<p>Just to focus on one part of your question. Whilst it might be possible for a neutron star to accrete material, or for two neutron stars to collide, in order to form black holes, this kind of event must be quite rare (although see below).</p>
<p>The distribution of measured neutron star and black holes masses can be fitted with an estimated true distribution. Here it is, from <a href="http://adsabs.harvard.edu/abs/2012ApJ...757...55O" rel="nofollow noreferrer">Ozel et al. (2012)</a>. You can see there is a distinct gap between the highest mass neutron stars (currently the record holder has a mass of about <span class="math-container">$2M_{\odot}$</span> and the smallest black holes (about <span class="math-container">$5M_{\odot}$</span>). This confirms slightly earlier work by <a href="http://iopscience.iop.org/0004-637X/741/2/103/article#apj403391s5" rel="nofollow noreferrer">Farr et al. (2011)</a>.</p>
<p><img src="https://i.sstatic.net/wkZQn.png" alt="Neutron star and black hole mass distributions from Ozel et al. (2012)." /></p>
<p>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 <span class="math-container">$2.83M_{\odot}$</span>, but the mass of any black hole they create would be lower, with the difference radiated away as neutrinos and gravitational waves.</p>
<p>Merging neutron stars (or merging neutron star + black hole binaries) are thought to be the progenitors of <a href="http://en.wikipedia.org/wiki/Gamma-ray_burst#Short_gamma-ray_bursts" rel="nofollow noreferrer">short duration gamma ray bursts</a> or so-called <a href="https://en.wikipedia.org/wiki/Kilonova" rel="nofollow noreferrer">Kilonova events</a> that are generally seen in high redshift galaxies. These typically last a second or less, but involve an energy release of about <span class="math-container">$\sim 10^{44}$</span> J. They may produce a black hole, or perhaps a more massive neutron star. There will also be a gravitational wave signature (a &quot;chirp&quot;) that could be detected by the next generation of gravitational wave experiments (now a reality). These black holes may be isolated and hence not represented in the mass distribution above. A further observational signature of these events may be in the form of the current levels of a number of heavy r-process elements, like Iridium and Gold, that <em>may</em> mostly be produced in these events.</p>
<p>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 <span class="math-container">$1.5M_{\odot}$</span>) and the maximum mass of a neutron star. We know that the latter is at least <span class="math-container">$2M_{\odot}$</span>, but it could be higher, perhaps <span class="math-container">$3M_{\odot}$</span>, 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 (<span class="math-container">$&gt;10^{18}$</span> kg/m<span class="math-container">$^3$</span>), 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 <span class="math-container">$2M_{\odot}$</span> neutron star) and become a black hole. Some sort of explosion seems unlikely, though a gravitational wave signature might be possible.</p>
<p>EDIT: An update on the NS/BH mass distribution above. I saw a talk recently at a conference - the explanation of the distribution has two broad thrusts; either the black holes are not produced in this mass range because of the physics of the progenitors, or there is a strong observational bias against seeing them. An example of the former explanation can be found in <a href="http://adsabs.harvard.edu/abs/2014ApJ...785...28K" rel="nofollow noreferrer">Kochanek (2014)</a>, who proposes that there is a class of &quot;failed supernovae&quot; between 16 and 25<span class="math-container">$M_{\odot}$</span> that do manage to eject their envelopes in <em>weak</em> transient events, but leave behind their helium cores to form the lowest mass 5-8<span class="math-container">$M_{\odot}$</span> black holes. Lower mass progenitors are then responsible for the neutron stars.</p>
<p>The observational bias is that the companions to the lowest mass black holes in binary systems may be <em>always</em> overflowing their Roche lobes. The resultant accretion signature swamps the companion spectrum and prevents a dynamical mass estimate (e.g. <a href="http://adsabs.harvard.edu/abs/1999ApJ...522..413F" rel="nofollow noreferrer">Fryer 1999</a>). The <a href="https://personal.sron.nl/%7Epeterj/gbs/survey.html" rel="nofollow noreferrer">Chandra Galactic Bulge Survey</a>
is attempting to find examples of quiescent, relatively low X-ray luminosity, eclipsing compact binaries, with which to measure a more unbiased black hole mass distribution.</p>
<p>Further Edit: There continue to be challenges and claims that there are &quot;low-mass&quot; black holes that could be formed via accretion-induced collapse of a neutron star (pointed out by Alexandra Veledina). For instance Cygnus-X3 has a claimed mass of <span class="math-container">$2.4^{+2.1}_{-1.1}\ M_{\odot}$</span> according to <a href="http://adsabs.harvard.edu/abs/2013MNRAS.429L.104Z" rel="nofollow noreferrer">Zdziarski et al. 2013</a>, but these observations lack the precision to be really sure yet.</p>
<p>Addition 30/6/20:</p>
<p>A <em>compact object</em> has now been found with a precise mass measurement between <span class="math-container">$2.5-2.67 M_{\odot}$</span>. It was detected through a gravitational wave signature during its merger with a much more massive black hole. This object was either the most massive neutron star found yet, or the least massive black hole (<a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab960f" rel="nofollow noreferrer">Abbott et al. 2020</a>).</p>

