# Hertzsprung-Russell Diagram

## Basics of the HR diagram

In a Hertzsprung-Russell diagram, each star is represented by a dot. One uses data from lots of stars, so there are lots of dots. The position of each dot on the diagram corresponds to the star's luminosity and its temperature
• The vertical position represents the star's luminosity.
• The horizontal position represents the star's surface temperature.

Sometimes the labels are a little different:

• The vertical position represents the star's luminosity.
• This could be the luminosity in watts.
• More commonly it is in units of the Sun's luminosity.
• In either case, a ``ratio scale'' is used.
• Absolute magnitude is also commonly used.
• The horizontal position represents the star's surface temperature.
• Sometimes this is labelled in by the temperature in Kelvins.
• Highest temperatures go to the left. (It's traditional.)
• Normally the temperature is given using a ``ratio scale.''
• Sometimes the stars spectral class (OBAFGKM) is used.
• One could also use a measure of color as seen through filters.

## Other uses of scatter plots

The Hertzsprung-Russell diagram us an example of a scatter plot. Such plots are useful outside of astronomy too.

## What one sees in the HR diagram

When data on the nearest stars to us or stars in a cluster are plotted in a HR diagram, here is what one sees.

## Brightness, size, and the HR diagram

We expect a relation between temperature and luminosity. How does that show up in the HR diagram?

## Mass and the HR diagram

There is a relation between mass and luminosity.

## Conclusion

Based on this evidence, we conclude
• Stars spend most of their lives as main sequence stars.
• During its lifetime, the surface temperature and luminosity stays pretty much constant.
• Something else could happen in the star birth process.
• Something else could happen in the star death process.
• The star's mass determines what the temperature and luminosity is during the star's main sequence lifetime.
• More mass -> hotter.
• More mass -> more luminous.
• Also, more mass -> bigger.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu