Main sequence turnoff
The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after the exhaustion of its main fuel. Red dwarfs are stars with solar masses of 0.08M-0.4M and are referred to as class M stars. Red dwarfs have sufficient hydrogen mass to sustain hydrogen fusion to helium via the proton-proton chain reaction. They do not have sufficient mass to create the temperatures and pressures necessary to fuse helium to carbon, nitrogen or oxygen. However, all their hydrogen is available to a class M star for fusion and the low temperatures and pressures mean the lifetimes of these stars on the main sequence from zero point to turn off point is measured in trillions of years. For example, the lifespan of a star with a solar mass of 0.1M is 6000Gy.1 This lifespan greatly exceeds the current age of the universe, thus 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 heated helium slowly cools by radiation. Gravity will contract the star from lack of expansive pressure from fusion until electron degeneracy pressure compensates. The cooling star is now off the main sequence and is known as a helium white dwarf.2
References:
1. Thermonuclear Reactions in Stars,By C Iliadis,Published by Wiley-VCH, 2006,ISBN 3527406026, 9783527406029,666 pages
2. Horizons: Exploring the Universe,By Michael A. Seeds,Published by Thomson Brooks/Cole, 2005,ISBN 0495010030, 9780495010036,560 pages