Neutron Stars

(Section Not Complete)

It is estimated that there are 10⁸ neutron stars in our galaxy. About 1000 of these have actually been observed by astronomers so far. Neutron stars typically have masses of around 1-2 solar masses and diameters of approximately 10 km. Thus, they have enormous densities that are similar to those encountered in the nucleus of the atom. In fact, in certain ways, neutron stars are similar to giant atomic nuclei the size of a city.

The first clear detection of neutron stars (but their existence had been forecasted theoretically) was in the discovery of radio pulsars in the 1960s. Although most neutron stars have been discovered as radio pulsars, the vast majority of the energy emitted by neutron stars is in very high energy photons (X-rays and Gamma-rays, with the highest energies exceeding 100 MeV) rather than radio waves. Typically only about 1/100,000 of their radiated energy is in the form of radio waves.

History of the idea of neutron stars and their discovery.

Baade and Zwicky predict the possible existence of a neutron star.

Oppenheimer works out the theory of neutron stars.

Virtual trips to black holes & neutron stars

Right image: white dwarf and neutron star on same scale

Java Applet: Orbits in Strongly Curved Spacetime

C-ship: Relativistic ray traced images (special relativity illustrations)

Magnetars.

More Magnetars

Mechanism for X-Ray Bursters

Because the gravitational field of a neutron star is much stronger than that of a white dwarf, the accretion under degenerate conditions leads to much higher temperatures than in the nova outburst. This in turn tends to produce X-rays rather than visible light in the thermonuclear runaway on the surface of the neutron star.

The adjacent figure shows a ROSAT X-ray satellite image of the Puppis Supernova Remnant (Ref). The supernova remnant, also known as Puppis A, is about 6000 light years away and is one of the brighter X-ray and radio sources in the sky. It is the remains of a supernova explosion that occurred about 4000 years ago. The supernova remnant glows with diffuse X-rays, but there is a bright point X-ray source near the center of the image (highlighted in the blowup). This is almost certainly the neutron star produced in the supernova explosion. (Its temperature, size, and brightness are what would be expected for a neutron star. The clinching information, observation of a pulsar, has not been supplied yet, however.)

By tracking the motion of the knots of material in the nebula and extrapolating the motion back it is possible to infer the place and time of the explosion. This can in turn be used to estimate how far the neutron star has traveled since the explosion and therefore its average velocity. By these means, the neutron star is found to have a space velocity of about 1000 km/s, suggesting that it was kicked out of the supernova explosion at high speed. Such high-velocity neutron stars are of particular interest for understanding supernova explosions. If the neutron star gets such a high kick velocity, this suggests that there is something unsymmetric in the explosion itself that sends the crushed core of the star in a particular direction with high velocity.