Are black holes neutron stars with more mass?

Question

As we know, a star that is too massive to become a white dwarf becomes either a neutron star or if it is even more massive, a black hole: neutron stars form with great gravity that comes with great mass, the gravity pushing down on its core, merging protons and electrons into neutrons making the core consist only of neutrons.

Take this into consideration, is it possible that the same thing happens to stars that become black holes, since their mass (and thus gravity) is so large that even light can't escape?

Answer 1

I would answer your question "yes and no" :)

"Yes": If you take a neutron star, and add mass to it, it will not stay a neutron star forever. Eventually, you will surpass the maximum mass of a neutron star, above which the neutron degeneracy pressure is no longer sufficient to support the star against the crushing inward force of gravity. Theoretically, the neutron star could collapse to a hypothetical form of matter even denser than a neutron star (such as a hypothetical quark star), however as far as we know in current physics, there is no stable matter that the neutron star will collapse into. Therefore, the matter in the neutron star has no option but to collapse all the way inward, until it forms a black hole.

"No": Black holes and neutron stars are quite different objects, despite the fact that they are the two densest star-like objects we know of. The main difference is that neutron stars are made of matter -- nuclear dense matter -- tightly packed into a star. A black hole is just an empty region of spacetime, surrounding a "singularity" -- a point with so much density that general relativity (our best theory of gravity) breaks down and does not give a reliable prediction. One implication of this is that neutron stars have a maximum mass, which is thought to be somewhere around 3 solar masses, while black holes can (in theory) be as heavy (or as light) as you like. In practice, the masses of black holes in the real Universe are determined by how they were formed -- there are "stellar mass black holes" (somewhere around 5-100 solar masses) formed from some kinds of stars, and "supermassive black holes" (millions of solar masses) in the centers of galaxies, as well as "intermediate mass black holes" with masses in between those extremes.

Answer 2

The answer is probably no. Most of the black holes that we know about have masses above 5 solar masses, whereas all of the neutron stars have mass below 2.5 solar masses. This gap suggests that just adding mass to neutron stars is not a common way to form black holes. See the summary plot below from Ozel et al. (2012).

If black hole formation by the simple addition of more mass to a neutron star was common, then we might expect to see many more objects populating the mass range just above the maximum supportable mass of a neutron star - which is thought to be around 2.2-2.5 solar masses, but almost certainly lower than about 3 solar masses.

What black holes and neutron stars share is progenitors that are massive stars. One school of thought is that there is a bifurcation at the ends of their lives, during the supernova core collapse that ends up either with a black hole above 5 solar masses or a neutron star of much lower mass. However, another school of thought is that although possibly rare, some low mass black holes, formed when neutron stars accrete more mass, are out there, but very difficult to spot observationally.