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]s, ] and ], showing the ] turn-off at different ages.]] ]s, ] and ], showing the ] turn-off at different ages.]]

The '''turnoff point''' for a ] refers to the point on the ] where it leaves the ] after the exhaustion of its main fuel. By plotting the turnoff point of the stars in ], one can estimate the cluster's age. The '''turnoff point''' for a ] refers to the point on the ] where it leaves the ] after its main fuel is exhausted{{snd}}the '''main sequence turnoff'''.

By plotting the turnoff points of individual stars in a ] one can ].


==Stars with no turnoff point== ==Stars with no turnoff point==
] are stars of 0.08-0.4 solar masses and are also referred to as class M stars. Red dwarfs have sufficient ] mass to sustain hydrogen ] to ] via the ], but do not have sufficient mass to create the temperatures and pressures necessary to fuse helium to ], ] or ] (see ]). 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.<ref>{{cite book | last = Iliadis | first = Christian | title = Nuclear Physics of Stars | publisher = Wiley-VCH | location = Weinheim | year = 2007 | isbn = 3-527-40602-6 }}</ref> This lifespan greatly exceeds the current age of the ], 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 fused ] stops and the remaining heated helium slowly cools by ]. ] will contract the star from lack of expansive pressure from fusion until ] compensates. The cooling star is now off the main sequence and is known as a helium ].<ref>{{cite book | last = Seeds | first = Michael | title = Horizons | publisher = Thomson Brooks/Cole | location = Australia | year = 2006 | isbn = 0-495-01003-0 }}</ref> ]s, also referred to as class{{nbs}}M stars, are stars of {{val|0.08|–|0.40|ul=solar mass}}. They have sufficient mass to sustain ]-to-] ] via the ], but they do not have sufficient mass to create the temperatures and pressures necessary to fuse helium into ], ] or ] (see ]). 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.<ref>{{cite book | last = Iliadis | first = Christian | title = Nuclear Physics of Stars | publisher = ] | location = Weinheim | date = 2007 | isbn = 978-3-527-40602-9 }}</ref> This lifespan greatly exceeds the current age of the ], 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 ] stops, and the remaining helium slowly cools by ]. ] contracts the star until ] compensates and it goes off the main sequence, i.e. becomes a ].<ref>{{cite book | last = Seeds | first = Michael | title = Horizons | publisher = Thomson Brooks/Cole | location = Australia | date = 2006 | isbn = 0-495-01003-0 }}</ref>


===References=== ==References==
{{Reflist}}
<references/>


] ]




{{star-stub}} {{stellar-evolution-stub}}

Latest revision as of 22:16, 10 December 2023

HR diagrams for two open clusters, M67 and NGC 188, showing the main sequence turn-off at different ages.

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 exhausted – the 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

Red dwarfs, also referred to as class M stars, are stars of 0.08–0.40 M☉. 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. 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, i.e. becomes a white dwarf.

References

  1. Iliadis, Christian (2007). Nuclear Physics of Stars. Weinheim: Wiley-VCH. ISBN 978-3-527-40602-9.
  2. Seeds, Michael (2006). Horizons. Australia: Thomson Brooks/Cole. ISBN 0-495-01003-0.


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