Stellar Classification

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The actual appearant colors of the star types discussed below.

In astronomy, stellar classification is a way of grouping stars by temperature and luminosity. A star is a ball of superheated gas called plasma. Star temperature can be measured by looking at its spectrum, the type of light that the star shines. Stars are also grouped into spectral types or classes by color. In general, a star's temperature determines its color, from red to blue-white. Blue-white stars are the hottest, while Red ones are the coolest. Each class has their own subclass as well, with Arabic Numerals. A G0 star would be the hottest type G and G9 with be the coolest type G.

Life Cycle of a Star

Stellar life cycles.

Stars are born in clouds of gas called a Nebula (plural: Nebulae) Nebulae are pulled together and become Protostars. Clouds of gas orbiting these stars then condense to form Planets, like how the Solar System was formed. The Star is then born and becomes a Main-Sequence Star. These stars live for up to 10 billion years and start to fuse hydrogen into the other elements of the Periodic Table. But as they reach heavier elements like Oxygen, they start cooling in temperature and become Red Giant stars, pushing their outer layers outward. Some stars expand even further into Red Supergiants by fusing metals. If the original post-giant star is more than 7 times larger than the Sun, then it will explode as a supernova for its death. Then it will become either a Neutron Star or a Black Hole, a bottomless pit that nothing can escape. If the original post-giant star is less than 7 times the size of the sun, it will become a white dwarf. White Dwarfs are small stars about the size of Earth, but they are hotter and were once the core of the progenitor star. Neutron Stars are the cores of original stars that have been crushed to the size of a city and they are packed up with Neutrons, the part of the atom that has no electrical charge. One 1 cm x 1 cm x 1 cm cube of their material would weigh as much as 5 fully-loaded cargo ships!

Stellar evolution phase

Temperature (Kelvin)

Diameter (D, or other when specified)
Nebula ~0 1 to 2,000 ly
Main Sequence Star 3,000 to 50,000 0.08 to 20
Red Giant 3,000 to 5,000 20 to 200
Red Supergiants 200 to 2,600
White Dwarf <4,000[1][2] to 150,000[3] 8,000 to 100,000 km
Neutron Star 600,000[4][5][6][7][8][9] 5 to 50 km
Black Hole 0.00000000000000000001 Any

Luminosity Class

Stars are also organized into another classification, this time for luminosity. This tells if the star is illegible to be called a dwarf, giant, supergiant, or hypergiant. It is also called the Yerkes spectral classification.

Temperature Class

The stellar classification used to classify stars by color or temperature has seven main classes and four other ones. It is called the Morgan-Keenan stellar classification. It was originally proposed by Italian astronomer Angelo Secchi, but has been improved by William Morgan and Philip Keenan.

Stellar Class

  • (example)
Secchi's original classification[10][11][12][13][14] Temperature

(in Kelvin)

Percentage (%) of known stars Conventional color appearance

(Celestia 1.6.1.'s impression)

LBV (luminous blue variable) n/a 7,500 to 50,000 0.000033
O 30,000 to 50,000
B I (Orion subtype), V 10,000 to 30,000 0.13
A I 7,500 to 10,000 0.7
  • Procyon
6,000 to 7,500 3
G II 5,000 to 6,000 7
K 4,000 to 5,000 12.1
Alpha centauri b-0.JPG
M III 2,400 to 4,000 77
Brown Dwarfs:[15]

  • L
  • T
  • Y
n/a 200 to 3,000 > 100
Comparison between the three types of brown dwarfs.
Carbon Stars
  • Y Canum Venaticorum
IV 1,000 to 3,000 unknown
Wolf-Rayet Stars n/a 30,000 to 200,000 unknown

Hertzsprung-Russell Diagram

Hertzsprung-Russel StarData.png

Star types are arranged in the Hertzsprung-Russell Diagram. This scattergraph was invented by Danish and American astronomers Ejnar Hertzsprung and Henry Russell in 1908. Stars farther away on the right are cooler in temperature, while stars that are near the top are more luminous and bright.

B-V Color Index

A graph comparing several values of the B-V Color Index, with other possible but non-black body colors for comparison

In astronomy, the B-V color index is a numerical expression that determines the exact "color" of a certain star. The smaller the color index, the more the star appears blue. For comparison, our Sun has an approximate B-V color index of 0.656[16], while Rigel has a color index of -0.03.[17] Red Giants have color indexes from 0.81 to 1.4 .[18]


  2. ISBN 0-333-75088-8
  5. ISBN 1-61233-765-1
  7. ISBN 0-387-33543-9
  9. A neutron star's density increases as its mass increases, and its radius decreases non-linearly. (archived image: A newer page is here: (specifically the image
  10. ISBN 0-521-25548-1
  13. pp. 62–63, Hearnshaw 1986.
  14. p. 60, Hearnshaw 1986.
  15. {{{1}}}
  18. ISBN 0-521-34787-4