When the "white" or all-wavelength rad center of a star passes through its somewhat cooler outer layers, these layers absorb certain frequencies corresponding to the elements present. Therefore, dark lines are seen in the spectrum of a star, which represent the sum of the absorption spectra of the elements in its outer region. The spectrum of a given element is the same whether seen in a laboratory or in light from a distant star.

We do not need to go to a star to determine its composition; it sends its analysis to us. This is why we can speak so confidently about the atomic composition of stars. It also is evidence for the statement, universally accepted but inherently untestable by direct means, thatthe basic laws of physics are the same throughout the universe.