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View Full Version : Why doesn't the Neutron Star Decay



m74z00219
2009-Jul-29, 07:10 AM
If free neutrons decay into a proton, electron, and antineutrino, with a short life time, how come neutron stars exist? Or...do protons "oscillate" back into neutrons via electron capture??

Also, I was just thinking about subjectivity of time. A neutron star, from our perspective, might be 5 billion years old. From the neutron star's perspective, it might only be 4 billion years old at the crust level...just musing.

Thanks,
m74

NorthernBoy
2009-Jul-29, 07:57 AM
Neutrons decay in free space because they have more energy than the daughter products. In a neutron star, they would not have.

This is just like the neutrons in a stable nucleus. Look at Iron 56, for example. Lots of neutrons in there, but you don't get them decaying every few minutes.

trinitree88
2009-Jul-29, 10:55 AM
Neutrons decay in free space because they have more energy than the daughter products. In a neutron star, they would not have.

This is just like the neutrons in a stable nucleus. Look at Iron 56, for example. Lots of neutrons in there, but you don't get them decaying every few minutes.

NorthernBoy. I think the magnetic field plays a role here, too. The radioactive half-life of a neutron varies with the ambient field and drops precipitously near 1011 to 1013 Gauss....just the values in a neutron star. Since this is a weak decay, and all weak interactions are polar asymmetric due to parity effects...the decay products should form a "tail" and exert an asymmetric force here. pete

SEE:http://www.innovations-report.com/html/reports/physics_astronomy/report-53103.html

Spaceman Spiff
2009-Jul-29, 09:00 PM
When a neutron decays, the products are a proton, an electron, and an anti-neutrino:

n --> p+ + e- + anti-neutrino.

In the nucleus of 56Fe, for example, there are 26 protons and 30 neutrons. For a neutron to decay, there would have to be an available unoccupied energy level at lower energy for the new proton to land -- there isn't any, they're all filled up (Fermi Exclusion Principle). So neutrons inside of nuclei are stable as long as there aren't too many extra neutrons such that the decay of one of them in some energy state allows the newly formed proton to exist in an unoccupied lower energy state.

In neutron stars, it's a bit different (I will only comment on the states of matter within them that we generally have a good handle on). In this case, the remaining free electrons are highly degenerate -- they've filled up all their free energy levels (2 by 2, spin up/down) all the way up to the Fermi level. The next wrung lies much higher in energy. The electrons occupy what is called the "Fermi Sea".

See the problem? For the neutron to decay within the electron Fermi Sea, an electron must be spit out -- but there are no quantum states available to that electron. So the Fermi Sea stabilizes the neutrons against decay.