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H-R Diagram

Having the information about spectral types was useful, but astronomers wanted to look for trends in the data. In 1911 Ejnar Hertzsprung plotted the absolute magnitude of stars against their colors. Two years later Henry Norris Russell independently did a similar graph using spectral types. Graphs of this type are known as Hertzsprung-Russell diagrams or H-R diagrams.

H-R diagram. Luminosity in solar units on the y-axis and Surface temperature in Kelvin on the x-axis. Luminosity increases going up the y-axis, and surface temperature decreases going along the x-axis. Diagonally across from top left to bottom right are the main sequence stars. Below the main sequence diagonal are the white dwarf stars. Above the main sequence diagonal line, are the giant stars towards the middle right of the plot. The supergiant stars are above this along the top of the plot.

H-R diagram. Image credit: Alice Hopkinson, LCO. Based on H-R diagram from ESO.

The most surprising thing about the H-R diagram was that the stars were not randomly scattered on it, but clustered in certain regions and along certain lines. The band that stretches across the diagram includes 90% of the stars in the night sky. This band is called the main sequence stars. The stars clustered at the upper right of the diagram include about 1% of the stars on the diagram, and are called giants and supergiants. Because of their cooler temperatures, they must be large to be as luminous as they are. The stars in the lower left of the diagram are called white dwarfs. They are very hot, but their luminosities are low, so they must be small. They make up about 9% of the stars on the diagram.