# Neutron star

What are they?

BigBang said:
What are they?

Neutron stars are stars that result from the implosion of a very heavy star. Such neutron stars have masses around 1.5 times the mass of the sun. The implosion happens because there is a mass known as the "Chandrasekhar mass", beyond which electron degeneracy pressure cannot hold a star up from gravity. Hence gravity becomes stronger then the counter-acting electron pressure and the star implodes. This mass is around 1.4 times the mass of the sun. Once it's passed (thus, during the implosion), the electrons and protons undergo inverse beta decay and form neutrons. This leads to a star that is instead held up by neutron degeneracy pressure, called a neutron star. There is also a mass beyond which neutron degeneracy pressure will fail and the object will collapse to a black hole.

regards
marlon

That helped alot...thanx

Astronuc
Staff Emeritus
Entropy said:
Actually, the forces at the very surface of the neutron star are not strong enough to insue neutron degradation. The outer most surface of a neutron star is acutally composed of various atomic nuclei, mostly iron isotopes. Then as you barrow deeper neutrons will start to break free and form a neutron superfluid but still have protons and electrons mixed in with it. Then as you get even deeper it will eventually become entirely 100% neutrons. As for the core, it may just continue to be neutrons as well, but it might be that their is further degradation of neutrons into a type of quark matter.
So then the light emitted from the surface of a neutron star should include the emission spectrum of Fe, and perhaps other elements.

As for the surface of a neutron star, I imagine there are 'layers', e.g. there is a surface/interfaces where the neutron 'gas' starts? I am trying to visualize an interface between the neutron region and the atomic region. I wonder how atoms make the transition from atoms to neutrons?

I would imagine that neutrons near the surface do decay - probably with a half-life similar to that on earth - but could the proton interact with another electron (electron capture) and be converted back into a neutron? That would explain why all the neutrons in a neutron star do not decay (they do, but then e-capture converts the protons back to neutrons).

Thought - The significance of the high gravity is that it produces a high mass density, which means a very large macroscopic cross-section for many types of reactions/interactions.

It appears the internal structure is open to discussion? Some baryonic mass or quark matter? Could hyperons form?

I understand that neutron stars are X-ray (gamma?) emitters, because of the high temperature (~106-107 K) of surface or gas in vicinity of star?

I also found an article - Physics of Neutron Stars, G Baym, C Pethick
Annual Review of Astronomy and Astrophysics, September 1979, Vol. 17, Pages 415-443 (doi: 10.1146/annurev.aa.17.090179.002215) - has anyone reviewed it?

Danger
Gold Member
Astronuc said:
It appears the internal structure is open to discussion? Some baryonic mass or quark matter? Could hyperons form?
As I mentioned to Entropy in the thread that you kifed his quote from, I've been unexposed to the subject for over 25 years and made a goof. I'd totally forgotten about the internal structure. Someone referred once to the core as possibly 'quark soup'. I've never heard the term 'hyperon'. Whazzzattt?

Astronuc said:
I understand that neutron stars are X-ray (gamma?) emitters, because of the high temperature (~106-107 K) of surface or gas in vicinity of star?
In general, yes, but it's a little more specialized. Bear in mind that along with its original mass, the collapsed star also retains its original magnetic field. And like the mass, it's incredibly concentrated. The parent star's angular momentum remains as well (more or less), so as it collapses it speeds up its spin. You therefore have an unimaginably strong magnetic field rotating at sometimes more than 5,000 rpm. In other words, a dynamo from hell. The synchrotron radiation alone from such a thing is enormous. The magnetic field also traps most radiation except for at the poles. If those poles are not aligned with the spin axis, then you get the lighthouse effect of twin high-energy beacons sweeping around at whatever the pulsar's rotation rate is.
Again, I'm pretty rusty on this stuff, so someone else should probably check my post.

Astronuc, I'm rather unfamiliar with these things (simply because I haven't learned them yet) but if a neutron were to decay what would it become?

~Kitty

Astronuc
Staff Emeritus
misskitty - free neutrons decay into a proton, electron and electron-associated anti-neutrino. This involves not only conservation of energy, but momentum, charge, and spin.

Protons and neutrons are baryons, and electrons and neutrinos are leptons, and their numbers are conserved too.

See for fun - http://en.wikipedia.org/wiki/Lepton
http://en.wikipedia.org/wiki/Lepton_number
http://en.wikipedia.org/wiki/Baryon
http://en.wikipedia.org/wiki/Baryon_number

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/parint.html

You will find much more of this discussion in the Nuclei & Particles subforum, and I see that you have already been there.

Thanks for the links Astronuc. I'm here to learn and thats why I asked.

~Kitty

A Neutron decays into a Proton + Electron + Neutrino. This is a particle decay mode. However, Beta plus decay commonly means the basic process p->n + e++v. It is a nuclear decay mode in that it can only happen if the proton is inside a heavier nucleus and the final state nucleus is more tightly bound; the process is forbidden in free space by energy conservation since a neutron alone is heavier than a proton.

marlon

BigBang said:
NeutronStar,... What are they?

