<p>In a white dwarf, the dense matter is not in its lowest energy configuration. Energy can still be extracted from the white dwarf material by fusion, provided it can be ignited.</p>

<p>What <em>exothermic</em> nuclear reactions would there be that could take place in a neutron star? The bulk of the material is in the form of neutrons with a small number of protons and electrons. At these densities, <em>that is</em> the most stable equilibrium composition. </p>

<p>If a neutron star gains mass in a gradual way, then the most likely course of events will be that its radius will decrease (that is what happens in objects supported by degenerate matter) until it reaches a General Relativistic instability where its collapse to a black hole is inevitable (when $R$ is somewhere between 1.25 and 2 times the Schwarzschild radius). It is possible that neutrons may transform before that into additional hadronic degrees of freedom or into quark matter, but these are endothermic processes that suck kinetic energy out of the neutron gas and only hasten the collapse.</p>

CC BY-SA 3.0
2016-05-03T20:20:15.003Z
What do you mean by "the dense matter is not at its lowest energy configuration"? Aren't the electrons occupying every energy level between the lowest possible one and the Fermi energy?
Sir Cumference
CC BY-SA 3.0
2016-05-03T20:47:39.467Z
@SirCumference Indeed the electrons are. But the energy density of the gas is the sum of the energy density of the nuclei plus the electrons.The energy density of the gas can in principle be lowered by fusing the carbon nuclei.
ProfRob
CC BY-SA 3.0
2016-05-04T02:07:01.850Z
I'm sorry, I still don't see why a neutron star can't undergo carbon fusion and lower its energy density. And why would energy density allow a white dwarf to explode as a type 1a supernova?
Sir Cumference
CC BY-SA 3.0
2016-05-04T05:59:23.710Z
@SirCumference (i) Because there isn't any carbon; (ii) because fusing all the carbon in a white dwarf yields more energy than the gravitational binding energy of the star.
ProfRob
CC BY-SA 3.0
2016-05-10T19:42:14.427Z
Just read that when a O-Mg-Ne white dwarf goes beyond the Chandrasekhar limit, oxygen fusion happens. Why doesn't this produce a similar result?
Sir Cumference
CC BY-SA 3.0
2016-05-10T19:50:52.837Z
@SirCumference Similar result to what?
ProfRob
CC BY-SA 3.0
2016-05-10T19:51:35.600Z
The Type 1a supernovae caused by carbon fusion in C-O white dwarves.
Sir Cumference
CC BY-SA 3.0
2016-05-10T20:13:54.423Z
@SirCumference The collapse of a O/Mg/Ne core is rapid (1 second) and results in a type of core-collapse supernova, leaving a neutron star remnant. Collapse is not a feature of type Ia thermonuclear detonations.
ProfRob
CC BY-SA 3.0
2016-05-10T20:17:06.063Z
Why would this leave a neutron star, but a carbon-oxygen white dwarf supernova wouldn't?
Sir Cumference
CC BY-SA 3.0
2016-05-10T20:30:04.407Z
@SirCumference Isn't this straying off your original question? Because C/O WDs ignite before they reach a high enough density to collapse. I have answered this at http://astronomy.stackexchange.com/questions/14740/why-does-the-chandrasekhar-limit-affect-white-dwarfs-differently/14747#14747
ProfRob
CC BY-SA 3.0
2016-05-03T20:14:36.073Z
2016-05-03T20:14:36.073Z
ProfRob
<p>Beyond the Chandrasekhar limit, white dwarfs become extremely hot. As a result, previously unfusable carbon can become fusable, causing nuclear reactions. This leads to a thermal runaway and ultimately a type Ia supernova.</p>
<p>However, this doesn't seem to happen in neutron stars. They can keep gaining mass until they become black holes. After a certain point, shouldn't the immense temperatures cause a thermal runaway like in white dwarfs, leading to their explosion?</p>

<supernova><stellar-evolution><neutron-star><temperature><degenerate-matter>
Why can't neutron stars ignite and explode?
Sir Cumference
# Why can't neutron stars ignite and explode?

(Asked by: username_0 on 2016-05-03)

Beyond the Chandrasekhar limit, white dwarfs become extremely hot. As a result, previously unfusable carbon can become fusable, causing nuclear reactions. This leads to a thermal runaway and ultimately a type Ia supernova.

However, this doesn't seem to happen in neutron stars. They can keep gaining mass until they become black holes. After a certain point, shouldn't the immense temperatures cause a thermal runaway like in white dwarfs, leading to their explosion?

**Tags:** supernova, stellar-evolution, neutron-star, temperature, degenerate-matter


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## 1 answers

### Answer by username_1 on 2016-05-03. Score: 14 | Accepted answer

In a white dwarf, the dense matter is not in its lowest energy configuration. Energy can still be extracted from the white dwarf material by fusion, provided it can be ignited.

What *exothermic* nuclear reactions would there be that could take place in a neutron star? The bulk of the material is in the form of neutrons with a small number of protons and electrons. At these densities, *that is* the most stable equilibrium composition.

If a neutron star gains mass in a gradual way, then the most likely course of events will be that its radius will decrease (that is what happens in objects supported by degenerate matter) until it reaches a General Relativistic instability where its collapse to a black hole is inevitable (when $R$ is somewhere between 1.25 and 2 times the Schwarzschild radius). It is possible that neutrons may transform before that into additional hadronic degrees of freedom or into quark matter, but these are endothermic processes that suck kinetic energy out of the neutron gas and only hasten the collapse.

> **Comments:**
> **username_0**: What do you mean by "the dense matter is not at its lowest energy configuration"? Aren't the electrons occupying every energy level between the lowest possible one and the Fermi energy?  
> **username_1**: @username_0 Indeed the electrons are. But the energy density of the gas is the sum of the energy density of the nuclei plus the electrons.The energy density of the gas can in principle be lowered by fusing the carbon nuclei.  
> **username_0**: I'm sorry, I still don't see why a neutron star can't undergo carbon fusion and lower its energy density. And why would energy density allow a white dwarf to explode as a type 1a supernova?  
> **username_1**: @username_0 (i) Because there isn't any carbon; (ii) because fusing all the carbon in a white dwarf yields more energy than the gravitational binding energy of the star.  
> **username_0**: Just read that when a O-Mg-Ne white dwarf goes beyond the Chandrasekhar limit, oxygen fusion happens. Why doesn't this produce a similar result?  
> **username_1**: @username_0 Similar result to what?  
> **username_0**: The Type 1a supernovae caused by carbon fusion in C-O white dwarves.  
> **username_1**: @username_0 The collapse of a O/Mg/Ne core is rapid (1 second) and results in a type of core-collapse supernova, leaving a neutron star remnant. Collapse is not a feature of type Ia thermonuclear detonations.  
> **username_0**: Why would this leave a neutron star, but a carbon-oxygen white dwarf supernova wouldn't?  
> **username_1**: @username_0 Isn't this straying off your original question? Because C/O WDs ignite before they reach a high enough density to collapse. I have answered this at http://astronomy.stackexchange.com/questions/14740/why-does-the-chandrasekhar-limit-affect-white-dwarfs-differently/14747#14747  

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