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Suppose by some miracle a large molecular cloud of pure lithium assembled itself in space, then gravitationally collapsed.

What would the result look like? Would it be a star of some sort (fusing the lithium into carbon perhaps)? Or would the result just collapse directly into either a neutron star or planet depending on the mass?

Note I am assuming the gas cloud is literally pure lithium, thus there will be no protons available for lithium burning (unless some process produces them from this pure lithium).

I understand that the scenario here would never occur in reality; I'm just curious about what our current laws of physics would predict about this scenario. (Specifically, I have read about the almost pure hydrogen stars from the early universe, extreme helium stars, and was then naturally wondering about whether lithium stars are possible.)

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    $\begingroup$ Without delving deep into nuclear physics calculations, I don't see why it wouldn't contract until some fusion that turns Lithium into Carbon or Oxygen kicks in and it reaches some sort of hydrodynamic equilibrium. $\endgroup$
    – ScienceSnake
    Commented Feb 27 at 16:06
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    $\begingroup$ Questions about impossible hypothetical scenarios are generally discouraged on this site. I suppose your question might be ok on Physics.SE, but it might be hard to find info about pure Li-7 + Li-7 fusion reactions. $\endgroup$
    – PM 2Ring
    Commented Feb 27 at 16:49
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    $\begingroup$ There is a theory for high lithium abundant stars but it is still in speculation; astronomy.stackexchange.com/questions/7844/… $\endgroup$
    – Nilay Ghosh
    Commented Feb 27 at 17:03
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    $\begingroup$ ENSDF does have some pointers to Li-Li nuclear reactions, although most involve Li-6 rather than Li-7 such as 6Li(6Li,$\gamma$)12C. The energy required to overcome the large Coulomb barrier means they require fairly energetic interactions. $\endgroup$
    – Jon Custer
    Commented Feb 27 at 19:48
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    $\begingroup$ @JonCuster though lower than the temperatures required to get the pp chain burning. $\endgroup$
    – ProfRob
    Commented Feb 28 at 7:14

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Depends on the mass of the star.

For a low-mass object, it will form a cooling lithium brown dwarf supported by electron degeneracy pressure before it gets hot enough to ignite lithium.

A higher mass object will contract and get hot enough to ignite lithium fusion. I would guesstimate, based on the likelihood that two $^7$Li nuclei might fuse to $^{14}$C, that deuterium ignites at $\sim (5-7)\times 10^5$ K and that the Coulomb barrier is 9 times higher, that temperatures of just $\sim 5\times 10^6$ K would be required (less than for the hydrogen pp chain, which is slowed by a weak interaction).

Details would likely depend on which Li isotope and the exact nuclear physics, but on the basis of the number above, the lowest mass lithium star would be between the threshold for deuterium ($0.012M_\odot$) and hydrogen ($0.075M_\odot$) burning stars. Anything below that would cool as a Li "brown dwarf".

The subsequent evolution of such an object is also going to be mass dependent. I wouldn't like to speculate much further, but it would involve either a cooling carbon white dwarf or carbon ignition and further burning phases. A lithium brown dwarf would just sit and cool.

Pure lithium stars aren't possible. Any lithium star that contains hydrogen will rapidly burn its lithium even before the pp chain ignites.

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