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We know the spectacular explosions of supernovae, that when heavy enough, form black holes. The explosive emission of both electromagnetic radiation and massive amounts of matter is clearly observable and studied quite thoroughly. If the star was massive enough, the remnant will be a black hole. If it wasn't massive enough, it will be a neutron star.

Now there's another mode of creation of black holes: the neutron star captures enough matter, or two neutron stars collide, and their combined mass creates enough gravity force to cause another collapse - into a black hole.

What effects are associated with this? Is there an explosive release of some kind of radiation or particles? Is it observable? What physical processes occur in the neutrons as they are subjected to the critical increase of pressure? What is the mass of the new black hole, comparing to its neutron star of origin?

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A neutron star must have a minimum mass of at least 1.4x solar masses (that is, 1.4x mass of our Sun) in order to become a neutron star in the first place. See Chandrasekhar limit on wikipedia for details.

A neutron star is formed during a supernova, an explosion of a star that is at least 8 solar masses.

The maximum mass of a neutron star is 3 solar masses. If it gets more massive than that, then it will collapse into a quark star, and then into a black hole.

We know that 1 electron + 1 proton = 1 neutron;

1 neutron = 3 quarks = up quark + down quark + down quark;

1 proton = 3 quarks = up quark + up quark + down quark;

A supernova results in either a neutron star (between 1.4 and 3 solar masses), a quark star(about 3 solar masses), or a black hole(greater than 3 solar masses), which is the remaining collapsed core of the star.

During a supernova, most of the stellar mass is blown off into space, forming elements heavier than iron which cannot be generated through stellar nucleosynthesis, because beyond iron, the star requires more energy to fuse the atoms than it gets back.

During the supernova collapse, the atoms in the core break up into electrons, protons and neutrons.

In the case that the supernova results in a neutron star core, the electrons and protons in the core are merged to become neutrons, so the newly born 20-km-diameter neutron star containing between 1.4 and 3 solar masses is like a giant atomic nucleus containing only neutrons.

If the neutron star's mass is then increased, neutrons become degenerate, breaking up into their constituent quarks, thus the star becomes a quark star; a further increase in mass results in a black hole.

The upper/lower mass limit for a quark star is not known (or at least I couldn't find it), in any case, it is a narrow band around 3 solar masses, which is the minimum stable mass of a black hole.

When you talk about a black hole with a stable mass (at least 3 solar masses), it is good to consider that they come in 4 flavors: rotating-charged, rotating-uncharged, non-rotating-charged, non-rotating-uncharged.

What we would see visually during the transformation would be a hard radiation flash. This is because during the collapse, the particles on/near the surface have time to emit hard radiation as they break up before going into the event horizon; so this could be one of the causes of gamma ray bursts (GRBs).

We know that atoms break up into protons, neutrons, electrons under pressure.

Under more pressure, protons and electrons combine into neutrons.

Under even more pressure, neutrons break down into quarks.

Under still more pressure, perhaps quarks break down into still smaller particles.

Ultimately the smallest particle is a string: open or closed loop, and has a Planck length, which is many orders of magnitude smaller than a quark. if a string is magnified so it is 1 millimeter in length, then a proton would have a diameter that would fit snugly between the Sun and Epsilon Eridani, 10.5 light years away; that's how big a proton is compared to a string, so you can imagine there are perhaps quite a few intermediate things between quarks and strings.

Currently it looks like several more decades will be needed to figure out all the math in string theory, and if there is anything smaller than strings then a new theory will be required, but so far string theory looks good; see the book Elegant Universe by Brian Greene.

A string is pure energy and Einstein said mass is just a form of energy, so the collapse into a black hole really breaks down the structure of energy that gives the appearance of mass/matter/baryonic particles, and leaves the mass in its most simple form, open or closed strings, that is, pure energy bound by gravity.

We know that black holes (which are not really holes or singularities, as they do have mass, radius, rotation, charge and hence density, which varies with radius) can evaporate, giving up their entire mass in the form of radiation, thus proving they are actually energy. Evaporation of a black hole occurs if its mass is below the minimum mass of a stable black hole, which is 3 solar masses; the Schwarzschild radius equation even tells you what the radius of a black hole is given its mass, and vice versa.

So you could transform anything you want, such as your pencil, into a black hole if you wanted to, and could compress it into the required size for it to become a black hole; it is just that it would immediately transform itself (evaporate) completely into a flash of hard radiation, because a pencil is less than the stable black hole mass (3 solar masses).

