In 1905, Danish astronomer Einar Hertzsprung, and independently American astronomer Henry Norris Russell, noticed that the luminosity of stars decreased from spectral type O to M. They developed the technique of plotting absolute magnitude for a star versus its spectral type to look for families of stellar type.

These diagrams, called the Hertzsprung-Russell or HR diagrams, plot luminosity in solar units on the Y axis and stellar temperature on the X axis, as shown below.

Notice that the scales are not linear. Hot stars inhabit the left hand side of the diagram, cool stars the right hand side. Bright stars at the top, faint stars at the bottom. Our Sun is a fairly average star and sits near the middle.

A plot of the nearest stars on the HR diagram is shown below:

Most stars in the solar neighborhood are fainter and cooler than the Sun. There are also a handful of stars which are red and very bright (called red supergiants) and a few stars that are hot, but very faint (called white dwarfs). We will see in a later lecture that stars begin their life on the main sequence then evolve to different parts of the HR diagram.

Several regions of the HR diagram have been given names, although stars can occupy any portion. The brightest stars are called supergiants. Star clusters are rich in stars just off the main sequence called red giants. Main sequence stars are called dwarfs. And the faint, hot stars are called white dwarfs.

On a log-log plot, the R squared term in the above equations is a straight line on an HR diagram. This means that on a HR diagram, a star's size is easy to read off once its luminosity and color are known.

The HR diagram is a key tool in tracing the evolution of stars. Stars begin their life on the main sequence, but then evolve off into red giant phase and supergiant phase before dying as white dwarfs or some more violent endpoint.

Thermonuclear Fusion:

Energy generation is the heart of stars. It provides the energy that we see as light, and it also supplies the heat and pressure that supports a stars' structure. The power source for stars is thermonuclear fusion.

Normally, particles with like charges (positive-positive or negative-negative) repel each other, this is called electrostatic repulsion. But at temperatures above 15 million degrees K, the motions of protons are high enough to overcome the electrostatic forces and the nuclei can ``fuse'' using the strong force.

The primary output from a thermonuclear reactions are photons in the form of gamma-rays, but a large number of other particles are produced as well. The simplest fusion reaction is the proton-proton chain, common in all main sequence stars. It has the following four stages: