Puppis A nebula and neutron star imaged at X-ray wavelengths by ROSAT.
Puppis A is a supernova remnant that is one of the brightest radio and X-ray sources in the sky. It lies about 6,000 light-years away in the constellation Puppis. Puppis A is its radio source designation and it is also known by the X-ray designation 2U 0821-42.
A neutron star has been found in association with this remnant that is believed to be the remains of the progenitor star that exploded some 4,000 years ago. Observational evidence suggests that the pre-explosion mass of the star was about 25 solar masses – the heaviest progenitor star known with which a neutron star has been linked. In order for the neutron star to have reached its present location, it has traveled at a speed of about 1,500 km/s. While neutron stars associated with several other supernova remnants seem to be traveling much faster, the one in Puppis A has a unique aspect. It is traveling across the sky in the opposite direction from the knots of ejecta, providing strong evidence of an asymmetric supernova explosion. The star's outer layers appear to have blown off preferentially in one direction, sending the central neutron star in the other. There is currently no satisfactory theory that explains how this could happen.