Main sequence
The main sequence is a plotted mathematical curve along which the majority of stars are located in a diagram based on their spectra and mass. Stars on this band are known as main-sequence stars or dwarf 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.
The main sequence does not follow a completely even curve; this is primarily because of the 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.
Main sequence data
Stellar
ClassRadius Mass Luminosity Temperature 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