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2020-06-30T17:35:54.353Z
Thanks for these updates, Rob.
PM 2Ring
CC BY-SA 4.0
2014-12-17T20:22:17.710Z
2023-10-01T13:27:03.577Z
2023-10-01T13:27:03.577Z
ProfRob
<p>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 <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit" rel="nofollow noreferrer">Chandrasekhar limit</a> on wikipedia for details.</p>

<p>A neutron star is formed during a <a href="http://en.wikipedia.org/wiki/Supernova" rel="nofollow noreferrer">supernova</a>, an explosion of a star that is at least 8 solar masses.</p>

<p>The maximum mass of a neutron star is 3 solar masses. If it gets more massive than that, then it will collapse into a <a href="https://en.wikipedia.org/wiki/Quark_star" rel="nofollow noreferrer">quark star</a>, and then into a black hole.</p>

<p>We know that 1 electron + 1 proton = 1 neutron;</p>

<p>1 neutron = 3 quarks = up quark + down quark + down quark;</p>

<p>1 proton = 3 quarks = up quark + up quark + down quark;</p>

<p>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.</p>

<p>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.</p>

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

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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, <a href="http://en.wikipedia.org/wiki/Kerr_metric" rel="nofollow noreferrer">rotating-uncharged</a>, non-rotating-charged, non-rotating-uncharged.</p>

<p>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).</p>

<p>We know that atoms break up into protons, neutrons, electrons under pressure.</p>

<p>Under more pressure, protons and electrons combine into neutrons.</p>

<p>Under even more pressure, neutrons break down into quarks.</p>

<p>Under still more pressure, perhaps quarks break down into still smaller particles.</p>

<p>Ultimately the smallest particle is a <a href="http://en.wikipedia.org/wiki/String_theory" rel="nofollow noreferrer">string</a>: 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.</p>

<p>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 <strong>Elegant Universe</strong> by Brian Greene.</p>

<p>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.</p>

<p>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 <a href="http://en.wikipedia.org/wiki/Hawking_radiation" rel="nofollow noreferrer">evaporate</a>, 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 <a href="http://en.wikipedia.org/wiki/Schwarzschild_radius" rel="nofollow noreferrer">Schwarzschild radius</a> equation even tells you what the radius of a black hole is given its mass, and vice versa.</p>

<p>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).</p>

<p>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.</p>

<p>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 <a href="http://en.wikipedia.org/wiki/Cataclysmic_variable" rel="nofollow noreferrer">the neutron star siphons off gas from the other star</a>, thus gaining mass.</p>

<p><img src="https://i.sstatic.net/VCcfj.jpg" alt="Cataclysmic variable binary"></p>

