- Turnoff point
The turnoff point for a
star refers to the point on theHertzsprung-Russell diagram where it leaves themain sequence after the exhaustion of its main fuel.Red dwarfs are stars of 0.08-0.4 solar masses and are also referred to as class M stars. Red dwarfs have sufficienthydrogen mass to sustain hydrogen fusion tohelium via theproton-proton chain reaction , but do not have sufficient mass to create the temperatures and pressures necessary to fuse helium tocarbon ,nitrogen oroxygen (seeCNO cycle ). However, all their hydrogen is available 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 of 0.1 solar masses is 6 trillion years. [cite book | last = Iliadis | first = Christian | title = Nuclear Physics of Stars | publisher = Wiley-VCH | location = Weinheim | year = 2007 | isbn = 3527406026 ] This lifespan greatly exceeds the current age of theuniverse , therefore all red dwarfs are main sequence stars. Even though extremely long lived, those stars will eventually run out of fuel. Once all of the available hydrogen has been fusedstellar nucleosynthesis stops and the remaining heated helium slowly cools byradiation .Gravity will contract the star from lack of expansive pressure from fusion untilelectron degeneracy pressure compensates. The cooling star is now off the main sequence and is known as a heliumwhite dwarf . [cite book | last = Seeds | first = Michael | title = Horizons | publisher = Thomson Brooks/Cole | location = Australia | year = 2006 | isbn = 0495010030 ]References
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