1. The Hertzsprung-Russell Diagram
Organizing Star Property Data

2. The H-R Diagram
The “H-R” Diagram was invented around 1910 to organize stars based on their properties.
The diagram is a graph.
Vertically the scale is increasing Luminosity
Horizontally the scale is Spectral Class/Temperature
From Hot (O class) to Cool (M class).

3. Equal Radius Lines
In general the hotter the star is the brighter it will be.
Thus you would expect stars of the same size but different temperatures to form a diagonal line called an equal radius line.
Equal Radius lines can be added to an H-R diagram

4. H-R Diagram Notice there are 4 groups: Most Stars are “Main Sequence”
Others are:
White Dwarfs,
Giants and
Supergiants

5. About the Groups
White Dwarfs are hot, but not that bright, so they must be small.
Giants and Supergiants are cool, but bright so they must be very large.
The Main-Sequence is steeper than an “equal radius line”
This is because large mass main sequence stars fuse fuel faster,
making them hotter, thus brighter

6. The Instability Strip
On the H-R Diagram Cepheid & RR Lyrae Variable Stars are found in an area called the “Instability Strip”