<p>Here is a nice drawing showing exactly that.</p>

<p>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.</p>

CC BY-SA 3.0
2014-12-21T22:20:47.543Z
Might I just mention two things: First off yes maybe his answer had some figures that included exceptions but I don't see why this requires in depth listing. Secondly I must mention that you are supporting string theory as if it is incontrovertible, which I am afraid is not true in the slightest. It is, of coarse, a legitimate theory, however you really must mention that.
trevorKirkby
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2015-06-02T04:00:23.567Z
I just want to add, what I find an interesting point here.   The largest observed Neutron Star is about 3 solar masses (and much more than that would get very close to the Schwarzchild radius, as the more mass you add, the smaller the Neutron star gets) and the smallest observed black holes are over 4 solar masses.   There is, as far as I know, still an unknown as to what happens between those masses, at least no observed examples, though I would think it's likely, too much more than 3 solar masses, a Neutron Star would collapse into a black hole.
userLTK
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2015-06-02T06:20:21.580Z
@userLTK The largest measured neutron star mass is 2 solar masses. The gap you comment on is dealt with in my answer and there are at least two classes of explanation for it. Neutron stars have a GR instability that makes them collapse well before they get near their Schwarzschild radii.
ProfRob
CC BY-SA 3.0
2014-12-17T19:23:02.043Z
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.
ProfRob
CC BY-SA 3.0
2014-12-17T19:32:09.707Z
(vi) Black holes appear observationally to have a minimum mass of about 4-5Msun (Ozel at al. 2012). (vii) The minimum stable mass for a black hole is definitely not 3Msun. (viii) GRBs are not caused by matter falling into black holes (or provide a reference for any work that says so). (ix) Black hole evaporation *may* be relevant for micro blackholes it is utterly irrelevant for stellar-sized black holes. (x) The paragraph about the pencil vanishing in a flash is nonsense.
ProfRob
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2017-04-29T14:28:43.697Z
I don't know why you speculate about possible components of quarks. Quarks (and leptons) are fundamental in the Standard Model, there's no evidence that they are composite particles. And even in string theory, a quark isn't _made_ of strings, it _is_ a string in a particular vibrational mode.
PM 2Ring
CC BY-SA 3.0
2013-10-24T09:48:50.153Z
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
CC BY-SA 3.0
2013-10-25T10:34:43.213Z
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.
CC BY-SA 3.0
2013-10-25T14:41:01.743Z
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
CC BY-SA 3.0
2013-10-24T09:22:33.430Z
2017-06-29T14:29:28.223Z
2017-06-29T14:29:28.223Z
jmarina
<p>We know the spectacular explosions of supernovae, that when massive 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.</p>
<p>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.</p>
<p>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?</p>

CC BY-SA 3.0
2016-03-25T23:26:26.660Z
Interesting. But the uncertainties on the masses still allow them to be 4 solar masses or more in both cases. It is clearly a subject with a lot more work to be done and these two papers both provide an interesting discussion along the lines of what I present in my answer.
ProfRob
<black-hole><neutron-star><astrophysics>
How does a neutron star collapse into a black hole?
SF.
# How does a neutron star collapse into a black hole?

(Asked by: username_0 on 2013-10-23)

We know the spectacular explosions of supernovae, that when massive 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?

**Tags:** black-hole, neutron-star, astrophysics
> **Comments:**
> **username_1**: Interesting. But the uncertainties on the masses still allow them to be 4 solar masses or more in both cases. It is clearly a subject with a lot more work to be done and these two papers both provide an interesting discussion along the lines of what I present in my answer.  

---

## 2 answers

### Answer by username_3 on 2013-10-24. Score: 24 | Accepted answer

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](https://en.wikipedia.org/wiki/Chandrasekhar_limit) on wikipedia for details.

A neutron star is formed during a [supernova](http://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/Kerr_metric), 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](http://en.wikipedia.org/wiki/String_theory): 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](http://en.wikipedia.org/wiki/Hawking_radiation), 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](http://en.wikipedia.org/wiki/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](http://en.wikipedia.org/wiki/Cataclysmic_variable), thus gaining mass.

![Cataclysmic variable binary](https://i.sstatic.net/VCcfj.jpg)

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.

> **Comments:**
> **username_3**: 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.  
> **username_0**: 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?  
> **username_3**: 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)  
> **username_1**: 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.  
> **username_1**: (vi) Black holes appear observationally to have a minimum mass of about 4-5Msun (Ozel at al. 2012). (vii) The minimum stable mass for a black hole is definitely not 3Msun. (viii) GRBs are not caused by matter falling into black holes (or provide a reference for any work that says so). (ix) Black hole evaporation *may* be relevant for micro blackholes it is utterly irrelevant for stellar-sized black holes. (x) The paragraph about the pencil vanishing in a flash is nonsense.  
> **username_4**: Might I just mention two things: First off yes maybe his answer had some figures that included exceptions but I don't see why this requires in depth listing. Secondly I must mention that you are supporting string theory as if it is incontrovertible, which I am afraid is not true in the slightest. It is, of coarse, a legitimate theory, however you really must mention that.  
> **username_5**: I just want to add, what I find an interesting point here.   The largest observed Neutron Star is about 3 solar masses (and much more than that would get very close to the Schwarzchild radius, as the more mass you add, the smaller the Neutron star gets) and the smallest observed black holes are over 4 solar masses.   There is, as far as I know, still an unknown as to what happens between those masses, at least no observed examples, though I would think it's likely, too much more than 3 solar masses, a Neutron Star would collapse into a black hole.  
> **username_1**: @userLTK The largest measured neutron star mass is 2 solar masses. The gap you comment on is dealt with in my answer and there are at least two classes of explanation for it. Neutron stars have a GR instability that makes them collapse well before they get near their Schwarzschild radii.  
> **username_2**: I don't know why you speculate about possible components of quarks. Quarks (and leptons) are fundamental in the Standard Model, there's no evidence that they are composite particles. And even in string theory, a quark isn't _made_ of strings, it _is_ a string in a particular vibrational mode.  