NeutronStar - an unknown entity that signs onto the physics forums every once in while to radiate a post. There is no known forumula for predicting the frequency of the posts radiated by a NeutronStar.

BigBang - an unknown entity that caused the universe to come into existence thus providing a medium for it to ask questions about itself.

just to add a little fun facts bigbang, a neutron star is very small, only about 15-20 km in diameter yet its density is $$10^{15}g/cm^3$$! making its gravitational force extremely strong. aswell you would weigh $$10^{11}$$ times more on a neutron star then you would on earth!
pretty much a neutron star is the closest to the black hole of the post-supernova stellar stages.

Astronuc said:
So then the light emitted from the surface of a neutron star should include the emission spectrum of Fe, and perhaps other elements.
sorry, no offence meant, but i dont see how thats possible...

Astronuc
Staff Emeritus
Yaaks said:
Astronuc said:
So then the light emitted from the surface of a neutron star should include the emission spectrum of Fe, and perhaps other elements.
sorry, no offence meant, but i dont see how thats possible...
No offense taken.

I made that statement based on -
The outer most surface of a neutron star is acutally composed of various atomic nuclei, mostly iron isotopes.
Perhaps I should have concluded the statement with a question mark (?). The temperatures are very high near the surface, so obviously Fe and other isotopes would be ionized - and recombination/ionziation would be a continuous process. Since the temperatures of the plasma however are in the keV range apparently, I would expect emissions are in the X-ray region of the EM spectrum.

DB said:
just to add a little fun facts bigbang, a neutron star is very small, only about 15-20 km in diameter yet its density is $$10^{15}g/cm^3$$! making its gravitational force extremely strong. aswell you would weigh $$10^{11}$$ times more on a neutron star then you would on earth!
pretty much a neutron star is the closest to the black hole of the post-supernova stellar stages.

Whoa. Why so small and so dense?! Are there any figures for estimating the gravitation of a neutron star?

~Kitty

Danger
Gold Member
misskitty said:
Whoa. Why so small and so dense?! Are there any figures for estimating the gravitation of a neutron star?
When the mass is great enough, electronic repulsion between the atoms isn't strong enough to overcome gravity. It keeps contracting until the electrons and protons are forced together to form neutrons. The degenerate neutron pressure then prevents further collapse (unless the mass is big enough to overcome that, and then you get a black hole).
The gravitational calculations are exactly the same as for any other body of the same mass.

Danger said:
The gravitational calculations are exactly the same as for any other body of the same mass.

doesnt the small diameter yet extreme density have a stronger effect of warping spacetime making its gravitational pull much stronger then a large star with the same mass?

SGT
DB said:
doesnt the small diameter yet extreme density have a stronger effect of warping spacetime making its gravitational pull much stronger then a large star with the same mass?
Since gravity is inversely proportional to the square of the distance, the surface gravity of a neutron star is several orders of magnitude stronger than the surface gravity of a large star, but at a distance greater than the radius of the normal star the gravitational pull should be the same.

Danger
Gold Member
SGT said:
Since gravity is inversely proportional to the square of the distance
Correct, and more significantly the distance from the centres of the masses involved. You can get much closer to the centre of a smaller body.

SpaceTiger
Staff Emeritus
Gold Member
Just a quick note that relativistic effects are non-negligible at the surface of a neutron star, so the force is no longer 1/r2, but the qualitative behavior is the same as is being discussed here.

As for emission lines, you usually don't get those in stars unless there is a significant extended region of hot gas beyond the star's photosphere. Stars with heavy stellar winds or interacting binaries will sometimes have emission lines, but most of the time, the spectrum is thermal+absorption. Isolated neutron stars that we can see are usually very hot, so the gas is too heavily ionized even for absorption lines (at least in the optical and UV).

So if we can't get the emission or absorption lines for a spectrum how do we know what elements are present in that star?

~Kitty

misskitty said:
So if we can't get the emission or absorption lines for a spectrum how do we know what elements are present in that star?

~Kitty
I may be wrong, but it's my understanding that a neutron star is made of degenerate matter. In other words, it doesn't contain any "elements" in the normal chemical sense. All of the electrons are in a degenerate state and therefore there are no actual "atoms" with electrons orbiting around them in normal sense of the periodic table. All of the electrons are packed into the lowest possible quantum states because of the overwhelming force of gravity.

Again, this is my understanding which may be wrong.

but, what will happen to neutron star at the end. it cannot approach absolute 0 even after radiating all the energy; if it does radiate. it has to loose energy and the eventually it will have nothing left. what will happen then?

Its probably me thats wrong. So in the star are there just free roaming particles flying around decaying as they go? What happens when they can't decay anymore?

~Kitty

SpaceTiger
Staff Emeritus