# Neutron star

1. Sep 19, 2007

### neg_ion13

My question is about the formation of a neutron star once fussion stops and it collapses. I recently saw a show about the life of a star and it said when gravity overcomes the energy released by fussion, the electrons are compressed toward the nucleus of the atoms and create neutrons. I recognize that the charge would be the same as neutron but an electron is a lepton and a proton is made of 2 U, and 1 D quarks. How does the combination of these two make a neutron?

2. Sep 19, 2007

### mgb_phys

It doesn't quite work like that, you can simply thing of a neutron as being a proton and an electron but you could equally think of a proton being a neutron and a positron.
Basically they are all made of matter and matter and energy can get swapped around.

A photon isn't made of electrons, but an electron and a positron combining make a gamma photon.

3. Sep 19, 2007

### neg_ion13

Are you saying that because the net charge is the same it becomes a neutron equivalent? It doesn't have to be be made from the same "ingredients" or parts as the neutron itself?

4. Sep 19, 2007

### genneth

There's more than just mass and electromagnetic charge that needs to be conserved. The process involved in beta decay (the opposite to what you've said) involves the weak force. To reinforce what mgb_phys said, in particle physics, particles aren't the whole story. In particular, relevant to this thread, when you put two particles together, you don't just get those two particles. However, certain things are always conserved, and from those you can work out the allowable "changes" (okay, I'm simplifying, but I think it gets the main gist). Furthermore, we've got a set of conserved quantities, and ways to figure out all the changes, such that it matches up with experiments pretty well.

5. Sep 19, 2007

### neg_ion13

I get what your saying. Thank you both very much.

6. Sep 19, 2007

### Astronuc

Staff Emeritus
Refer to this on the weak force and neutron decay
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html#c4

In the case of electrons in a collapsed stellar core, protons will combine with neutrons via 'electron capture', which actually occurs with some natural terrestrial nuclides.

EC reaction is p + e- -> n + $\nu_e$

and see - Transformation of Quark Flavors by the Weak Interaction
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/qrkdec.html#c1

7. Sep 20, 2007

### neg_ion13

Wow, I really had no idea how much was involved in that process. Those links were great thanks a lot.

8. Sep 21, 2007

### Astronuc

Staff Emeritus
Should read "protons will combine with electrons to form neutrons via 'electron capture' "

Free neutrons will decay eventually, but in a neutron star, the resulting protons could absorb another electron and become a neutron again. On the other hand, I'm not sure about the lifetime of a neutron in a neutron star, and I am not sure anybody is. It's a whole other form of matter that we cannot recreate in our terrestrial labs simply because of the enormous masses and densities involved. We can't even reproduce confined stellar plasmas, which have the density of water, since the pressures are too great for anything that man can devise.

The other part of the story is the strong force

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c2

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/feyns.html

9. Oct 13, 2007

### -RA-

I find neutron stars fascinating as they're so different from other stars.

i do have some questions about the properties of neutron stars though that i cant get my head around.

-doesn't nuclear chemistry say that the island of stability does not allow you to pack neutrons that close?
-if you add neutrons to the nucleus of atoms dont you need to add a proportional amount of protons to keep that atom stable? dont atoms that contain over approximately 1.5 neutrons per proton spontaneusly undergo radioactive decay transformations that bring their compositions closer to this ratio?
-dont individual neutrons decay into a proton in less than 14 minutes? ie making real 'Neutronium' impossible to sustain

Last edited: Oct 13, 2007
10. Oct 13, 2007

### Astronuc

Staff Emeritus
Well the mass-energy density in a neutron star, with neutrons packed at roughly the nuclear mass density, is a rather unique state of matter. We certainly don't experience that around our part of the universe, except in the nuclei of atoms.

Well it's nuclear physics. There are only so many stable nuclides and so many radionuclides with long half-lives. Beyond Bi-209, there are not stable elements, although Th-232 and U-238 have very long half-lives.

Free neutrons spontaneously decay by beta emission, and neutrons apparently do not form pairs which would seem to be necessary in order to be stable.

The deuteron (np) is a stable configuration. He-3 (ppn) is stable, but T (pnn) is not and decays by beta emission.

(pp) is not possible due to Coulomb repulsion, (pn) is stable, and (nn) does not form. So clearly, to be stable, there needs to be (udu)(dud) pairing.

Last edited: Oct 13, 2007
11. Oct 13, 2007

### Parlyne

It's certainly true that there are limitations on how many nucleons can be bound together by nuclear forces. However, neutron stars are not bound by nuclear forces. They're bound by gravity. And, so far as anyone knows, there is no fundamental limitation on the amount of material that can be bound by gravity.

12. Oct 13, 2007

### -RA-

that clears that one up, i did not suspect that gravity would ever be strong enough to significantly effect particles the size of neutrons due to gravities comparative weakness to other forces. i suppose its going to very hard to get close enough to a body with that much mass to test its true composition anyway.

13. Oct 13, 2007

### Astronuc

Staff Emeritus
Gravity is very strong in neutron stars - they are just very massive and concentrated (very dense) objects.

Coincidentally - The Physics of Neutron Stars - http://arxiv.org/abs/astro-ph/0405262
Authors: J.M. Lattimer, M. Prakash