The virial theorem is a theorem that states the average kinetic energy of a star is about half its gravitational potential energy in a cluster. We can use this theorem to determine the mass of star clusters.

## Cepheid Variables

There are stars known as Cepheid variables that change in brightness over time. The rate at which their brightness varies is proportional to their brightness, which allows us to determine the distance to galaxies millions of light years away.

## Rotating Stars

Some stars rotate very quickly. So quickly that they bulge significantly at their equator. Is there a maximum rate of rotation? We can use a simple model to find out.