When a massive star dies, it probably blasts itself to bits as a supernova. All that’s left is its tiny but heavy core – a neutron star or black hole. But astronomers aren’t quite sure which stars are likely to end up in the two categories.

All stars shine by “fusing” elements in their cores to make heavier elements. Stars like the Sun convert hydrogen to helium. But the heaviest stars continue making new elements all the way up to iron. At that point, the core collapses. The outer layers fall in, then rebound in spectacular fashion – as a supernova.

The core’s collapse might stop when it forms a neutron star – an extremely dense ball only a few miles in diameter. But it might not stop – the core might collapse even more, forming a black hole.

The dividing line isn’t clear – astronomers aren’t sure which stars will form neutron stars, and which will make black holes.

There also appears to be a gap in those dead stars. The heaviest neutron stars yet seen are about twice the mass of the Sun. The lightest black holes, on the other hand, are more than three times the Sun’s mass. That could simply mean that we just haven’t found anything between them – neutron stars and black holes are hard to study. But it certainly means that scientists have more work to do to explain the fates of some of the heaviest stars in the universe.

Some massive stars might form black holes without exploding; more about that tomorrow.

Script by Damond Benningfield