 Early in the nineteenth century, German chemist Joseph Fraunhofer invented the spectroscope, an instrument to automatically separate light and mark the wavelengths. In so doing he discovered that when he spread sunlight into a spectrum, the spectrum was crossed by a great number of fine dark lines. He had no idea what these dark lines were, but today we know that they were the key to learning what stars are made of. Remember the red and green light of the aurora borealis and the structure of molecules we discussed in our segment on the heliosphere? The aurora is a good example of light being emitted as electrons change energy levels. But for our purposes here we want to examine what happens when light from the center of a star passes through the gases in the outer layers on its way to us. Here's how it works. When a photon with exactly the right energy level hits an electron orbiting a nucleus, its entire energy is transformed to the electron which jumps to a higher energy level with a larger quantum number. The photon is eliminated. This creates absorption lines in a star's spectrum as light from the star travels through the star's atmosphere. Every atom and molecule has its own spectral line signature. So by observing the absorption lines in a star's spectrum we can tell what elements are present. When scientists discovered connections between groups of spectral lines and star temperatures, they developed a set of spectral classifications to highlight star content.