This is why the CERN experiment could never have created a black hole to swallow the Earth - a subatomic black hole, even one with the mass of the entire Earth, or the Sun, would evaporate before swallowing anything; there is not enough mass in our solar system to make a stable (3 solar mass) black hole.

A simple way for a neutron star to become more massive in order to be able to turn into a black hole is to be part of a binary system, where it is close enough to another star that the neutron star and its binary pair orbit each other, and the neutron star siphons off gas from the other star, thus gaining mass.

Cataclysmic variable binary

Here is a nice drawing showing exactly that.

Matter falling into a black hole is accelerated toward light speed. As it is accelerated, the matter breaks down into subatomic particles and hard radiation, that is, X-rays and gamma rays. A black hole itself is not visible, but the light from infalling matter that is accelerated and broken up into particles is visible. Black holes can also cause a gravitational lens effect on the light of background stars/galaxies.

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Only your mention of quark star as an intermediate phase is on-topic regarding my question. –  SF. Oct 24 '13 at 9:38
    
well, I gave a possible answer to the question in your headline; in your question you actually asked 5 more questions; I addressed only some of those with my last paragraph. You could break up your question into multiple questions. –  jmarina Oct 24 '13 at 9:48
    
In short, summarizing - we don't know how collapse of neutron star into a quark star looks like (just the border conditions for that to occur), and we have no clue about these collapsing into black holes, at all. Right? –  SF. Oct 25 '13 at 10:34
    
I think it is safe to say that no one on this planet has observed exactly how the force that holds 3 quarks together into a neutron breaks down under pressure to free the quarks of each other, or how the force that holds the quark together breaks down to release smaller particles down to the strings; but as far as the special effects are concerned, I would definitely expect a hard radiation flash, which could possibly include some photons throughout the spectrum including visible (some GRBs do emit visible light) –  jmarina Oct 25 '13 at 14:41
    
I'll just list the inaccuracies of this answer: (i) Neutrons stars must be more massive than 1.4Msun. Not true and several are known not to be. The Chandrasekhar mass depends on composition - supernovae cores are not made of carbon (for which 1.4 Msun is appropriate). (ii) The maximum mass of a neutron star is at least 2Msun (the highest measured). We do not know how much higher, but general relativity places an upper bound of about 3Msun. (iii) Nobody knows if quark stars exist. (iv) Neutron stars are not only made of neutrons. (v) The neutrons in a neutrons star are already degenerate. –  Rob Jeffries Dec 17 at 19:23

Just to focus on one part of your question. Whilst it might be possible for a neutron star to accrete material, or for two neutrons stars to collide, in order to form black holes, this kind of event must be quite rare.

The distribution of measured neutrons star and black holes masses can be fitted with an estimated true distribution. Here it is, from Ozel et al. (2012). You can see there is a distinct gap between the highest mass neutron stars (currently the record holder has a mass of about $2M_{\odot}$ and the smallest black holes (about $5M_{\odot}$).

Neutron star and black hole mass distributions from Ozel et al. (2012).

The merging of neutron stars must happen though. The obvious example is the Hulse-Taylor binary neutron star system, where the two object are spiralling together, presumably by the emission of gravitational waves, and will merge in about 300 million years. The combined mass of the 2 neutron stars is $2.83M_{\odot}$.

Merging neutron stars (or merging neutron star + black hole binaries) are thought to be the progenitors of short duration gamma ray bursts that are generally seen in high redshift galaxies. These typically last a second or less, but involve an energy release of about $\sim 10^{44}$ J. They may produce a black hole, or perhaps a more massive neutron star. There will also be a gravitational wave signature (a "chirp") that could be detected by the next generation of gravitational wave experiments.

As for accretion onto an existing neutron star - well it looks quite rare because there may be a large gap between the highest masses at which neutron stars are produced in supernovae (maybe $1.5M_{\odot}$) and the maximum mass of a neutron star. We know that the latter is at least $2M_{\odot}$, but it could be higher, perhaps $3M_{\odot}$, the maximum allowed by General Relativity. As to the outcome of this hypothetical event, well sticking to non-speculative physics, the most likely thing to happen would be the production of massive hyperons in the neutron star core at sufficiently high densities ($>10^{18}$ kg/m$^3$), which would lead to an instability (due to the removal of degenerate neutrons that are providing the majority of support); the neutron star may then slip inside its event horizon (about 6km for a $2M_{\odot}$ neutron star) and become a black hole. Some sort of explosion seems unlikely, though a gravitational wave signature might be possible.

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