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I believe it is the case that stars as low as about 0.08 solar masses undergo the proton-proton chain reaction. So if you were to have a gas of more than about that mass of hydrogen in a location of space it should eventually collapse and heat up, undergoing this reaction.

I have a question about a slightly different secnario (partly as I want, if possible, to scare my friends). If you got a sphere of water, of sufficient mass, would the protons in the water undergo the proton-proton chain? I would imagine that the centre would get very hot under gravitational compression, sufficient for a proton soup (is that correct? ). Would the temperature at the core be hot enough to ignite a proton-proton chain? Or would it collapse into a neutron star, possibly in a sort of supernova-like event?

I ask this, because I honestly am intererested in the answer. Plus, it would be fun to scare my friends into realising that, under very very extreme conditions, at least, all the water inside their bodies could undergo a nuclear reaction.

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If you put enough baryonic matter in one place - where the value of "enough" depends on composition - gravity will make it hot enough to separate all electrons from all nuclei.

If you put a much larger "enough" in one place - where now the only relevant composition detail is nuclides, not whatever chemical bonds were initially present before the first aforementioned cutoff - your favourite exothermic fusion reactions between present nuclides will occur.

If you put enough water or meat in one place, hydrogen therein will fuse as you've proposed. But the much smaller requirement needed to destroy all chemistry would have led to a scary scenario long before that.

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    $\begingroup$ To pick a minor nit: putting enough baryonic matter in one place means gravity will guarantee high pressure, and thus high density, not necessarily a high temperature. However, what is certainly also true is that, if the matter is originally in a low-density state (such as liquid water), its collapse to a high-density state under gravity will release plenty of heat, possibly even enough to fully ionize it as you describe. And yes, this will typically happen long before thermonuclear fusion starts. […] $\endgroup$ Oct 9, 2021 at 21:09
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    $\begingroup$ … But if that doesn't happen, and you let the resulting huge lump of hot matter cool down enough, you'll eventually end up in a state where chemistry — even some rather interesting chemistry — is again possible. And AFAIK it should even be technically possible for matter at sufficiently high pressure and low temperature (e.g. inside very old white dwarf stars) to end up in a state where both (exotic) chemistry and fusion (not thermonuclear but pycnonuclear!) are possible. $\endgroup$ Oct 9, 2021 at 21:10

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