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Universe main sequence stars are often called dwarf stars.

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Main sequence stars of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. Stars on this band are known as main-sequence stars or Main sequence dwarf stars.

Main sequence stars.
Hertzsprung-Russell diagram: Main sequence stars.

This line is so pronounced because both the Spectral type and the luminosity depend only on a star's mass (to zeroth order) as long as it is fusing Hydrogen-and that is what almost all stars spend most of their "active" life doing.

At closer inspection, one notices that the main sequence is not exactly a line but instead somewhat fuzzy. There are many reasons for this fuzziness, the most important one still being observational uncertainties which mainly affect the distance of the star in question but range all the way to unresolved Binary stars.

But even perfect observations would lead to a fuzzy main sequence, because mass is not a star's only parameter. Chemical composition and-related-its evolutionary status also move a star slightly on the main sequence, as do close companions, rotation, or magnetic fields, to name just a few. Actually, there are very metal-poor stars (subdwarfs) that lie just below the main sequence although they are fusing hydrogen, thus marking the lower edge of the main sequence's fuzziness due to chemical composition.

Astronomers will sometimes refer to the "zero age main sequence", or ZAMS. This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age. Stars usually enter and leave the main sequence from about when they are born or when they are starting to die, respectively.

Our Sun is a main-sequence star-it has been one for about 4.5 billion years and will continue to be one for another 4.5 billion years. It has the spectral classification of G2 V. After the hydrogen supply in the core is exhausted, it will expand to become a red giant.

The total main sequence lifetime of a star can be estimated from its mass relative to the Sun's as follows:

where is the mass of the sun,M is the mass of the star and tms is the star's estimated main sequence lifetime in years. The lightest stars, of less than a tenth of solar mass, may last over a trillion years, whilst the heaviest last only a few tens of thousands of years.

Main sequence stars data.

The table below shows typical values for stars along the main sequence. The values of luminosity (L), radius (R), and mass (M) are relative to the Sun. The actual values for a star may vary by as much as 20-30%. The coloration of the stellar class column gives an approximate representation of the star's photographic color.

R/R? M/M? L/L? K
O2 16 158 2,000,000 54,000
O5 14 58 800,000 46,000
B0 5.7 16 16,000 29,000
B5 3.7 5.4 750 15,200
A0 2.3 2.6 63 9,600
A5 1.8 1.9 24 8,700
F0 1.5 1.6 9.0 7,200
F5 1.2 1.35 4.0 6,400
G0 1.05 1.08 1.45 6,000
G2 1.0 1.0 1.0 5,700
G5 0.98 0.95 0.70 5,500
K0 0.89 0.83 0.36 5,150
K5 0.75 0.62 0.18 4,450
M0 0.64 0.47 0.075 3,850
M5 0.36 0.25 0.013 3,200
M8 0.15 0.10 0.0008 2,500
M9.5 0.10 0.08 0.0001 1,900

"Oh Be A Fine Girl, Kiss Me" is a phrase used to aid memory of the sequence.

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