Neutron stars are what are thought to cause pulsars. A pulsar is an astronomical radio source that emits pulses of radiation, as seen from earth. The regularity of the pulses initially caused the discoverers Jocelyn Bell Burnell and Antony Hewish to speculate that the signals were from another civilization. The pulses were so regularly spaced that it was thought that pulsars could be used as a time standard. However, it was soon seen that the rate of pulse emission slows, but then abruptly increases in events known as glitches. The current model (highly simplified) for a pulsar is a neutron star, one in which the electrons have been pushed down into the nucleuses by extreme gravitational forces to combine with the protons to form neutrons. Material falling onto the surface of the neutron star gets superheated and shoots off in the form of a jet. If the jet doesn’t coincide with the rotational axis of the star, the jet will precess around the axis. If the earth happens to lie on the cone of precession, radiation will arrive at the earth and the pulsar can be detected. The pulsing effect is due to the fact that the jet only points at the earth once per precession. An analogy is that of a lighthouse; although the beam is always on, one can see the beam only when it is pointing directly at the observer. The slow decrease in pulse rate is due to the fact that the pulsar is emitting energy, some of which comes from the kinetic energy of rotation. As the star slows, the crust of the star is placed under stress. When the strain becomes too much, the surface breaks and resettles at a smaller distance from the center; this is called a star‐quake. Since the mass of the star is now closer to the axis of rotation, the moment of inertia decreases, and, much like the skater pulling in her arms, the angular speed increases. A neutron star with angular speed ωi = 70.4 rad/sec experiences a glitch such that its angular speed increase by a factor Δω/ωi = 2.0110‐6. If the initial radius were 11km, by how much was the star’s radius decreased? Assume that the star is a uniform sphere if you like, although this assumption is not really necessary
Neutron stars are what are thought to cause pulsars. A pulsar is an astronomical radio source
that emits pulses of
the discoverers Jocelyn Bell Burnell and Antony Hewish to speculate that the signals were from
another civilization. The pulses were so regularly spaced that it was thought that pulsars could
be used as a time standard. However, it was soon seen that the rate of pulse emission slows, but
then abruptly increases in events known as glitches.
The current model (highly simplified) for a pulsar is a neutron star, one in which the electrons
have been pushed down into the nucleuses by extreme gravitational forces to combine with the
protons to form neutrons. Material falling onto the surface of the neutron star gets superheated
and shoots off in the form of a jet. If the jet doesn’t coincide with the rotational axis of the star,
the jet will precess around the axis. If the earth happens to lie on the cone of precession,
radiation will arrive at the earth and the pulsar can be detected. The pulsing effect is due to the
fact that the jet only points at the earth once per precession. An analogy is that of a lighthouse;
although the beam is always on, one can see the beam only when it is pointing directly at the
observer.
The slow decrease in pulse rate is due to the fact that the pulsar is emitting energy, some of
which comes from the kinetic energy of rotation. As the star slows, the crust of the star is placed
under stress. When the strain becomes too much, the surface breaks and resettles at a smaller
distance from the center; this is called a star‐quake. Since the mass of the star is now closer to
the axis of rotation, the moment of inertia decreases, and, much like the skater pulling in her
arms, the angular speed increases.
A neutron star with angular speed ωi = 70.4 rad/sec experiences a glitch such that its angular
speed increase by a factor Δω/ωi = 2.0110‐6. If the initial radius were 11km, by how much was
the star’s radius decreased?
Assume that the star is a uniform sphere if you like, although this assumption is not really
necessary
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