 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 this connection. Every star we have seen so far fits into one of these classifications. Our sun is spectral class G and has around 67 elements in its photosphere. Here are a few identified by their spectral signature. Turns out that hydrogen is 50 to 80% of most stars and combined with helium they make up 96 to 99% of all stars.