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Creation of spinning neutron stars and pulsars

  1. Dec 27, 2014 #1
    I've gotten a bit confused about the creation of the neutron star/pulsar, so I was hoping someone could point me in the right direction :)

    As fusion stops, when reaching the iron phase, the outer layers (hydrogen, helium, carbon... etc.) gets pulled in-wards do to gravity. This creates a bounce effect on the hard surface of the neutron star (Which I suppose is formed from the iron core?), which in turn sends out a shockwave, which then is halted for a bit, and then "re-ignited" due to neutrinos coming from the outer layer of the neutron star - which the creates the supernova explosion. So far so good, or am I missing some details?

    Now, if I'm not mistaken, neutron stars and pulsars are pretty much the same. They both spin rapidly, but pulsars emit it's two giant light beams due to the strong magnetic fields and synchotron radiation from charged particles in that field (Although I'm not quite sure how, and why, it loses its magnetic field and rotation speed though?).

    So, what I've read, the fast rotation comes from conservation of angular momentum, and the large magnetic field comes from compressing the original stars magnetic field into a smaller volume, i.e. increasing its strength. But, is the conservation, and magnetic fields, from the full star, with the hydrogen, helium, carbon... layers, or is it only a small collapse in the core that gives rise to these?

    Hope you understand the questions, and I've made myself clear :)

    Thanks in advance.
  2. jcsd
  3. Dec 27, 2014 #2


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    In a type Ib, Ic, or IIa supernova, it's really only the core which collapses (hence why it's called a core collapse supernova), the outer envelopes don't really even have time to react over the dynamical timescales of the collapse of the core (several milliseconds - compare with the fact that light takes several minutes to reach from one side of a super giant star to the other). So the envelope of the star really has nothing to do with the collapse, it's just blown off by the explosion itself.

    The shockwave arises when the outer core hits the inner core. The two pieces of the core actually detach from one another because the inner core collapses much quicker. When the inner core turns into a neutron star, it acts like a piston and drives a shockwave through the outer core (which then stalls as you said).

    There's actually a lot more happening inside a supernova than simple neutrino shock re-heating, but that's probably not relevant to your question.

    Because the outer envelope is detached, more or less, from the formation of the neutron star, it is really only the core's angular momentum and magnetic fields which are residual inside the neutron star. But even then, the core is ~1.5-2 solar masses so that's still quite a lot of material, a lot of angular momentum, and a lot of magnetic fields which you have to compress into ~10km in size.
  4. Dec 28, 2014 #3
    Thanks for the enlightening, it really made many of my questions clear :)
    But, if you don't mind, I would like to know the neutrino thing, if it's not too much to ask? I've read a lot of things about it, but it hasn't really been very detailed, so I pretty much just end up getting something like: "Neutrinos are formed, or else neutrons would "boil", and these neutrinos drive the stalled shockwave from before..."

    Again, thank you :)
  5. Dec 28, 2014 #4


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    The shock reheating is actually a field of active research. Basically what happened was that neutrino shock reheating was successful in 1-D simulations (done in the 80's), but when people made more sophisticated simulations in 2-D (and recently 3-D) it was found that simple neutrino shock reheating was not successful in reviving the shock wave. The neutrino needs to deposit heat into the shockwave, but if you deposit too much heat in a thin layer of material, what happens is you heat the shockwave to too high temperatures and then it just re-emits neutrinos instead of driving a more robust explosion. So, things like convective heating, or the effects of rotation in the star, or magnetic fields are being investigated now. The current state of the art, that I am aware of, is something called the SASI - standing accretion shock instability. Basically, with this scheme, you use neutrinos to heat material at what is called the "gain radius", and then convective instabilities develop which allow convection to drive heat up to the shockwave, and move cooler shock-wave material back down to the gain radius where you heat it up with neutrinos again. This scheme distributes the heating over a larger area so that you don't run into the problem of having one thin layer of material heating up too much. 2-D simulations seem to suggest robust explosions once the SASI is accounted for...of course this doesn't mean we will have robust explosions once 3-D simulations are performed lol.
  6. Dec 28, 2014 #5
    Once again, thank you very much :)
    It was very informative.
  7. Dec 28, 2014 #6


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