---

### Answer by username_1 on 2014-12-17. Score: 24

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

The distribution of measured neutron star and black holes masses can be fitted with an estimated true distribution. Here it is, from [Ozel et al. (2012)](http://adsabs.harvard.edu/abs/2012ApJ...757...55O). 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 $5M\_{\odot}$). This confirms slightly earlier work by [Farr et al. (2011)](http://iopscience.iop.org/0004-637X/741/2/103/article#apj403391s5).

![Neutron star and black hole mass distributions from Ozel et al. (2012).](https://i.sstatic.net/wkZQn.png)

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 $2.83M\_{\odot}$, but the mass of any black hole they create would be lower, with the difference radiated away as neutrinos and gravitational waves.

Merging neutron stars (or merging neutron star + black hole binaries) are thought to be the progenitors of [short duration gamma ray bursts](http://en.wikipedia.org/wiki/Gamma-ray_burst#Short_gamma-ray_bursts) or so-called [Kilonova events](https://en.wikipedia.org/wiki/Kilonova) 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 (now a reality). These black holes may be isolated and hence not represented in the mass distribution above. A further observational signature of these events may be in the form of the current levels of a number of heavy r-process elements, like Iridium and Gold, that *may* mostly be produced in these events.

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.

EDIT: An update on the NS/BH mass distribution above. I saw a talk recently at a conference - the explanation of the distribution has two broad thrusts; either the black holes are not produced in this mass range because of the physics of the progenitors, or there is a strong observational bias against seeing them. An example of the former explanation can be found in [Kochanek (2014)](http://adsabs.harvard.edu/abs/2014ApJ...785...28K), who proposes that there is a class of "failed supernovae" between 16 and 25$M\_{\odot}$ that do manage to eject their envelopes in *weak* transient events, but leave behind their helium cores to form the lowest mass 5-8$M\_{\odot}$ black holes. Lower mass progenitors are then responsible for the neutron stars.

The observational bias is that the companions to the lowest mass black holes in binary systems may be *always* overflowing their Roche lobes. The resultant accretion signature swamps the companion spectrum and prevents a dynamical mass estimate (e.g. [Fryer 1999](http://adsabs.harvard.edu/abs/1999ApJ...522..413F)). The [Chandra Galactic Bulge Survey](https://personal.sron.nl/%7Epeterj/gbs/survey.html)
is attempting to find examples of quiescent, relatively low X-ray luminosity, eclipsing compact binaries, with which to measure a more unbiased black hole mass distribution.

Further Edit: There continue to be challenges and claims that there are "low-mass" black holes that could be formed via accretion-induced collapse of a neutron star (pointed out by Alexandra Veledina). For instance Cygnus-X3 has a claimed mass of $2.4^{+2.1}\_{-1.1}\ M\_{\odot}$ according to [Zdziarski et al. 2013](http://adsabs.harvard.edu/abs/2013MNRAS.429L.104Z), but these observations lack the precision to be really sure yet.

Addition 30/6/20:

A *compact object* has now been found with a precise mass measurement between $2.5-2.67 M\_{\odot}$. It was detected through a gravitational wave signature during its merger with a much more massive black hole. This object was either the most massive neutron star found yet, or the least massive black hole ([Abbott et al. 2020](https://iopscience.iop.org/article/10.3847/2041-8213/ab960f)).

> **Comments:**
> **username_2**: Thanks for these updates, Rob.  

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