A standard tool in astronomy. It is a plot of a star's luminosity (y-axis) against its temperature (x-axis). Perversely the temperature decreases along the x-axis*. This is for historical reasons (astronomy is plagued with historical interest).

A normal hydrogen burning star such as our sun, occupies a place somewhere low and in the middle of the diagram. Giant luminous stars that are very hot are on the top left of the diagram. Red Giants are not so hot but their size gives them enormous luminosities. This places them at top right hand corner of the diagram. A white dwarf is very hot, but also very dim due to its tiny size. They live on the far bottom left of the diagram.

As a star evolves it moves about on the HR diagram. Stars spend most of their lives burning hydrogen. As their metallicity increases they creep slowly leftward, luminosity depends on metallicity. Most stars spend most of their lives along this track and it is called the main sequence. When hydrogen burning is exhausted helium burning sets in. The luminosity and size jump and the star pops off the main sequence. As the envelope expands the star cools and races over to the Red Giant branch. If the star is big enough it will explode into a supernova and drop straight down to join the white dwarfs.

The HR diagram has a lot of devious uses. You can plot the HR diagram for a globular cluster (GC) and by looking at the positions of its stars on the HR diagram you can determine the age of the GC.

Some stars have very odd tracks on the HR diagram. Binary star systems will weave a complicated pattern as the two stars co-evolve.

The contents of a stellar atmosphere can radically change the position of a star on the HR diagram. We are only now beginning to understand the complex molecular reactions that go on in stellar atmospheres. An industry has developed about calculating stellar atmospheres and about plotting tracks on the HR diagram.

* As bad as this is, we astronomers thank ourselves that we were not the ones responsiable for giving the electron a negative charge.