In my previous article I discussed the possibility that the Universe contains a strange type of star called a Planck star, which is prevented from collapsing due to the effects of quantum gravity. However they are not the only type of exotic star which may exist, and certainly not the only kind that can be searched for with modern telescopes.

Another interesting possibility that is being studied by the theoretical physics community is the dark star. 

Before I go into details though, I must clarify an unfortunate duplication of names. Prior to the development of the theory of general relativity and of the discovery of black holes, some physicists and astronomers discussed the possibility of a star in nonrelativistic classical physics which was so massive that Newtonian gravity would prevent light from escaping. These objects have some similarities to black holes, but are not identical. However since we know that general relativity is the correct theory of gravity and not Newtonian gravity, such objects have not been studied or searched for in much detail for the past century. These objects were referred to as dark stars as well, but they are not the dark star that are of interest to the astrophysics community currently.

The modern dark star is fueled by dark matter. As I have discussed in a number of previous articles, there is a problem with our understanding of the Universe, and specifically of what it is made of. All known forms of matter, such as atoms and molecules, constitute less than 5% of the energy in the Universe. The remainder is composed of a strange form of matter that does not interact with light, known as dark matter, and an even stranger form of energy known as dark energy. It is the former that we are interested in for the moment.

We know dark matter exists, and we know that it can clump together. Large clouds of dark matter are what holds galaxies together. However that is about all we know about this odd substance. We have some strong theories that suggest it could be supersymmetric in nature, or could be caused by higher dimensions in spacetime, but experimental data on its nature has so far eluded us. But we do know that it can form clumps.

And so if dark matter can form clumps, then there should exist objects in the Universe that have dark matter cores. Ordinary stars form when clouds of atomic matter condense into spheres, and the immense pressures that are created start fusing the atoms together. This creates light and heat, and forms all of the stars that we see in the sky. By similar arguments, clouds of dark matter should also condense into similar objects, but with different properties. These are known as dark stars.

Such dark stars would not be able to operate through nuclear fusion, since dark matter is not made of nuclei. However their mass would possibly trap nuclei which could then gain sufficient energy to fuse together.  The reaction energies would be different, and the densities probably (but not necessarily) lower. Another variation is that the dark matter cores would add heat to the star as well.  Our best models of dark matter, known as weakly interacting massive particles or WIMPs, need to annihilate with each other to generate and maintain the correct density in the early Universe, and this would also imply that the high density and high energy WIMPs inside the dark star would have a high annihilation rate. The products of these annihilations - whether high energy gamma rays or other massive particles - would then interact with the nuclei and generate different types of nuclear reactions. Until we have a better understanding of dark matter though, we cannot easily predict what these differences will be.

At present it is believed that if such dark stars do exist, they will have been primarily generated in the early Universe before the ordinary stars were formed. However there are competing models of dark stars at present, and in some models they will continue to form in the present cosmological era while in others they are suppressed by the formation of ordinary stars.

However in all of these models they are predicted to be far heavier, larger, and brighter than our own Sun. If they exist, then they would be seen as very bright objects in the sky that do not follow the well established life cycles of normal stars. And that means that we should be able to search for them with modern telescopes. In fact one of the missions of the new James Webb space telescope (which will hopefully replace the Hubble and produce even better data and images) will be to search for exotic stars such as these in the Universe. 

Dark stars are potentially very interesting objects. We know dark matter exists, and we know that it could condense into objects that are very similar to stellar cores. And we know that if these dark stars exist, then they will be observable in the next generation of space telescopes.

It will be interesting watch this research over the next few years.