<p>No, it's not stable without the huge pressure from gravity. There is a minimum stable size, but it's definitely much greater than a spoon.</p>

CC BY-SA 3.0
2015-04-28T00:43:49.150Z
Would the neutrons go back to being an electron and a proton? Or is that not possible because the neutrinos that were created at the same time are no longer around?
matryoshka
CC BY-SA 3.0
2015-04-28T01:25:31.073Z
@Grace, free neutrons spontaneously decay to protons and electrons and neutrinos. Look up beta decay for more information.
Spencer
CC BY-SA 3.0
2015-04-28T08:12:48.023Z
@Spencer free neutrons in a *low density* environment spontaneously decay into protons and electrons. They don't in neutron star material (or not readily).
ProfRob
CC BY-SA 3.0
2015-04-28T20:49:03.417Z
@RobJeffries, thats true but the environment is not likely to stay in its high density state without the gravitational force of a full neutron star there to keep it that way.
Spencer
CC BY-SA 3.0
2015-04-27T23:37:24.240Z
2015-04-27T23:37:24.240Z
Florin Andrei
<p>If we take neutron star material and somehow transport it somewhere for examination (say the Earth!), the results would be catastrophic. At say a density of $\sim 10^{17}$ kg/m$^{3}$ the neutrons have a number density of $\sim 6\times 10^{43}$ m$^{-3}$ an internal kinetic energy density of $3 \times 10^{32}$ J/m$^{3}$ (calculated using the relevant equations for an ideal gas of degenerate neutrons at this density). So even in a tablespoonful (say 20ml, which would have a mass of 2 billion tonnes!), there is $6\times10^{27}$ J of kinetic energy (15 times more than the Sun emits in a second, or a few billion atom bombs) and this will be released <strong>instantaneously</strong>.  </p>

<p>The energy is in the form of around $10^{39}$ neutrons travelling at around 0.1-0.2$c$. So roughly speaking it is like half the neutrons (about a billion tonnes) travelling at 0.1$c$ ploughing into the Earth. If I have done my Maths right, that is roughly equivalent to a 50km radius near-earth asteroid hitting the Earth at 30 km/s.</p>

<p>The neutrons in a dense neutron star gas are relatively stable (beta decay is blocked by electron degeneracy). The expansion described above would allow beta decay into protons and electrons, but as this happens on timescales of 10 minutes, it is hardly relevant to the initial destruction. However, you would end up after a few tens of minutes with an expanding cloud of ionised hydrogen a few light minutes across.</p>

<p>The minimum possible size to gravitationally bind neutron star material is thought to be around $0.15 M_{\odot}$ (see <a href="https://physics.stackexchange.com/questions/143166/what-is-the-theoretical-lower-mass-limit-for-a-gravitationally-stable-neutron-st/143174#143174">here</a>). The equilibrium <em>electron</em> density (there are always some electrons and protons present in neutron star material) for lower masses is too low to block neutron beta-decay.</p>

CC BY-SA 3.0
2016-05-24T15:50:02.433Z
Impressive. This should be in the next "What If" installment.
Peter - Reinstate Monica
CC BY-SA 3.0
2015-09-20T13:33:05.620Z
What if we picked *inert* neutronium? (btw, "A free neutron will decay with a half-life of about 10.3 minutes"[->](http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html) so it shouldn't blow up all instantly... although the "decay radiation" intensity of a spoon of this material would probably overshadow any atom bombs even without chain reaction.
SF.
CC BY-SA 3.0
2015-09-20T13:46:34.443Z
@SF You have completely misunderstood my answer. The material blows up (expands at a significant fraction of $c$) because of its massive degeneracy pressure and hence kinetic energy. Nothing to do with neutron decay at all. What is "inert neutronium"?
ProfRob
CC BY-SA 3.0
2015-09-20T13:50:28.910Z
Okay, now I'm getting it - you confused me with "kinetic energy" which is usually associated with *motion* while in this case it's more of a sub-atomic counterpart of elasticity.
SF.
CC BY-SA 3.0
2015-09-20T14:50:28.833Z
@SF No it's motion. Pressure, kinetic theory etc. The neutrons have huge momenta.
ProfRob
CC BY-SA 3.0
2015-04-28T08:11:43.380Z
2016-12-29T17:43:53.457Z
2017-04-13T12:40:15.377Z
ProfRob
<p>I've heard people say that a tablespoon of neutron star would weigh over a billion tons. If we ever <em>could</em> take a tablespoon of one would it still remain intact with the same density? </p>

<astrophysics><neutron-star>
Would a tablespoon of a neutron star remain intact?
matryoshka
# Would a tablespoon of a neutron star remain intact?

