Stellar Evolution

We have a surprisingly complete understanding of how stars change over time, even though they change slowly over millions and billions of years. Watch this brief video where Carl Sagan concisely describes the evolution of stars.
Read 22.1 Evolution from the Main Sequence to Red Giants to understand how stars change in temperature, luminosity, and radius as they age.
Explore 22.2 Star Clusters to learn about the three types of star clusters—globular, open, and associations—and their significance in testing theories of stellar evolution.
It is very surprising, but using our knowledge of stellar evolution and HR Diagrams, we can obtain reliable measurements of the age of stars! How do astronomers do that? The secret is to create HR diagrams of star clusters. A cluster of stars contains thousands of stars that all formed at the same time. Like a group of runners starting at the same moment, the stars all “turned on” at the same time, but they evolve at different rates. Some stars (the more massive ones) evolve more quickly, becoming red giants first while less massive stars remain in the main sequence. When astronomers create an HR diagram for a star cluster, they can see what percentage of the stars are already red giants (how many stars have “finished the race”), which provides an accurate measure of the relative age of the whole cluster.
Over time, more and more of the stars in a cluster become red giants. Watch this elegant video of the Omega Centauri Cluster, which shows how an HR Diagram can be constructed from an image of a star cluster.
Let’s examine a few simulations showing how the evolution of a star cluster appears on an HR Diagram. Watch the teacher video Star Evolution.
Read 22.3 Checking Out the Theory to review how the H–R diagram of star clusters changes over time and how astronomers determine the age of a cluster.