Observations suggest that a Type II supernova should occur roughly every 50 years in our Milky Way. In doing so, they enrich the ISM with the heavy elements that are so important to our everyday existence. It has been discovered recently that the heavy element abundance in intergalactic gases is comparable to that within galaxies themselves. Evidence that Type II supernovae make neutron stars was discovered about 30 years ago when pulsating sources of radio waves were found. Once it became clear that they were not beacons from extraterrestrial civilizations, the radio sources were named pulsars.
Most astronomers agree that the clocklike pulses are emitted from spinning neutron stars.
The Crab Nebula and its pulsar offer a well-known example of a neutron star within the ejecta of a documented supernova explosion. More than 500 pulsars have been identified, and theory suggests that there may be up to one billion neutron stars in our galaxy. These exceptional stars lose mass at such high rates that only a helium core remains by the time they leave the main sequence. These relatively rare objects are known as WoIf-Rayet stars. However, their iron cores are so massive that their collapse cannot he halted by neutron degeneracy.