelengths of the star's light. Because each chemical species has a unique set of energy levels, these particles absorb only the specific wavelengths that correspond to the energy required to excite them from one quantum state to another.123

As a result, these absorbed wavelengths appear as dark lines in the star's spectrum. By observing these missing wavelengths, astronomers can identify the specific molecules present, as well as measure their density and velocity. This method is effective because the absorption lines produced by interstellar gas are typically distinct from the spectral lines originating from the star itself, allowing researchers to isolate and study the material located along the line of sight.21