HR Charts, Types of Stars, and Spectral Class
HR ChartsHR charts are diagrams that show similarities and differences between stars. They were created by astronomers, Enjar Hertzsprung and Henry Norris Russell. HR charts display the different stages of a star's life cycle.
HR Chart On an HR chart, the position of the star tells its luminosity and temperature.The vertical axis shows the luminosity or absolute magnitude of the stars. Luminosity measures a star's brightness. The horizontal axis shows the star's temperature. Temperature is usually measured in Kelvin.
Types of StarsMain Sequence stars are the most common stars and are located in the center of an HR chart. Their energy comes from nuclear fusion, as they turn hydrogen into helium. Main Sequence stars have average temperature and luminosity. Our Sun is a Main Sequence star.As stars get older, they become giants and supergiants which are located above the Main Sequence on an HR chart. Giants and supergiants have low amounts of hydrogen. Blue giants have very high temperatures and high luminosity. Red supergiants have low temperatures and high luminosity. Red giants have low temperatures and average luminosity.Dwarf stars are located beneath the Main Sequence on an HR chart. Their nuclear fuel supply is low and they are nearing the end of their lifetime. White dwarfs are hotter than red dwarfs, but they have similar luminosity.
 Red Giant |  Blue Giant |  White Dwarf |
Spectral ClassIn Spectral Classes, stars are grouped by their spectral characteristics. Light from the star is looked at through a prism. The prism produces a rainbow of colors with absorption lines. The lines show the chemical elements of the star. Then, the stars are grouped into classes like the one below.
Quiz1. What does the position of a star on an HR chart show?
| size and temperature | luminosity and temperature |
| size and velocity | luminosity and velocity |
2. What does luminosity mean?
3. What type of star is the Sun?
| white Dwarf | red supergiant |
4. Where are giants and supergiants located on an HR chart?
| below Main sequence | in Main sequence |
| above main sequence | to the right of main sequence |
5. What object is used to classify stars in the Spectral Class?
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