What's a Pulsar?

[~*~Kit_The_Kat~*~]

Neutron Stars
Neutron stars are about 10 km in diameter and have the mass of about 1.4 times that of our Sun. This means that a neutron star is so 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 300,000 times that of 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 or in with a normal star. When a neutron star is in an x-ray binary, astronomers are able to measure its mass. From a number of such x-ray binaries, neutron stars have been found to have masses of about 1.4 times the mass of the sun. Astronomers can often use this fact to determine whether an unknown object in an x-ray binary 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 pulse because they rotate!

imagine.gsfc.nasa.gov/Images/basic/xray/pulsar.gif [/img]

A diagram of a pulsar, showing its rotation axis
and its magnetic axis.

[~*~Kit_The_Kat~*~]

Pulsars were first discovered in late 1967 by 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 their two 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 beam of light from the jet sweeps 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".


X-ray Observations of Pulsars
Some pulsars also emit X-rays. In pulsars which are part of supernova remnants, the x-rays result from the jets of material streaming out along the magnetic field of the pulsar. For pulsars in x-ray binaries, the x-rays are due to material from the companion star streaming down the magnetic poles.
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.

imagine.gsfc.nasa.gov/Images/advanced/einstein_crab_on.gif [/img]

Crab Pulsar "On"

imagine.gsfc.nasa.gov/Images/advanced/einstein_crab_off.gif [/img]

Crab Pulsar "Off"