Q&A: Dark Matter

Q:
Why does the production of neutron stars and stellar black holes imply the release of a lot of energy and heavy elements, and why does this make them unlikely dark matter candidates?

A:
Although there is still considerable difference of opinion of the details, the general view is as follows. Stars that begin with ten times as much mass as the Sun or more will ultimately undergo a catastrophe in which their core, or central region will collapse. This collapse forms a neutron star – or if the star is very massive, a black hole – and releases a tremendous amount of gravitational energy in the process, like water falling over a dam. See Supernovas

The energy released in the collapse also creates an enormous pressure that blows away the outer layers of the star. These outer layers are rich in elements such as carbon, nitrogen, oxygen, etc. that have been produced by nuclear reactions inside the star over the course of millions of years.

If there had been a sufficient number of massive stars to produce the number of neutron stars and black holes needed to explain the dark matter, the number of supernovas associated with their creation would have produced a huge amount of energy and heavy elements, which is not observed.

Stellar evolution calculations suggest that a very massive star (greater than about 40 solar masses) could collapse totally into a black hole without a supernova explosion, in which case the amount of energy and heavy elements released would be much smaller. However, such massive stars are observed to be quite rare at present. Whether they were more abundant in an earlier era billions of years ago when galaxies were forming is still an open question, though it is considered highly unlikely that there would be enough of these objects to explain the dark matter.

For more information see Dark Matter Mystery