Spectral Class

Astronomers classify stars by their spectral class. The spectral class of a star is dependent upon its color and temperature.

• The coolest stars are red (class M) and orange (class K), while the hottest stars are blue (class O and class B).
• Each spectral class is then subdivided, 0 to 9.
  • The higher the number, the cooler the star within that category.

The sun is a class G yellow star, composed primarily of hydrogen and helium.

Hertzsprung-Russell Diagram

In the early 1900s, Danish astronomer Enjar Hertzsprung and American astronomer Henry Norris Russell independently came up with a diagram that shows the relationship between properties of stars, such as color, temperature, and luminosity or brightness. It is called the Herztzsprung-Russell diagram and is useful in studying the properties of stars.