Neutron Stars

Recall that one of the possible endpoints of stellar evolution is a neutron star. Furthermore, there is evidence for the existence of a neutron star.

Why should one think that pulsars are neutron stars?

Are neutron stars left behind by supernovae?

Also recall that, at least theoretically, a type II supernova explosion leaves a massive core of neutrons behind. Let's look at this..

Why should a neutron star be rotating so fast?

As the stellar core shrinks, it must rotate faster in order to conserve its angular momentum.

Neutron stars should also have big magnetic fields

Recall that electrically conducting fluids tend to trap magnetic field lines. The fluids can move across the field lines only very slowly, and conversely the field lines can move across the fluid only very slowly. (Cf. the reason why sunspots are dark.)

So what happens when an electrically conducting sphere collapses to a much smaller size?

The magnetic field lines get concentrated in a smaller surface area, so the magnetic field is stronger.

Note: keeping the magnetic field requires an electrically conducting star. Neutrons are electrically neutral, but the theory of neutron stars suggests that there are enough protons and electrons in the star to provide the conductivity -- in fact super good conductivity.

Some evidence for strong magnetic fields associated with neutron stars: the white light produced by the Crab Nebula appears to be the sort of light made when electrons move along cureved paths through a strong magnetic field.

Lighthouse model for the pulses

A model for how the pulses are produced:


The glitch in the period vs time graph above is an example of a phenomenon that is observed in other pulsars too. It appears that neutron stars have a (thin) solid surface that can break suddenly in a ``starquake.'' Then the star gets a little smaller and its rotation rate speeds up accordingly.

Millisecond pulsars

Some pulsars with periods of just a few milliseconds have been found.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA