In general we classify main sequence stars as one of the following types: O,B,A,F,G,K or M. This scale is broken into sub classes numbered 0-9.
  • O Blue above 25,000 K
  • B Blue 11,000 - 25,000
  • A Blue 7,500 - 11,000 Vega
  • F Blue to White 6,000 - 7,500
  • G White to Yellow 5,000 - 6,000
  • K Orange to Red 3,500 - 5,000
  • M Red less than 3,500
For example our sun is a G2 class star.

When a star reaches ZAMS, or Zero age main sequence, It can be classified as a certain type from the above list. At this time the star is composed of 24% helium 75% hydrogen and 1% metal. The star in general does not change spectral type. It can change temperature and luminosity by a considerable amount but usualy not enough to change spectral type. It is a common misconception that stars move along the main sequence because astronomers tend to say "traveling on the main sequence" giving the impression of movement when the movement is small during the main sequence lifetime. The main sequence is a fairly uneventful part of a stars life. It fuses hydrogen into helium until it starts to run out of hydrogen which cause core contraction and is the start of the end for the star.

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