4 ms·

What about the neutrons in a neutron star -- do they also decay into protons? Aren't neutron stars long-lived?

ars 11y ago
One way to think about it: They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.) Another way to think about it: They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into. You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so. That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).
- swombat 11y ago
  > They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable. The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
  - piannucci 11y ago
    GP is correct-- the neutron degeneracy pressure prevents the stellar remnant from getting any smaller, but it is the electron degeneracy pressure which makes beta decay energetically unfavorable. In order to be produced by neutron beta decay, an electron must enter a valid state. Pauli exclusion (electrons are fermions) requires the electron to enter a state at least as energetic as the electron Fermi level, which due to the neutron star's gravity is comparable to the rest mass of the electron. Neutron decay is energetically favorable, but not that favorable. The reverse process accelerates and equilibrium is reached.
reo9Sahx 11y ago
Neutron stars are just mostly neutrons. Like atomic nuclei, they also contain protons. (In their core, they could potentially even contain quarks or hyperons, although recent neutron star measurements make this a bit more unlikely)
- deleted 11y ago
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ars 11y ago

One way to think about it: They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.) Another way to think about it: They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into. You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so. That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).

swombat 11y ago
> They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable. The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
- piannucci 11y ago
  GP is correct-- the neutron degeneracy pressure prevents the stellar remnant from getting any smaller, but it is the electron degeneracy pressure which makes beta decay energetically unfavorable. In order to be produced by neutron beta decay, an electron must enter a valid state. Pauli exclusion (electrons are fermions) requires the electron to enter a state at least as energetic as the electron Fermi level, which due to the neutron star's gravity is comparable to the rest mass of the electron. Neutron decay is energetically favorable, but not that favorable. The reverse process accelerates and equilibrium is reached.

reo9Sahx 11y ago

Neutron stars are just mostly neutrons. Like atomic nuclei, they also contain protons. (In their core, they could potentially even contain quarks or hyperons, although recent neutron star measurements make this a bit more unlikely)

deleted 11y ago
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