The Birth and Lifetime of Stars (2)

Imagine the protostellar material becoming denser and heating up, hence moving left to right on the HR diagram, eventually finding a place on the main sequence.  Where they end up is dependent on their original mass. After a lifetime on the main sequence, what happens to them then? It depends on their mass.

Reminder – the proton-proton chain is is the nucleosynthesis most commonly found in stellar processes. You might like to transpose this into a series of nuclear equations…

  1. Two mass-1 isotopes of H undergo a simultaneous fusion and beta + decay to produce a positron (immediately annihilated with an electron to form 2 gamma rays + 1.02MeV)), a neutrino, and a mass-2 isotope of H (deuterium) +0.42MeV
  2. The deuterium fuses with another mass-1 isotope of H to produce He-3 and a gamma-ray +5.49MeV
  3. Two He-3 isotopes produced in separate implementations of steps (1) and (2)  most commonly fuse to form a He-4 nucleus plus two protons and a further 12.6MeV, but there are other, temperature-dependent pathways.

Eventually, the main sequence star’s main store of H is used up. Without radiation pressure, it contracts under gravity. This contraction releases GPE, heats the core up, the remaining H at the periphery fuses, expands and cools. The star increases in size, becoming a Red Giant, huge and cool. What happens next depends on initial mass. Check this diagram out – it’s important for exam purposes.

Screen Shot 2014-04-07 at 20.05.55

An HR diagram is below. We recall that temperature increases from right to left. Notice how our sun, for example,  when it runs out of H will eventually loop off the main sequence on an HR diagram heading north towards the giants section, as the radiation pressure due to He fusion pushes the remaining H outwards, the consequent red giant’s orbit will at least engulf Mars and probably further, thereafter looping back down again below main sequence and ending up as a white dwarf

Neutron stars and black holes aren’t shown – this is why.

  •  Neutron stars are the collapsed cores of supergiants that have exploded as supernovae. They are about 6-20 km across with average densities of a million tonnes per cubic centimetre . With temperatures of the order of 1,000,000K, they would fall far off to the left of the diagram.
  • Superdense black holes, which may be created out of supernovae from the most massive stars, emit no light on their own and cannot be seen. Their surroundings may become visible if they accrete mass from a binary companion, but they still cannot be placed on an HR diagram.Screen Shot 2014-04-09 at 18.03.03

When somebody puts on too much weight there is an increased risk of heart attack; when a white dwarf star puts on too much weight (i.e. adds mass), there is the mother of all fatal heart attacks, a supernova explosion. The greatest mass a white dwarf star can have before it goes supernova is called the Chandrasekhar limit. About 1.4 solar masses is the limit.

Read more:

“Where do Stars Go When They Die?” is a podcast which might answer a few questions








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