The Main Sequence

Soon after their formation, stars settle into a relatively narrow band on the HR diagram that is known as the main sequence. In typical HR diagrams, 90 percent of all stars fall on the main sequence. As shown in the righthand figure, this band runs from from the upper left (bright, blue, hot) to lower right (faint, red, cool) portion of the diagram.

Masses
The location of a star on the main sequence is determined mainly by its mass. The brightest stars in the upper left portion of an HR diagram can have up to about 100 solar masses (though masses above 20 solar units are rare). Conversely, the faintest main sequence stars at the lower right can be small as about 1/10 solar mass.

Characteristics
The range of intrinsic brightness (luminosity) on the main sequence is very large, much larger than the range of masses; there is a luminosity ratio of about a billion between the brightest and faintest main-sequence stars. Stars remain on the main sequence for most of their existences; this period can be of order a million years for the most luminous stars and perhaps several hundred billion years for the least luminous (much longer than the present age of the Universe). This is why most stars are on the main sequence; it is the same reason why you are much more likely to see intermediate age people in a crowd of average people than babies or the very old. Because stars spend most of their life with luminosities and temperatures that place them on the main sequence, at any one time a snapshot of many stars will find most of them on the main sequence. The following table gives the typical properties of main sequence stars divided according to spectral class.

Main Sequence Spectral Class Properties
Spectral Class Mass (Solar Units) Luminosity (Solar Units) Temperature (K) Radius (Solar Units) Time on Main Sequence (Million Years)
O5 40 400,000 40,000 13 1.0
B0 15 13,000 28,000 4.9 11
A0 3.5 80 10,000 3.0 440
F0 1.7 6.4 7,500 1.5 2,700
G0 1.1 1.4 6,000 1.1 8,000
K0 0.8 0.46 5,000 0.9 17,000
M0 0.5 0.08 3,500 0.8 56,000