Main sequence turnoff

They have sufficient mass to sustain hydrogen-to-helium 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.

[1] This lifespan greatly exceeds the current age of the universe, therefore all red dwarfs are main sequence stars.

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, i.e. becomes a white dwarf.