Neutron stars

[eoinmac]

Just need some general info on neutron stars, wikipedia's page was a little too advanced and need something a little more basic please. Their compostion, importance in cosmology, and other useful info would help, thanks!

[mulreay]

Neutron stars are one of the most highly dense (viewable) objects in the universe. A tea spoon of Neutron star would way approx 1 billion tonnes. These stars are remnants of a massive star (4-8 times the size of our sun) which has (died). The gravitational field of said star can be up to the magnitude of 2 x 10 to the power of 11. With a possible magnetic field a million times greater than earth.

These are generally super nova remnants (stars that have blown off there outer layer) when the star has burnt up all it's nuclear fuel.

Hope this helps... if you need more help please ask

[eoinmac]

Thanks for the response, do you know what is the state of matter on the neutron stars? Is it a liquid, gas, solid, plasma or what? And why have they not turned into black holes?

[Sisyphus]

Thanks for the response, do you know what is the state of matter on the neutron stars? Is it a liquid, gas, solid, plasma or what? And why have they not turned into black holes?

 

There is a lot of information here:

 

http://en.wikipedia.org/wiki/Neutron_star

 

Particularly the "structure" section.

 

They haven't turned into black holes because their gravity is not strong enough. A black hole is an object whose gravity is so strong that no other force can prevent it from collapsing into itself. In a neutron star, the forces that prevent neutrons from occupying the same location are still stronger than the gravity compressing it, so it can't collapse completely.

[eoinmac]

Seems like little is sure about them at the moment... Interesting objects though! Has anyone heard anything about the theorized life cycle of a neutron star? I wonder if at the end of a neutron star's life there is a chance for a black hole to form

[Royston]

Seems like little is sure about them at the moment...

 

True, the evidence for neutron stars is very indirect, so it's more of an observational issue.

 

However there is a wealth of information you can grab from light curves, and it's a case of back tracking i.e we have available solar models (a culmination of observation, lab experiments and maths) and the emission detected from these bodies. So certain systems will give a distinctive light curve, which agree with current models. There's always exceptions of course.

 

Has anyone heard anything about the theorized life cycle of a neutron star?

 

Well, being a stellar remnant and due to the longevity of stellar objects, trying to pin down the death of a neutron star is very hard. I can expand on that if you want.

 

I wonder if at the end of a neutron star's life there is a chance for a black hole to form

 

Binary systems are common, so for a neutron star with a high mass donor (the donor being a higher mass orbiting star) i.e the neutron star is gaining mass due to the donor overfilling it's Roche lobe, it could in principle, gain enough mass to form a black hole, depending on the stability of the system. Any terms you're not sure of, just google or ask.

Edited June 25, 2010 by Snail

[eoinmac]

Thanks snail, do you know more about the end of a theorized neutron star? Would there be a super nova explosion?

[iNow]

You may learn more by reviewing sites such as this:

 

http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_deathhigh.html#sdhighneutron

 

http://mitworld.mit.edu/video/158

[eoinmac]

 

http://mitworld.mit.edu/video/158

 

 

Awesome site, thanks!

[Airbrush]

Thanks snail, do you know more about the end of a theorized neutron star? Would there be a super nova explosion?

 

No, neutron stars never super nova, I think. Neutron stars are in their final state, and will just remain the same while slightly evaporating over quadrillions of years as they lose energy. Or, if left alone by other neutron stars or black holes, can neutron stars change form in any way over the LONG term?

[Moontanman]

I would think they could loose heat, a neutron star should radiate away it's heat quickly due the it being so small.

[Ophiolite]

I would think they could loose heat, a neutron star should radiate away it's heat quickly due the it being so small.

Whoa! If it is small it has less surface area from which to radiate. You are thinking in terms of rabbits losing temperature faster than elephants because of body size. That's a conduction/heat generation thing, not a radiation thing.

[Moontanman]

Whoa! If it is small it has less surface area from which to radiate. You are thinking in terms of rabbits losing temperature faster than elephants because of body size. That's a conduction/heat generation thing, not a radiation thing.

 

I disagree take a steel ball the size of a BB, a golf ball, and a soccer ball, heat them in a kiln till the are white hot and let them cool off in a vacuum. the BB will cool off quicker than the golf ball sized ball and Much quicker than the soccer ball sized steel ball.

[insane_alien]

but the soccer sized steel ball will radiate heat faster due to its larger surface area.

 

the BB just contains less heat is all.

 

a neutron star on the otherhand has the same heat as a stellar core but a far smaller surface area due to its immense density therefore it cools slower.

[Ophiolite]

a neutron star on the otherhand has the same heat as a stellar core but a far smaller surface area due to its immense density therefore it cools slower.

Exactly so.

[Moontanman]

Again, I disagree, try this thought experiment, take the before mentioned steel balls and bring them to the ambient temperature. transfer exactly 100 watts of energy into each one and allow them to cool back to ambient in a vacuum, the tiny ball will still cool to ambient faster than the other two and the medium sized one will cool faster than the big one but slower than the small one. The small object has far more surface area per cubic unit of volume than the bigger objects....

Edited July 1, 2010 by Moontanman
sp

[insane_alien]

Again, I disagree, try this thought experiment, take the before mentioned steel balls and bring them to the ambient temperature. transfer exactly 100 watts of energy into each one and allow them to cool back to ambient in a vacuum, the tiny ball will still cool to ambient faster than the other two and the medium sized one will cool faster than the big one but slower than the small one. The small object has far more surface area per cubic unit of volume than the bigger objects....

 

actually with 100 watts the smaller ball will be hotter than the larger ball.

 

and again, neutron stars and stellar cores do not have the same heat capacity as stellar cores. so using a comparison of two identically composed materials isn't really appropriate.

