# Can a star collapse into a black hole before it finishes burning its hydrogen fuel?

I don't see why a 50 solar mass star couldn't do so for the following reason. The pressure beyond some depth is so high that the matter below it is degenerate matter despite the very high temperature which is much denser than the rest of the star. Because of high pressure, it has a large core of electron degenerate matter that has another core of neutron degenerate matter in it. As it goes further through its cycle, the core of neutron degenerate matter keeps growing until it's so large that it starts a runaway effect of starting to collapse into a black hole.

• There is theoretically such a thing as a quasi-star, though at more than 1000 solar masses.
– Eth
Mar 15, 2019 at 17:33

That's not the way massive stars evolve. Since they have high mass, they do not need to contract so much to get to fusion temperature, so they are actually much farther from being degenerate than stars like the Sun (which is already pretty far from being degenerate). Also, as long as there is hydrogen in the core to fuse, the star will not suffer a net loss of heat-- the fusion will replace any heat being lost. With no net loss of heat, there is also no contraction, and no movement toward higher degeneracy. So the answer to your question is no-- massive stars will not collapse into black holes as long as they have hydrogen to fuse, because collapse requires a net loss of heat, and fusion prevents that. Formation of supermassive black holes is rather different-- that could happen without the hydrogen ever getting to fusion temperature until the black hole has already swallowed it.

• According to learnastronomyhq.com/articles/…, the amount of hydrogen in the sun left to fuse is lessening over time, which in the short run causes it to cool then the cooling causes it to contract creating a higher concentration of hydrogen gas which causes faster fusion and thus a higher temperature on the inside preventing further collapse. Maybe a 50 solar mass star could start a runaway collapsing effect while it still has lots of hydrogen fuel left because it slowly contracts with age. Nov 21, 2016 at 0:41
• No, even high-mass stars like 50 solar masses have core fusion temperatures where are nowhere near what it takes to make the electrons relativistic. As long as the electrons are not relativistic, there is no problem with hydrostatic equilibrium, so there is no collapse. Nov 21, 2016 at 23:21

Your scenario could not work. Your description of quasi-equilibrium with a core of neutron-degenerate material, surrounded by electron degenerate material is a description of a neutron star inside a $\sim 50$ solar mass envelope.

Neutron-degenerate material is incapable of supporting anything more massive than 2-3 solar masses, let alone a 50 solar mass star.

The sequence of events is that an inert (iron) core is compressed and is electron degenerate. As more iron is deposited from an Si burning shell then the core grows beyond its Chandrasekhar mass and it collapses. The collapse may be halted providing the neutron degenerate material is only having to support less than 2-3 solar masses. Any more than this, then collapse to a black hole seems inevitable.

A further factor to consider is how your neutron degenerate core could form. There can be no smooth, quasi-equilibrium transition between electron degenerate and neutron degenerate material because neutronisation ensures that the adiabatic index falls below 4/3 (the threshold for any kind of stability) at densities between about $10^{13}$ and $10^{17}$ kg/m$^3$. That is why neutron stars are much smaller than white dwarfs and why the formation of a neutron degenerate core releases vast quantities of gravitational potential energy and causes supernovae.

• This answer is probably useless and can't be improved without a major rewrite. It does not answer the question. Instead, it explains that a core of neutron degenerate matter of sufficient mass can't support itself. That explanation does not answer the question because a star can't have a core of neutron degenerate matter before it's finished burning almost all of the hydrogen in its core. Nov 23, 2017 at 22:04
• @user46757 This is an absurd comment. YOUR OWN QUESTION posits that the star has a neutron degenerate core "before it has finished burning its hydrogen fuel". Yes, a ridiculous scenario, and I point out why it is not possible. Nov 23, 2017 at 23:33
• I made that comment because you missed saying that a smaller neutron core can't collapse into a black hole and instead said a larger one could Nov 23, 2017 at 23:49
• "The collapse may be halted providing the neutron degenerate material is only having to support less than 2-3 solar masses. Any more than this, then collapse to a black hole seems inevitable." @user46757. Ridiculous. Nov 23, 2017 at 23:58

Other answers have detailed why a ~50 solar mass star won't create a black hole while still burning hydrogen. However, this can sort of theoretically happen with big enough stars.

A quasi-star is a theoretical star of more than 1000 solar masses, that may (or may not) have existed among the very first stars of the Universe, and could be an origin for intermediate-mass black holes.

For massive stars, the extreme temperatures in the core of the protostar would blow away the remainder of the still-accreting gas, and limit the mass of the star to ~150 solar masses. But above ~1000 solar masses, the still-accreting mass is big enough to overcome radiation pressure and solar winds and keep falling, until gravity is enough to form a black hole in the core of the protostar.

As such, quasi-stars would produce energy from the accretion disk of the black hole in their core, instead of fusion. However, like in fusion stars, energy from the core is pushing the remaining gas outward, creating an equilibrium and preventing all of it from falling in the black hole at once.

With a lifespan of a few million years, bigger than Neptune's orbit and as bright as a galaxy, they would have been rather spectacular.

There is a lot of good information in @KenG's answer, but it does somewhat miss the end result, I think. Yes, we do think that a star can collapse to a BH before completely expending its Hydrogen. The requisite for collapse depends on the conditions in the core: you need to reach an Iron core (with sufficient mass), in which you no longer have fussible material, and eventually cannot support itself. This can happen while there is still hydrogen in the envelope, and indeed, that should generally be the case as hydrogen burning will often still be occurring in outer shells. See for example the attached figure.

• I should say that I mean hydrogen fuel in the core. If you count the hydrogen in the envelope, then certainly any type II supernova still has hydrogen there when the core collapses, that defines type II supernovae. What my answer is trying to say that any star that is fusing hydrogen in its core is strongly protected against collapse. But you are right, it can be fusing in shells, and absent in the core, and that does not protect against collapse. Nov 21, 2016 at 23:24