(Asked by: username_0 on 2015-04-27)

I've heard people say that a tablespoon of neutron star would weigh over a billion tons. If we ever *could* take a tablespoon of one would it still remain intact with the same density?

**Tags:** astrophysics, neutron-star


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## 2 answers

### Answer by username_3 on 2015-04-28. Score: 14 | Accepted answer

If we take neutron star material and somehow transport it somewhere for examination (say the Earth!), the results would be catastrophic. At say a density of $\sim 10^{17}$ kg/m$^{3}$ the neutrons have a number density of $\sim 6\times 10^{43}$ m$^{-3}$ an internal kinetic energy density of $3 \times 10^{32}$ J/m$^{3}$ (calculated using the relevant equations for an ideal gas of degenerate neutrons at this density). So even in a tablespoonful (say 20ml, which would have a mass of 2 billion tonnes!), there is $6\times10^{27}$ J of kinetic energy (15 times more than the Sun emits in a second, or a few billion atom bombs) and this will be released **instantaneously**.

The energy is in the form of around $10^{39}$ neutrons travelling at around 0.1-0.2$c$. So roughly speaking it is like half the neutrons (about a billion tonnes) travelling at 0.1$c$ ploughing into the Earth. If I have done my Maths right, that is roughly equivalent to a 50km radius near-earth asteroid hitting the Earth at 30 km/s.

The neutrons in a dense neutron star gas are relatively stable (beta decay is blocked by electron degeneracy). The expansion described above would allow beta decay into protons and electrons, but as this happens on timescales of 10 minutes, it is hardly relevant to the initial destruction. However, you would end up after a few tens of minutes with an expanding cloud of ionised hydrogen a few light minutes across.

The minimum possible size to gravitationally bind neutron star material is thought to be around $0.15 M\_{\odot}$ (see [here](https://physics.stackexchange.com/questions/143166/what-is-the-theoretical-lower-mass-limit-for-a-gravitationally-stable-neutron-st/143174#143174)). The equilibrium *electron* density (there are always some electrons and protons present in neutron star material) for lower masses is too low to block neutron beta-decay.

> **Comments:**
> **username_5**: What if we picked *inert* neutronium? (btw, "A free neutron will decay with a half-life of about 10.3 minutes"[->](http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html) so it shouldn't blow up all instantly... although the "decay radiation" intensity of a spoon of this material would probably overshadow any atom bombs even without chain reaction.  
> **username_3**: @SF You have completely misunderstood my answer. The material blows up (expands at a significant fraction of $c$) because of its massive degeneracy pressure and hence kinetic energy. Nothing to do with neutron decay at all. What is "inert neutronium"?  
> **username_5**: Okay, now I'm getting it - you confused me with "kinetic energy" which is usually associated with *motion* while in this case it's more of a sub-atomic counterpart of elasticity.  
> **username_3**: @SF No it's motion. Pressure, kinetic theory etc. The neutrons have huge momenta.  
> **username_4**: Impressive. This should be in the next "What If" installment.  

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### Answer by username_1 on 2015-04-27. Score: 5

No, it's not stable without the huge pressure from gravity. There is a minimum stable size, but it's definitely much greater than a spoon.

> **Comments:**
> **username_0**: Would the neutrons go back to being an electron and a proton? Or is that not possible because the neutrinos that were created at the same time are no longer around?  
> **username_2**: @Grace, free neutrons spontaneously decay to protons and electrons and neutrinos. Look up beta decay for more information.  
> **username_3**: @username_2 free neutrons in a *low density* environment spontaneously decay into protons and electrons. They don't in neutron star material (or not readily).  
> **username_2**: @RobJeffries, thats true but the environment is not likely to stay in its high density state without the gravitational force of a full neutron star there to keep it that way.  

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