Star Clocks
HOW DO WE TELL TIME in the Milky Way? To understand the history of our galaxy, its star systems, and even its elements and molecules, we need a way to put a timeline to events — a timeline that spans billions of years. The Milky Way didn’t form in one sudden burst of star formation, nor has it always been forming stars at the same rate. It didn’t start rich in the heavier elements that make up most of the physical material we interact with on a day-to-day basis, either. These elements (carbon, oxygen, silicon, iron etc.) are all the products of the lives and deaths of stars, built up over time and multiple generations. To understand the history of star formation and enrichment, we need a means of telling time.
Likewise, we’ve now discovered thousands of planets orbiting other stars. A whole cohort of scientists is devoting their lives to the search for Earth-like planets, ultimately trying to answer the question, “How unusual is Earth?” The answer requires more than just a tally of how many Earth-size planets there are in Earth-like orbits around Sun-like stars (we think the answer is ‘a lot’), but also where in their evolution those planets are. Earth is 4.6 billion years old; we’ve only had an oxygen-rich atmosphere for about 2.4 billion years. Complex animal life has only existed for 600 million years or so. If we’re judging based on what our world looks like today, then a very young Earth would not have been Earth-like at all. Without a sense of time, it’s hard to put our own Solar System in context.
Stars are our galactic clocks. They are bright and therefore easy to
