Main sequence

When one plots the locations of some not too biased sample of stars in a diagram of luminosity? and [spectral type]? (a so-called Hertzsprung-Russell-Diagram?), one finds that most of the stars lie on the main sequence, a gently bent line from upper left to lower right.

This line is so pronounced because both the spectral type and the luminosity depend on a star's mass only to zeroeth order as long as it is fusing hydrogen and that a star spends most of its "active" life doing just that.

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 the only parameter a star has after all. Chemical compositon and -- related -- its evolutionary status also move a star slightly on the main sequence.

Stars usually enter and leave the main sequence from above when they are born or they are starting to die, respectively. See stellar evolution