Teach Astronomy - Evolving HR Diagrams

http://www.teachastronomy.com/ The HR diagram is a plot of stellar properties, luminosity and photospheric temperature. It's a frozen snapshot in time, but over tens of millions to billions of years the main sequence population changes as stars exhaust their hydrogen and leave the main sequence to become giants, dwarfs, supernovae, and collapsed objects. This process can be used to measure age. Remember that when astronomers talk about stars, their position on the main sequence, and their movement on an HR diagram, they're referring to a plot of stellar properties. The stars themselves are not moving in physical space. This is just the statistical way of discussing the properties of a set of stars.