What's left after a Type II supernova?
If theories are correct, the collapsed core should remain as a neutron star.
Like a white dwarf, a neutron star is thought to have a maximum mass before
it becomes unstable. That mass is around 3 
. A neutron star below this limit should just sit around and cool off like a
white dwarf. But neutron stars are so tiny that even a very hot one would have
a very low luminosity ( 
). So how could you possibly observe one?
In 1967, a graduate student in England named Jocelyn Bell was looking at data from a radio telescope and found, much to her surprise, that one radio source was emitting a pulse of radiation every 1.33 seconds. For a while, astronomers thought this might be a signal from intelligent extraterrestrials: ``little green men.'' Soon, however, many more of these sources were found, including one in the Crab supernova remnant that emits 30 pulses a second. Astronomers now believe that these sources, dubbed pulsars, are rapidly spinning neutron stars with strong magnetic fields. The rotating magnetic field produces an electric current, in much the same way that an electric generator operates here on Earth. As the electrons in the current are accelerated, they emit electromagnetic radiation in a sort of conical beam. The radiation can have a broad range of wavelengths, from radio waves through X-rays. Each time that beam sweeps by us, we see a burst of radiation (akin to the flashing of a lighthouse beacon).
How do we know that the pulsars are neutron stars? The
fact that there's
a pulsar in the Crab supernova remnant is suggestive, because
we'd expect a
neutron star to be left behind. Also, the Crab pulsar is slowing
down
at a rate that is consistent with an age of about 1000 years
(plus
light travel time), which is when the supernova was observed.
The
sharpness of the pulses indicates that they come from a region
roughly 100 km across, since otherwise the finite travel time for
light
to move across the region would smear out the pulse. The masses
of
pulsars in binary systems can be calculated (from the observed or
deduced motions of the stars) and give masses between
1.4 and 1.8 , too massive for a white dwarf but just right
for a neutron star. Finally, all other reasonable explanations,
such
as oscillation or rotation of white dwarfs, do not explain the
rapidity
and regularity of the pulses.
Why do pulsars have strong magnetic fields? When the iron
core
collapses to become a neutron star, the magnetic field lines
remain
trapped inside it--they get scrunched up. This increases the
strength
of the magnetic field by a huge factor ( 
to 
).
What provides the energy for a pulsar? The root source is the rapid rotation of the neutron star, which results from the collapse of a much larger, rotating core. Again, the idea is that when an object collapses, the angular momentum must stay constant, so the object must spin faster. As the pulsar radiates energy, then, we expect its rotation to gradually slow down. Eventually the rotation is too slow to produce much radiation and the ``lighthouse beacon'' gradually fades from view. By carefully monitoring pulsars over periods of months, we can learn more about the interiors of neutron stars.
Start:Stars
