Galaxies are orbited by hundreds and in some cases thousands of globular clusters, spherical collections of gravity-bound stars. The stars in these clusters seem to have a slightly different chemical composition from stars in galaxies and a group of researchers have put forward a new explanation. Globular clusters were the birthplaces of the most massive stars that ever existed.
After the Big Bang, only hydrogen and a dash of helium existed in the universe. All the other elements were created in stellar processes. Some elements can be created during the regular life of the star, others during supernova explosions, and others in collisions between neutron stars. Globular clusters mess with this picture a bit because they have elements that should have been produced in much hotter stars.
As reported in the Monthly Notices of the Royal Astronomical Society, a potential explanation could be supermassive stars. These objects would have a mass of over 10,000 times the mass of our Sun. The supermassive stars are believed to be hot enough to create these elements and pollute the other stars in the globular cluster.
"What is truly novel in our model is that the formation of the supermassive stars and the globular clusters are intimately linked, and this new mechanism is the first model that can form enough material to pollute the cluster, and with the correct abundances of different elements, which has been a long-standing challenge," lead author Professor Mark Gieles, of the University of Surrey, said in a statement.
The team suggests that these stars formed through a runaway collision process. Globular clusters are tightly packed, making it likely that these stars interact and end up merging into supersized objects. The researchers propose that the supermassive stars formed early on in globular clusters' lifetimes and quickly went through their hydrogen reservoirs, exploding into supernovae.
"There have been many attempts to solve this problem that has puzzled astronomers for decades and I believe that this is the most promising explanation that has been proposed so far," added co-author Professor Henny Lamers from the University of Amsterdam.
To find out if this model is indeed correct, we will need to find these massive stars. And to do so we need to look further back in time to when the universe was still young. It is possible that the next generation of space telescopes will be capable of spotting these tremendous objects.