Neutron Stars and Pulsars
Neutron Stars
A neutron star is about 20 km in diameter and has the
mass of about 1.4
times that of our Sun. This means that a neutron star is so
dense that on
Earth, one teaspoonful would weigh a billion tons! Because of its
small size and high density, a neutron star possesses a surface
gravitational field about 2 x 1011 times that of Earth. Neutron stars
can also have magnetic fields a million times stronger than the
strongest magnetic fields produced on Earth.
Neutron stars are one of the possible ends for a star. They result
from massive stars which have mass greater than 4 to 8 times that of our
Sun. After these stars have finished burning their nuclear fuel,
they undergo a supernova
explosion. This explosion blows off the
outer layers of a star into a beautiful supernova remnant. The
central region of the star collapses under gravity. It collapses
so much that protons and electrons combine to form neutrons. Hence
the name "neutron star".
Neutron stars may appear in supernova remnants, as isolated objects, or in
binary
systems. Four known neutron stars are thought to have planets. When a neutron
star is in a binary system,
astronomers are able to measure its mass. From a number of such
binaries seen with radio or X-ray telescopes, neutron star masses
has been found to be
about 1.4 times the mass of the Sun. For binary systems containing an
unknown object, this information helps distinguish whether the object
is a neutron star or a black hole, since black holes are
more massive than neutron stars.
What is a Pulsar and What Makes it Pulse?
Simply put, pulsars are rotating neutron stars. And pulsars appear to pulse
because they rotate!
A diagram of a pulsar, showing its rotation axis
and its
magnetic axis
Pulsars were discovered in late 1967 by graduate student Jocelyn Bell Burnell as
radio sources that
blink on and off at a constant
frequency. Now
we observe the brightest ones at almost every
wavelength of
light. Pulsars are spinning
neutron stars
that have jets of particles moving almost at the
speed of
light streaming out above their magnetic poles. These jets produce very
powerful beams of light. For a similar reason that "true north" and
"magnetic north" are different on Earth, the magnetic and rotational
axes of a pulsar are also misaligned. Therefore, the beams of light
from the jets
sweep around as the pulsar rotates, just as the spotlight in a lighthouse
does. Like a ship in the ocean that sees only regular flashes of light, we see
pulsars "turn on and off" as the beam sweeps over the Earth. Neutron
stars for which we see such pulses are called "pulsars", or
sometimes "spin-powered pulsars," indicating that the source of
energy is the rotation of the neutron star.
X-ray Observations of Pulsars
Some pulsars emit X-rays.
Below, we see the famous Crab
Nebula, an
undisputed example of a neutron star formed during a supernova explosion. The
supernova itself was observed in 1054 A.D. These
images are from the
Einstein X-ray observatory. They show the diffuse emission of the Crab
Nebula surrounding the bright pulsar in both
the "on" and "off" states, i.e. when the magnetic pole is
"in" and "out" of the line-of-sight from Earth.
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| Crab Pulsar "On" |
Crab Pulsar "Off" |
A very different type of pulsar is seen by X-ray telescopes in some
X-ray binaries. In these cases, a neutron star and a normal star form
the binary system. The strong gravitational force from the neutron
star pulls material from the normal star. The material is funneled
onto the neutron star at its magnetic poles. In this process, called
accretion, the material becomes so hot that it produces X-rays. The
pulses of X-rays are seen when the hot spots on the spinning neutron
star rotate through our line of sight from Earth. These pulsars are
sometimes called "accretion-powered pulsars" to distinguish
them from the spin-powered pulsars.
Last Modified: December 2006
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