Question:
I read today about "millisecond pulsars" which is a star (neutron?)
which rotates very rapidly, at the rate of "several thousand times per second."
I would like to know how large one of these stars might be. If the
circumference at the equator is hundreds of miles or more, then the stellar
material would be moving almost at the speed of light. How can this be? My
only guess is that the stars are extremely small.
Replies:
A good deduction! All of my astronomy texts say that a neutron star of
about one solar mass should have a radius of about 10 km.; thus the relativistic
"speed limit" is not violated. The above figure of 10 km. Is based on a
calculation using a simple model, including an assumption of constant density an d a limiting mass; more
sophisticated models allow for density to vary with depth. These models take
limits of approximately 0.1 to roughly 2-3 solar masses as the limits of a
neutron star's mass, with radius decreasing as a function of mass (that is, more massive neutron stars
should have smaller radii). As one approaches the 0.1 lower limit, the models
predict a thicker, less-dense crust and a limit radius of about 50 km. The lower
mass limit comes about as the minimum to get past a quantum effect called electr on degeneracy
which balances gravity in a white dwarf; the upper limit marks the point where
gravity just overwhelms another quantum effect, neutron degeneracy, which
balances gravity in a neutron star.
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