The Hertzsprung-Russell Diagram Organizing Star Property Data

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).

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

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

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

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