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Many different types of stars exist. Some types of stars are really just different stages of a star’s evolution. Some types are different because the stars formed with much more or much less mass than other stars, or because they formed close to other stars. The Sun is a type of star known as a main-sequence star. Eventually, main-sequence stars such as the Sun swell into giant stars and then evolve into tiny, dense, white dwarf stars.

Main-sequence stars and giants have a role in the behavior of most variable stars and novas. A star much more massive than the Sun will become a supergiant star, then explode as a supernova. A supernova may leave behind a neutron star or a black hole.

The H-R diagram compares the brightness of a star with its temperature. The diagonal line running from the upper left to the lower right is called the Main Sequence. Stars lying on the Main Sequence are blue when they are bright and red when they are dim. Stars in the upper right (called Red Giants) are very bright, but still appear red. Stars near the bottom (known as White Dwarfs) are white, but not very bright. This diagram was developed independently by Ejnar Hertzsprung, a Danish astronomer, and Henry Norris Russell, an American astronomer.
Encarta Encyclopedia

In about 1910 Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently worked out a way to graph basic properties of stars. On the horizontal axis of their graphs, they plotted the temperatures of stars. On the vertical axis, they plotted the brightness of stars in a way that allowed the stars to be compared. (One plotted the absolute brightness, or absolute magnitude, of a star, a measurement of brightness that takes into account the distance of the star from Earth. The other plotted stars in a nearby galaxy, all about the same distance from Earth.) The resulting Hertzsprung-Russell diagram, also called an H-R diagram or a color-magnitude diagram (where color relates to temperature), is a basic tool of astronomers.


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