In this lecture we’ll talk about neutron
stars, how they were discovered, and what
can happen to a neutron star in a close binary
system.
Neutron stars are even more bizarre than white
dwarfs.
The possibility that neutron stars might exist
was first proposed in the 1930s, but many
astronomers thought the idea was crazy.
We now have evidence, that neutron stars do
indeed exist.
A neutron star is the ball of neutrons crated
by the collapse of the iron core in a massive
star supernova.
You may recall, that at the end of a star’s
life, an iron core with the mass comparable
to that of our Sun and a size larger than
that of Earth collapses into a ball of neutrons
just a few kilometers across.
The collapse stops only because the neutrons
have a degeneracy pressure of their own.
Like white dwarfs, neutron stars resist the
crush of gravity with degeneracy pressure.
White dwarfs are supported by electron degeneracy
pressure.
In neutron stars, it’s neutrons rather than
electrons that are being squished, so we call
this pressure neutron degeneracy pressure.
The force of gravity at the surface of a neutron
star is incredible.
Neutron stars cram roughly 1.3 to 2.5 solar
masses into a region of about 20 kilometers
across.
Matter is packed so tightly that a sugar-cube-sized
amount of neutron star material would weigh
more than 1 billion tons, about the same as
Mount Everest!
A neutron star would fit into the Washington,
DC beltway, but it would be a very bad idea
to bring a neutron star anywhere near Earth.
If you did, the neutron stars enormous surface
gravity would quickly destroy our entire planet.
It would squash Earth in to a shell no thicker
than your thumb on the surface of the neutron
star.
The first observational evidence for neutron
stars came in 1967, when a 24-year old graduate
student named Jocelyn Bell discovered an unusual
source of radio waves.
Bell and her advisor, Anthony Hewish, had
built a radio telescope.
Bell was looking at the data and found a source
that was putting out very regular pulses of
radio waves coming from somewhere near the
direction of the constellation Cygnus.
The pulses came at intervals so precise, they
almost seem artificial.
For some time, the phenomenon was half-jokingly
nicknamed "LGM" for "Little Green Men".
The pulsing wasn’t due to extraterrestrials.
It was discovered that the pulses were coming
from a spinning neutron star we now call a
pulsar.
Pulsars are neutron stars left behind by supernova
explosions
This is an image of Jocelyn Bell as a graduate
student with her radio telescope.
Her thesis supervisor Antony Hewish shared
the Nobel Prize with another astronomer, Martin
Ryle.
Bell was excluded, despite having been the
one to observe the pulsars.
More pulsars were soon discovered within supernova
remnants, including the pulsar at the center
of the Crab nebula.
But What makes a pulsar pulse?
As the iron core of a massive star shrinks
into a neutron star, it spins faster and faster.
The magnetic field lines within the core get
all bunched up, and the magnetic field strengthens.
The intense magnetic field directs beams of
radiation out along the magnetic poles.
If the neutron star’s magnetic poles are
not lined up with its rotation axis, the beams
of radiation sweep around like a light house.
The idea is something like this.
Like lighthouses, neutron stars emit a fairly
steady beam of light, but we see a pulse of
light only when the beam sweeps past Earth.
The radiation beams sweep through space like
lighthouse beams as the neutron star rotates.
In order to see a pulsar, the neutron star
has to be oriented such that its beams sweep
past us.
If we are seeing the system from overhead,
for example, we won?t be able to see the pulses.
Therefore we say that all pulsars are neutrons
stars, but not all neutron stars are pulsars.
Binary systems in which both objects are neutron
stars have been used to test Einstein’s
general theory of relativity.
Relativity predicts that neutron stars emit
gravitational waves- ripples of space-time.
This causes the orbits of neutron stars to
shrink and gradually brings the neutron stars
closer together.
Observations have found that the orbital decay
does occur at precisely the rate that Einstein’s
theory predicts, giving scientists confidence
that gravitational waves really exist.
Pulsar timing was also used in the first confirmed
discovery of extrasolar planets.
In 1992, measurements showed that pulsar has
a slightly varying pulsation rate.
An analysis revealed that the changes in the
pulsar’s pulsation rate could be explained
by gravitational tugs of three orbiting planets.
This was unexpected.
It seems highly unlikely that planets could
have survived a supernova explosion.
Astronomers therefore suspect the planets
formed after the explosion.
Matter falling toward a neutron star forms
an accretion disk, just as in a white dwarf
binary, but a neutron star has a much greater
gravitational field.
Therefore in-falling matter will release far
more energy.
The temperatures get so high in the inner
regions of the accretion disk that it emits
X rays.
Close binaries that contain accreted neutron
stars are often called x-ray binaries.
Like accreting white dwarfs that occasionally
erupt into novae, accreting neutron stars
sporadically erupt with a pronounced spike
in luminosity.
Because these eruptions release energy primarily
in the form of X rays.
We call them X-ray bursts.
X-ray bursts arise from the ignition of helium
fusion on the neutron star in a close binary
system.
Typical x-ray bursters flare every few hours
to every few days.
Each burst lasts only a few seconds, but during
those seconds the system radiates 100,000
times as much power as the Sun, all in X rays.
That’s all for the bizarre world of neutron
stars, pulsars, and x-ray bursts.
Take care, I'll talk to you soon.
