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 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 pulse because they rotate!
A diagram of a pulsar, showing its rotation axis
and its magnetic axis
Pulsars were first 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 also 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.
|Crab Pulsar “On”||Crab Pulsar “Off”|
A very different type of pulsar is seen by X-ray telescopes in some X-ray binaries. In this case, 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.