Production of Neutron Stars

Neutron stars are made primarily from the crushed cores of Type II supernovae. The adjacent figure shows a ROSAT X-ray satellite image of the Puppis Supernova Remnant. 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 Neutron Star?
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 as inferred from the X-ray observations are what would be expected for a neutron star produced about 4000 years ago. No star can be seen at this position at optical wavelengths, further comfirming the neutron star interpretation.

Technically Speaking: Kick Velocities for Neutron Stars

High space velocity neutron stars are of particular interest for understanding supernova explosions. If the neutron star has a high space velocity, this suggests that there is something asymmetric in the explosion itself that "kicks" the crushed core of the star in a particular direction with high velocity. Thus, high space velocities for neutron stars are termed kick velocities. This is important because there are other indications that Type II supernova explosions may be asymmetric, and many neutron stars have unusually high kick velocities.

The Space Velocity
By tracking the motion of the knots of material in the Puppis A remnant 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 in the opposite direction from the general motion of the supernova remnant. This suggests that it was kicked out of an asymmetric supernova explosion at high speed (see the adjacent box).