The turnoff point for a star refers to the point on the Hertzsprung–Russell diagram where it leaves the main sequence after its main fuel is exhaustedthe main sequence turnoff.
By plotting the turnoff points of individual stars in a star cluster one can estimate the cluster's age.
Stars with no turnoff point
, also referred to as classM stars, are stars of . They have sufficient mass to sustain
hydrogen-to-
helium Nuclear fusion via the proton–proton chain reaction, but they do not have sufficient mass to create the temperatures and pressures necessary to fuse helium into
carbon,
nitrogen or
oxygen (see
CNO cycle). However, all their hydrogen is available for fusion, and low temperature and pressure means a lifetime measured in trillions of years. For example, the lifespan of a star of 0.1 solar masses is six trillion years.
This lifespan greatly exceeds the current age of the
universe, therefore all red dwarfs are main sequence stars. Even though extremely long lived, those stars will eventually run out of fuel. Once all the available hydrogen has been fused stellar nucleosynthesis stops, and the remaining helium slowly cools by
radiation.
Gravity contracts the star until electron degeneracy pressure compensates and it goes off the main sequence, becoming a
white dwarf.
