<P> In high - mass main - sequence stars, the opacity is dominated by electron scattering, which is nearly constant with increasing temperature . Thus the luminosity only increases as the cube of the star's mass . For stars below 10 M, the opacity becomes dependent on temperature, resulting in the luminosity varying approximately as the fourth power of the star's mass . For very low - mass stars, molecules in the atmosphere also contribute to the opacity . Below about 0.5 M, the luminosity of the star varies as the mass to the power of 2.3, producing a flattening of the slope on a graph of mass versus luminosity . Even these refinements are only an approximation, however, and the mass - luminosity relation can vary depending on a star's composition . </P> <P> When a main - sequence star consumes the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume . Stars with less than 0.23 M, are predicted to directly become white dwarfs when energy generation by nuclear fusion of hydrogen at their core comes to a halt . In stars between this threshold and 10 M, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen - burning shell . In consequence of this change, the outer envelope of the star expands and decreases in temperature, turning it into a red giant . At this point the star is evolving off the main sequence and entering the giant branch . The path which the star now follows across the HR diagram, to the upper right of the main sequence, is called an evolutionary track . </P> <P> The helium core of a red giant continues to collapse until it is entirely supported by electron degeneracy pressure--a quantum mechanical effect that restricts how closely matter can be compacted . For stars of more than about 0.5 M, the core eventually reaches a temperature where it becomes hot enough to burn helium into carbon via the triple alpha process . Stars with more than 5--7.5 M can additionally fuse elements with higher atomic numbers . For stars with ten or more solar masses, this process can lead to an increasingly dense core that finally collapses, ejecting the star's overlying layers in a Type II supernova explosion, Type Ib supernova or Type Ic supernova . </P> <P> When a cluster of stars is formed at about the same time, the life span of these stars will depend on their individual masses . The most massive stars will leave the main sequence first, followed steadily in sequence by stars of ever lower masses . 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 . The current position where stars in this cluster are leaving the main sequence is known as the turn - off point . By knowing the main sequence lifespan of stars at this point, it becomes possible to estimate the age of the cluster . </P>

The hr diagram main sequence of stars has a lower limit for mass because