The standard explanation for this is that the atoms in the atmosphere of the star absorb some of the light which is emitted, and each element absorbs only some distinct, characteristic wavelengths of light. (talkorigins.org)
Since different elements and compounds absorb light at characteristic wavelengths, astronomers can determine what chemicals make up a planet's atmosphere by measuring the relative amounts of light that come in at particular wavelengths. (discovermagazine.com)
Spectrometers can be tuned to detect the wavelengths at which scientists know a given element emits and absorbs light; scientists can then determine the element's presence by whether it emits or absorbs light of certain characteristic wavelengths. (keckobservatory.org)