It is well known that our Milky Way galaxy has a spiral shape. Perhaps it is less well-known that the Milky Way is surrounded by about 150 dense star clusters. To picture this better, if we imagine that a beehive is the Milky Way galaxy, then the bees orbiting the beehive in all directions would be the dense star clusters.
It was in the year 1919 that Harlow Shapley counted these star clusters, also known as globular clusters. Interestingly, he found that there were about twice as many stars in one direction compared to the anti-direction, from which he inferred that the Sun + Earth system lies about two-thirds of the way out from the center.
This resulted in a shift in our philosophical outlook, for with this one exercise humans were humbled out of their assumed ‘central’ placement in our own galaxy.
A new use for globular clusters is discussed in this week’s issue of Nature magazine. In this study, the speeds of rapidly-rotating dead stars or “pulsars” are measured. The question is then asked, “How much mass needs to be at the center of the star cluster in order to explain their rapid speeds?” It turns out that the central masses need to be very large, around millions of times the mass of the Sun.
In the study they find that this large mass must be, in turn, confined to a very small space. As no such large and bright object is seen in the cluster center, the most likely option is that there is an intermediate mass black hole at the center of this one globular cluster.
It is not a new idea to test for potential intermediate mass black holes, but it is an idea that is famously hard to test given that black holes are well……black! If it is true that intermediate mass black holes exist, then it is our best bet for finding what was the “seed” of our own Milky Way galaxy when it was born more than 13 billion years ago.