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