What
is a Pulsar?
You may well ask. In fact, all of our parents have, as well as those
of our friends who deign to take an interest in physics. For those
with some technical background, the Princeton physics department offers
an explanation.
For the rest of you, we will attempt to explain it in lay terms.
A
Star is Born... and then it dies.
Well, you needn't
look so surprised. Everything that can be born can die. We
won't get into the birth here, as it's irrelevant to the definition of
a pulsar and you can find a perfectly good explanation, with a lovely picture,
at NASA.
So, a star is born. Its fire is sustained by fusing hydrogen atoms
(this gives off a tremendous amount of energy). Eventually, it runs
out of hydrogen, and moves onto helium fusion, and then so on down the
periodic
table. However, fusion of iron and any heavier elements requires
energy input, not output, so once a star's core is reduced to iron, unless
it happens to have a humongous power generator on hand and some very skilled
technicians, it's...well, you might call it stuck. The only force
acting on it now is gravity,
and since a star is tremendously massive, it attempts to collapse in on
itself. However, there is a limit to its capacity for density, and
the collapse is very quickly met by an outward force due to the pressure
of the subatomic particles that make up the star.
If the mass left is less than 1.4 times the mass of the sun, the star fizzles
out. If greater, the opposing forces create a tremendous explosion
known as a supernova
(don't look here
or here).
If the mass is greater than 2-3 solar masses, its gravity is so strong
that nothing can hold it up from collapsing back in completely on itself,
down to the size of a point (theoretically at least, this is of infinitely
small size). This is known as a black
hole (do NOT look here).
With a final (post-fusion) mass anywhere in between these masses, a neutron
star forms. After the supernova, what hasn't been blown away
collapses back in on itself. Again, we have two opposing forces,
but this time they are gravity and neutron degeneracy
pressure: protons and electrons have been melded together into neutrons,
and neutron degeneracy pressure is much greater than electron degeneracy
pressure. Equilibrium is eventually reached and a stable neutron
star is created (calling it a star may seem deceptive, since we just told
you that its life as a star was over, but astronomers
are notoriously flexible with the English
language).
So far, all neutron stars discovered have been pulsars. It has a
very powerful magnetic
field which is not aligned with the star's axis of rotation, so as
the neutron star rotates (all stars rotate, in case you didn't know that),
the magnetic field gets twisted up. Neutron stars, unlike normal
stars, tend to rotate at incredibly
high speeds: they will go through one full rotation in a fraction of
a second, while it takes the sun (a quite ordinary sort of star) about
a month. As you can imagine, the magnetic field gets very tangled,
very quickly. The magnetic field emits radiation at radio wavelengths
(sometimes at X-ray wavelengths--you can find information on that from
Ignacio
Negueruela. Better to first read
his overview of X-ray binaries). This radiation is emitted through
a beam which sweeps past us, rather like a lighthouse beam. This
manifests itself as very rapid pulses: hence the name pulsar.
We have no pictures
here because pulsars do not emit at visible wavelengths. Use your imagination.
For more information on pulsars, try
Princeton's
pulsar resource page
Jodrell
Bank's pulsar page
The
MPIfR group at Bonn
Precision
Pulsar Astrophysics at the Naval Research Laboratory
And
one of our own alumni, Josh Kempner
I
want to go home!
Last Updated June 1998