As the composition of its interior changes, the star departs the main sequence slowly at first and then more rapidly.
The main sequence ends at about spectral type G, however, and does not extend up through the A, B, and O spectral types, though occasionally a few such stars are found in the region normally occupied by the main sequence.
Indeed, main sequence stars at least eight times larger than our sun will eventually have a core that leads to iron.
The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars formed at the beginning of the cluster.
By knowing the main sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster.
The temperature at the core of a main sequence or giant star is at least on the order of 107 K.
This effect results in a broadening of the main sequence band because stars are observed at random stages in their lifetime.
Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.
Thus the stars will evolve in order of their position on the main sequence, proceeding from the most massive at the left toward the right of the HR diagram.
