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 because of a little something called the HR diagram. Over the summer, I wrote two other posts on what I called the “most important graph in astrophysics.” Today, I want to finish the series by explaining how the HR diagram shows us how stars age and evolve. Stellar evolution: a star’s life cycle You can read the first and second posts here and here , respectively.…