We now know they had witnessed a supernova explosion that happened 6,000 light years from earth — relatively close by cosmic standards.

For these most massive of stars, the final collapse gives rise to one of the rarest and most spectacular sights in the universe — a supernova explosion.

After the supernova explosion, the ejected material expands and becomes a supernova remnant.

This delay, called a light echo of the supernova explosion also allowed the astronomers to measure the spectral signature of the light from the explosion.

The notion that pulsars were neutron stars, leftovers from a supernova explosion, had been proposed in 1933.