Transcript: Spectroscopy is the key to chemical composition to determining what a star is actually made of. There are two issues. One is detecting the presence of an element, and the second is the amount of that element. The presence of an element is determined by measuring one or more spectral features that exactly match the wavelengths of features of the element as seen in a lab. This is the idea of a spectral fingerprint, unique to each element in the periodic table. The amount of the element depends on the strength of the lines. In general, wider and deeper absorption lines correspond to a larger abundance or more atoms of that particular element. The actual abundance needs a physical model for the star to be accurately determined.