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Hertzsprung-Russell Diagram

Putting the Stars in Order

Luisa Simmons

Basics

This diagram was first made independently in 1911 by Danish astronomer Ejnar Hertzsprung (1873-1967 and American astronomer Henry Norris Russell (1877-1957) in 1913. The form of the diagram we use today is Russell's.

Summary of Stellar Properties
Stellar Luminosities: 10-4 to 106 Lsun
Stellar Radii: 10-2 to 103 Rsun
Stellar Temperatures: 3000 to >50,000 K
Stellar Masses: 0.1 to ~50 Msun

The horizontal axis represents stellar surface temperature. The temperature corresponds with the spectral type: OBAFGKM
These categorize stars by their surface temperature, O naturally being the category for the hottest stars and M for the coolest ones.

The vertical axis represents stellar luminosity, based off the sun’s luminosity (L sun). This corresponds with luminosity class:
I           Supergiants
II          Bright Giants
III        Giants
IV        Subgiants
V         Main-sequence stars
We use both spectral type and luminosity class to fully classify starts. For example our sun is classified as a G2 V

!This diagram also allows one to determine stellar radii:
L = R2s T4
Which, when we isolate R:
R= √L / (4p s T4)
All we need to know is the luminosity and surface temperature because all the other variables are constant.

R-H Diagram

Luminosity increases upward and surface temperature increases to the left, therefore stars near the upper right are cool and luminous and stars near the lower left are hot and dim.