Just like you and me, stars change over time. By studying the characteristics of stars, like their temperature and luminosity, astrophysicists figured out how stars evolve over time. This amazing insight is the primary lesson of the Hertzsprung-Russell (HR) diagram. Human beings, as the species Homo sapiens , have been around for about 300,000 years. That turns out to be about 100 million nights during which somebody, somewhere looked up at the dark sky and asked, "What are those twinkly lights?" Given all those nights and all those people asking pretty much the same question, it is pretty remarkable that we happen to live in one of the first generations that actually knows the answer. Here in the 21 st century, we know for sure what stars are, and a key reason we have that knowledge is be
