With a name that’s like something out of Star Trek, it should come as no surprise that strange things are happening in Palomar 5. Astronomers set their sights on this globular cluster to try and understand how tidal streams (long stretches of stars) form. But they found something unsettling—over 100 tightly packed, stellar-mass black holes.
Palomar 5 is a globular cluster—basically a group of very old stars packed into a spherical shape. It’s located around 80,000 light-years from Earth, and is one of the 150 globular clusters known to orbit around the Milky Way. If that’s not weird enough for you, astronomers believe that all of the stars in globular clusters form at once, and Palomar 5’s stars date back to the universe’s beginnings.
So what does this have to do with tidal streams, the light-years-long stretches of stars the astronomers can’t explain? Well, astronomers have an interesting hypothesis; what if tidal streams are disrupted globular clusters? Are globular clusters doomed to stretch out into a long line of stars due to some internal or external force?
There may never be a satisfactory answer for this question, but scientists can’t just sit on their hands. In this case, the best idea is to find a globular cluster that’s associated with a tidal stream—Palomar 5 is the only one that fits the bill, as far as astronomers are aware.
Researchers at the University of Barcelona decided to run some N-body simulations, which use existing data to simulate the history of stars in Palomar 5. Because black holes are formed from dying stars and are known to slingshot stars through space, the astronomers decided to include black holes in some of their simulations.
The results are, frankly, a bit terrifying! Researchers’ simulations show that there may be over 100 black holes within Palomar 5, and that these black holes may be launching the cluster’s stars into the tidal stream pattern that we’re so interested in. If this data is correct, then 20% of Palomar 5’s collective mass is made up of black holes—if it makes you feel any better, black holes contain several times more mass than our Sun, so it’s not like Palomar 5 is just black holes.
Not yet, at least. The University of Barcelona’s simulations show that Palomar 5’s black holes will continue slinging stars into a tidal stream until there’s nothing left of the globular cluster, save for a few hundred black holes orbiting a galactic center.
Should this worry us? No, not at all. Those who live on Earth today will die long before humans encounter a black hole (assuming that it ever happens). Even if our Sun decided to spontaneously off itself, it’s much too small to become a black hole. If anything, the newly found black holes are just one of many incredibly disturbing things that we know about the world outside of Earth.