Name: Unknown
Status: Other
Age: N/A
Location: N/A
Country: N/A
Date: Prior to 1993
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 and 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 electron 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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