[syhprum (OP)]

Much of the mystery of black holes derives from the calculation that the familiar nuclear material formed of up and down quarks cannot be sufficiently compressed to form a Neutron star with an escape velocity of more than c.
Would it be possible to calculate the density of material formed from the heavier quarks and the resulting escape velocity from a star formed of such matter.
is there any way that the familiar nuclear material could transmute to a more dense form?

[Kryptid]

Sounds like a kind of quark star called a strange star: https://en.wikipedia.org/wiki/Strange_star

Quark stars are expected to be slightly smaller and more dense than regular neutron stars (an upper mass limit of 2.5 solar masses vs. 2 for a neutron star, and a radius below 10-12 km vs. above 10-12 km for a neutron star: http://hosting.astro.cornell.edu/~dong/a6511/presentations/SnellQuarkStarReport.pdf)

They wouldn't be black holes, though. I don't think any physical structure could be stable inside of an event horizon unless it was literally impossible for that structure to be compressed further.

[evan_au]

is there any way that the familiar nuclear material could transmute to a more dense form?

As Kryptid said, there are several ways this might be possible.

To discover whether this actually happens, the NICER X-Ray telescope on the ISS is monitoring pulsars in an attempt to measure their mass and radius, which will allow calculation of their density. This will reveal whether a more dense form of nuclear matter is stable enough to withstand the intense gravitational forces attempting to crush the neutron star into a black hole.

See: https://heasarc.gsfc.nasa.gov/docs/nicer/
https://en.wikipedia.org/wiki/Neutron_Star_Interior_Composition_Explorer

a Neutron star with an escape velocity of more than c

In general relativity, spacetime becomes rather twisted inside the event horizon of a black hole - the dimensions of space and time are interchanged, to some extent. This means that it is impossible for an object (eg the body of a neutron star) to avoid the central singularity, just as it is impossible for you to avoid time ticking forwards.

Theoreticians don't think that the structure of a neutron star could be maintained once it becomes a black hole.
- The forces keeping baryons apart is mainly the strong nuclear force, carried by massless gluons
- Being massless, they can travel at the speed of light, holding up the core of the neutron star: exerting force out, in and sideways
- But once the escape velocity exceeds the speed of light, the gluons can't travel "outwards", and so can't exert a force to keep the neutron star interior from collapsing in on itself.
- This implies that a neutron star can't have an escape velocity greater than c at the surface, or at any point in the interior.

If anything is to resist being crushed to a point at the central singularity, it will probably be some sort of quantum indeterminacy.
However, current theories of quantum gravity are plagued by awkward infinities, just like classical general relativity. So we really don't know what happens right at the center of a black hole.
See: https://en.wikipedia.org/wiki/Strong_interaction#Behavior_of_the_strong_force

[evan_au]

Some recent astronomical news that may shed further light on this topic:

A recent gravitational wave event was observed, where one of the participants in the merger had a mass around 2.6 solar masses. This is either:
- the lightest black hole ever observed
- or an impossibly heavy neutron star
- or perhaps evidence for some type of quark star, which is stable at higher masses than a pure neutron star.
- it's not really possible to distinguish these interpretations from just this one event (although, if they detected an expanding shell of debris, that would imply that it wasn't two black holes colliding)

See: https://www.scientificamerican.com/article/mystery-object-blurs-line-between-neutron-stars-and-black-holes/