This is a basic question, but it's been bugging me. In the Wikipedia article for lithium burning, it states that:

Stars, which by definition must achieve the high temperature (2.5 × 10^6 K) necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion.

I would imagine that with lithium having more protons, it would have a stronger Coulomb repulsion and require higher temperatures to fuse with hydrogen. Now, this article is pretty sketchy since it cites no sources, and I would normally dismiss this. But according to here, lithium burning occurs in protostars, before hydrogen fusion even takes place. How could lithium fuse with hydrogen at temperatures that low?


The slowest reaction rate in the pp chain determines how quickly hydrogen can "burn" in the core of a sun-like star. That rate-determining step is actually the fusion of two protons to form deuterium via the diproton and a weak interaction decay.

The fusion of lithium, whereby it fuses with a proton and then splits into two Helium nuclei is actually part of the PPII series of reactions that convert helium 3 into helium 4. It does not involve weak interactions and therefore the cross-section at a given temperature is much higher than the first step in the pp chain, regardless of the stronger coulomb repulsion between the reactants. It is therefore initiated at lower temperatures - as you say, Li "burning" occurs in low-mass pre main sequence stars well before hydrogen "burning" commences (but after deuterium burning).

  • $\begingroup$ Hmm...so if a star's core is hot enough for lithium burning, but not for hydrogen burning, could it still be considered a protostar? $\endgroup$ – Sir Cumference Jun 5 '16 at 12:53
  • $\begingroup$ @SirCumference A star doesn't know it is a star until it starts hydrogen burning. There is little physics that distinguishes between a 0.1 solar mass "star", that will eventually burn H after contracting for about 200 million years, and a 0.02 solar mass brown dwarf, when both are 10 million years old. I suppose pedantically you could distinguish a protostar from a brown dwarf based on its mass. $\endgroup$ – ProfRob Jun 5 '16 at 13:26
  • $\begingroup$ So why does helium require such high temperatures to fuse, even when it has a lower Coulomb repulsion than lithium? It doesn't require $\beta^+$ decay. $\endgroup$ – Sir Cumference Jun 13 '16 at 5:10
  • $\begingroup$ @SirCumference (a) Coulomb repulsion between two He nuclei is larger than between Li and a proton. (b) You have to get THREE in the same place at once - it's the "triple alpha reaction". $\endgroup$ – ProfRob Jun 13 '16 at 7:40
  • $\begingroup$ Why is that? It's two protons vs three protons. $\endgroup$ – Sir Cumference Jun 13 '16 at 11:38

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