 

EDI: whoops, forgot to address the main point(i'm a bit tired)

 

anyway, the neutron star will have more thermal energy due to collapse so it will be much hotter which will give it an initial faster cooling rate however, lets look at a simple case.

 

we have a neutron star and a dead stellar core. same mass, same thermal energy and it should be approximately the same heat capacity. so roughly the same temperature (lets say to within 10000K)

 

the neutron star will take longer to reach ambient as it has less thermal transfer area. this is the point, the smaller the area for thermal transfer, the less heat will be transfered.

Edited July 1, 2010 by insane_alien

[Moontanman]

So the cube square law does not apply?

[insane_alien]

the cube-square laws are only appropriate for things with the same density.

[Moontanman]

the cube-square laws are only appropriate for things with the same density.

 

To be honest with you I thought that was what I was saying... But it see your point, a neutron star would not cool as fast as a less dense object of similar size....

[eoinmac]

radiating away will take a very long time for a neutron star right?

[insane_alien]

yes, a very long time. as in, much longer than the age of the universe before it gets close to equilibrium.

[Moontanman]

yes, a very long time. as in, much longer than the age of the universe before it gets close to equilibrium.

 

I'm not sure what you are getting at with this IA, it will take many billions of years for the earth to cool to the same temp as the cosmic backround, neutron stars cool very fast, millions of degrees in just a 1000 years.

 

http://en.wikipedia.org/wiki/Neutron_star

 

The temperature inside a newly formed neutron star is from around 1011 to 1012 kelvin.[5] However, the huge number of neutrinos it emits carries away so much energy that the temperature falls within a few years to around 1 million kelvin.[5] Even at 1 million kelvin, most of the light generated by a neutron star is in X-rays. In visible light, neutron stars probably radiate approximately the same energy in all parts of visible spectrum, and therefore appear white.

 

http://www.astro.umd.edu/~miller/nstar.html

 

Thermal history

At the moment of a neutron star's birth, the nucleons that compose it have energies characteristic of free fall, which is to say about 100 MeV per nucleon. That translates to 10^12 K or so. The star cools off very quickly, though, by neutrino emission, so that within a couple of seconds the temperature is below 10^11 K and falling fast. In this early stage of a neutron star's life neutrinos are produced copiously, and since if the neutrinos have energies less than about 10 MeV they sail right through the neutron star without interacting, they act as a wonderful heat sink. Early on, the easiest way to produce neutrinos is via the so-called "URCA" processes: n->p+e+(nu) [where (nu) means an antineutrino] and p+e->n+nu. If the core is composed of only "ordinary" matter (neutrons, protons, and electrons), then when the temperature drops below about 10^9 K all particles are degenerate and there are so many more neutrons than protons or electrons that the URCA processes don't conserve momentum, so a bystander particle is required, leading to the "modified URCA" processes n+n->n+p+e+(nu) and n+p+e->n+n+nu. The power lost from the neutron stars to neutrinos due to the modified URCA processes goes like T^8, so as the star cools down the emission in neutrinos drops sharply.

 

When the temperature has dropped far enough (probably between 10 and 10,000 years after the birth of the neutron star), processes less sensitive to the temperature take over. One example is standard thermal photon cooling, which has a power proportional to T^4. Another example is thermal pair bremsstrahlung in the crust, where an electron passes by a nucleus and, instead of emitting a single photon as in standard bremsstrahlung, emits a neutrino-antineutrino pair. This has a power that goes like T^6, but its importance is uncertain. In any case, the qualitative picture of "standard cooling" that has emerged is that the star first cools by URCA processes, then by modified URCA, then by neutrino pair bremsstrahlung, then by thermal photon emission. In such a picture, a 1,000 year old neutron star (like the Crab pulsar) would have a surface temperature of a few million degrees Kelvin.

 

Near the center of a neutron star, depending on the equation of state the density can get up to several times nuclear density. This is a regime that we can't explore on Earth, because the core temperatures of 10^9 K that are probably typical of young neutron stars are actually cold by nuclear standards, since in accelerators when high densities are produced it's always by smashing together particles with high Lorentz factors. Here, the thermal energies of the particles are much less than their rest masses. Anyway, that leaves us with only theoretical predictions, which (as you might expect given the lack of data to guide us) vary a lot. Some people think that strange matter, pion condensates, lambda hyperons, delta isobars, or free quark matter might form under those conditions, and it seems to be a general rule that no matter what the weird stuff is, if you have exotic matter then neutrino cooling processes proportional to T^6 can exist, which would mean that the star would cool off much faster than you thought. It even appears possible in some equations of state that the proton and electron fraction in the core may be high enough that the URCA process can operate, which would really cool things down in a hurry. Adding to the complication is that the neutrons probably form a superfluid (along with the protons forming a superconductor!), and depending on the critical temperature some of the cooling processes may get cut off.

[insane_alien]

I'm not sure what you are getting at with this IA, it will take many billions of years for the earth to cool to the same temp as the cosmic backround, neutron stars cool very fast, millions of degrees in just a 1000 years.

 

http://en.wikipedia.org/wiki/Neutron_star

 

cooling slows down as the temperature drops. yes, it drops like a stone initially, but if it kept up that extreme rate of cooling it'd hit zero K in a few thousand years.

 

it follows an exponential curve. and a neutron star will cool much slower than earth when it gets down to a similar temperature. due to its MUCH smaller surface area.

 

these neutron stars are only about 10km across. that makes their surface area tiny. and rate of cooling is directly proportional to surface area.

[Moontanman]

So if you had a neutron star that was as big as the earth (diameter, i know this is not possible) it would cool faster than a neutron star 10 miles in diameter? This would violate the cube square law, a small object has more surface area per unit of volume than a large object.