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What Are White Giant Stars?

A white, giant star is an uncommon object in the universe. Most giant stars have used up a percentage of their hydrogen, cooled down and expanded. The star then leaves what's called the main sequence, which is where most stars, including the sun, are found. A white giant is not only very large, but very hot and may or may not be on the main sequence.

Temperature and Radius
- The radius of a giant star is between 10 and 100 times that of the sun, and its luminosity is between 10 and 1,000 times that of the sun. The temperature of a white star can range from 7,500 to 10,000 degrees Kelvin or 13,532 to 18,032 degrees Fahrenheit. Because they are so hot and burn their fuel so quickly these stars will probably only last a few more million years or so. By contrast the sun is about half way through its 10-billion-year lifespan.
Characteristics
- Each star has its own spectral type, which scientists determine by analyzing the colors into which the star's light can be separated. Stars are classified from hottest to coolest as O B A F G K M. A and F stars are white. The sun is a G star, which means it's yellow. Further classifications fine tune the type of star under consideration.
Designations
- Johann Bayer, a German stellar cartographer, created Uranometria, a sky atlas published in 1603. He used Greek letters to name the brightest star in a given constellation. The stars were lettered in order of brightness, with alpha usually being the brightest star.
Notable White Giants
- Thuban is a white, giant star in the constellation Draco. It's Bayer designation is Alpha Draconis, alpha because it used to be the brightest star in the constellation. (Gamma Draconis is really the brightest.) Alpha Draconis is about 309 light years away from the sun and about 300 times as luminous. Gamma Ursae Minoris, or Pherkad, is a star in Ursa Minor. It's 480 light years from the sun, is 15 times larger and 1,100 times as luminous. It's part of the Little Dipper asterism. Its name means calf and it was given this name because it keeps as close to the pole star as a calf does to its mother. Deneb, or alpha Cygni, is a white supergiant. The star at the head of the Northern Cross, it's the brightest star in the constellation Cygnus. It has a luminosity 250,000 times greater than the sun, has about 200 times the radius and about 20 times the mass. It's about 1,400 light years away. Even though it's so far away it's so large and bright that it can still be seen from the earth.

Astronomy