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Can stellar black holes be a category, or subset, of quark stars?

Strange stars are so called not because they're weird (though they are) but because the extraordinary pressures within them should turn some of their constituent quarks into a heavy type of quark named "strange." Neutrons and protons are made of two lighter types called "up" and "down." (The names were chosen whimsically and have no particular meaning.)

"It's possible that all the stars we have been calling neutron stars are in fact strange stars" or hybrids of the two, said Turner. If strange matter does convert everything it touches into more strange matter, these stars would consist of the stuff right up to their perfectly hard-edged surfaces, with nothing but vacuum above. If it doesn't, the stars could be hybrids with strange-matter cores and neutron-star outer layers.

from https://skyandtelescope.org/astronomy-news/quark-matter-stars-said-found/

The visible quark stars such as the one depicted would be quark-neutron hybrids. This would conveniently answer the question "what is black hole substance made of".

If this is certain to not be the case, what (if any) known substance can black holes de made of?

Note, I said a subset - "...strange stars exhibit a low-luminosity but high-temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disc, may be the key signature to differentiate massive strange stars from the black hole." in https://academic.oup.com/mnras/article/400/3/1632/962273

https://physicsworld.com/a/calculations-point-to-massive-quark-stars/

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    $\begingroup$ Should read these answers. astronomy.stackexchange.com/questions/2441/… $\endgroup$
    – James K
    Commented Mar 26, 2021 at 4:55
  • $\begingroup$ @JamesK Thank you, I did, great post. Here's the thing. Black holes are made of something, surely. What's smaller than neutrons? Quarks. Thus, perhaps black hole matter may be a subset of pure quark star material. $\endgroup$
    – Mark Besser
    Commented Mar 26, 2021 at 12:17

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Black holes are not made of anything... at least nothing in our current understanding of reality.

We understand black holes in terms of General Relativity, the theory of gravity. In this theory there are some "vacuum solutions" These are solutions that have no matter at any point in spacetime. One solution is "flat space", but another solution is a solution with a singularity in time and space. This solution was discovered by Schwatzchild, and for long was thought to be a mathematical curiosity but not physically real.

But it was later discovered that when a star collapses under gravity it reaches this state. All the matter falls towards the singularity and we can't describe what happens later than the singularity in time. From the outside it seems as if all the mass of the star is concentrated at one point, but there is actually nothing inside, just the distorted spacetime.

In General relativity nothing can exist in a black hole. For anything falling beyond the event horizon, all directions lead to the singularity. It is as unavoidable as "next Wednesday". No ball of matter could exist. In normal space, two particles can remain separate. Inside a black hole this isn't possible, since both particles end up at the singularity in a finite (and short) amount of time.

Now we aren't certain if this truly describes reality, because we lack a theory of gravity that takes account of quantum mechanics. It might be that the singularity is real. It might be that something happens (with strings or something) that prevents the ultimate collapse to a singularity. Whatever it would be outside our current models of reality. Not "quarks".

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  • $\begingroup$ Thank you. I always had the interpretation that singularities were a mathematical construct with no physical reality (not unlike a baseball diamond of size 400 square units, that has a side of 20 units, the other root of 400 being -20 but there is no physical distance of -20 other than the vector), and black holes were simply objects with an escape velocity above 300k km/s, neutron stars already reaching half of that. $\endgroup$
    – Mark Besser
    Commented Mar 27, 2021 at 16:32
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    $\begingroup$ A key fact about the interior of black holes is that spacial dimensions become "time-like" You have to go forward in time, you can't go back. Inside a black hole, you have to go forwards in space. There is no "backwards" or "away from the singularity" If something string-like happens to prevent ultimate collapse, then the final object would be very very small, probably much smaller than a single proton. We know enough about matter at larger scales and how gravity works to rule out any form of matter that we currently understand well. $\endgroup$
    – James K
    Commented Mar 27, 2021 at 16:38
  • $\begingroup$ The “from the outside it seems as if all the mass of the star is concentrated at one point” thing is misleading, Appearance of a black hole is highly symmetric (namely, a hole without angular momentum is spherically symmetric), but no reliable implications on where does “all the mass is concentrated” can be made of that. The gravity of any spherically symmetric (movement included) body is Schwarzschildean, right? m $\endgroup$
    – Incnis Mrsi
    Commented Jul 26, 2022 at 16:09
  • $\begingroup$ The “it seems as if all the mass of the star is concentrated at one point” thing is misleading, Outside appearance of a black hole is highly symmetric (namely, a hole without angular momentum is spherically symmetric), but no reliable implication on where does “all the mass is concentrated” ensues. The gravity of any spherically symmetric (movement included) body is Schwarzschildean, right? Moreover, spacetime of a hole resulting from collapse is different from ideal models (Schwarzschild, Kerr etc., having two event horizons, both future and past), and the former isn’t “vacuum” everywhere. $\endgroup$
    – Incnis Mrsi
    Commented Jul 26, 2022 at 16:51
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The structure of black holes is currently unknown,depending largely on the theory of quantum gravity - therefore, your question is not currently answerable

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