"The gas blobs that evolved into the Milky Way consisted merely of hydrogen and helium (and a smattering of lithium), the elements that were created in the Big Bang. All the other elements were literally created by the stars. Unlike the medieval alchemists, the stars can actually transmute one element into another—they are prodigious chemical factories. Nevertheless, even today hydrogen and helium make up about 98 percent of the normal matter in the universe. It’s the distribution of the elements that make up the final 2 percent that makes all the difference to studies of galactic evolution."