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  • What makes it burn, what kind of fusion/decay is happening there ?
  • Another question is, what suppose to happen with neutron star on long run ? What if it cools, then what degenerated matter looks like after it cools ? will the gravitational equilibrium be ruined after some burn time ? how it explodes if it can explode at all ?

Thank you

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9  
It doesn't burn. –  Cedric H. Nov 11 '10 at 13:02
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@cedric, you should post that as an answer –  Grant Crofton Nov 11 '10 at 16:49
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3 Answers

up vote 9 down vote accepted

With normal stars, the "burning", that is to say the fusion reaction, produces a pressure that counteracts the pull of gravity to keep the star from collapsing. But with neutron stars, the protons and electrons in the star have combined into neutrons*.

The Pauli exclusion principle causes the neutrons to resist further compression. That is, the neutrons, being identical fermions, can't all be put in the same state. So to get them closer and closer together you have to go into higher and higher energy states. Thus, there is an energy cost in compressing the star, and this results in a sort of pressure called "degeneracy pressure".

It is this pressure that stabilizes the neutron star against collapse (assuming it doesn't have enough mass to overcome this pressure and become a black hole). So they don't need to "burn" to maintain their stability, and so far as I know, they don't. At least not in the sense of a normal star where you have atomic nuclei fusing.

  • Note: Neutrons aren't made of protons and elections, but this transformation can happen by means of the weak nuclear force. Normally neutrons aren't stable outside of the atomic nucleus -- instead the transformation would go the other way and a free neutron would decay into a proton and electron (there's also an anti-electron neutrino produced). But under the intense gravitational pressure in a collapsed star, the neutrons are stable, which allows us to end up with neutron stars.

Edit: This is of course a very approximate picture. The link posted by Thomas Thernel has much more detail. One good point to emphasize is that, as you might expect, the density is greater at the center of a neutron star than at its outskirts, so the star won't really be all neutrons... you'll have more neutrons closer to the center, and more ordinary atomic nuclei further out. Apparently some interesting sorts of structures can form from the remaining nuclei, even at the point where it's 90-95% neutrons.

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Thank you for the answer. –  user299 Nov 12 '10 at 4:50
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I think this is a good almost-popular introduction to neutron stars, the processes expected to occur therein and their evolution.

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Thanks for the link. I had a rough idea of how neutron stars work (as given in my answer), but this business of "spaghetti" and "lasagna" like structures is really fascinating. –  Tim Goodman Nov 11 '10 at 19:00
    
Thank you. Nice link –  user299 Nov 12 '10 at 0:52
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Tim Goodman's answer is right, and I just want to add a couple of things:

Though Neutron stars do not "burn" anything, for the most part, they DO shine, thanks to the fact that they are the remnants of the core of a star, and thus, when they are born, are roughly as hot as the center of a star. They will thus shine in the same way that a hot poker shines when you stick it in a fire.

Second, neutron stars have a maximum possible mass, dictated by the rules of general relativity and the mass of a neutron. If a neutron star has extra mass beyond this dumped on its surface (say, because it is orbiting an ordinary star, and that star accretes mass onto the NS), they can explode in a violent way. To my understanding, this has not been observed, but it has been known to happen in White Dwarfs, which are held together by a very similar principle to neutron stars. The end result is a supernova, and then a black hole.

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Thank you. I assume they stay cold forever, when nothing happens –  user299 Nov 14 '10 at 20:13
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Yes, that's right. They just cool off and reach a final, static state at the same temperature as the space around them. –  Jerry Schirmer Nov 14 '10 at 21:41
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