There are many things hidden in our Milky Way galaxy – one of them is black holes. A study published in the scientific journal Nature on July 5 discovered the presence of over 100 stellar black holes hidden within a cluster of stars moving through the Milky Way.
The cluster in question, known as “Palomar 5,”is about 80,000 light-years away, with stars as far apart as 30,000 light-years within the cluster – creating a stellar stream across the Milky Way. However, the black holes have caught the eye.
Mark Gieles, an astrophysicist at the Cosmos Sciences Institute of the University of Barcelona and lead author of the study, said: “The number of black holes is about three times higher than what scientists expected to find, which means that more than 20% of the total mass of the cluster is made up of black holes. Each of them has a mass about 20 times that of the Sun and was formed during supernova explosions at the end of the lives of massive stars, when the cluster was still very young.”
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The discovery helps identify Palomar 5 as a tidal stream rather than a globular cluster. The difference lies in the distribution of stars – globular clusters are made up of stars that all formed around the same time, while tidal streams have stars of varying ages.
Until now, the origin of the tidal streams has only been speculated upon. Now, with further analysis of Palomar 5, Gieles' team has been able to use N-body simulations to reconstruct the orbits and evolutions of the stars that make up the cluster.
This discovery helps us understand the mysterious star cluster, Palomar 5, and how it might evolve in the future. It may also help us understand the fate of other similar globular clusters of stars in our Milky Way. It may also help us explain the strange phenomena recently found in our Milky Way, the thin streams of stars that do not seem to be associated with such a cluster.
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Large clusters full of black holes, like Palomar 5, could have created these unexplained flows, according to the researchers.
Gieles pointed out the following: “We don’t know how these streams form, but one idea is that they are disrupted clusters of stars. However, none of the recently discovered streams have been associated with a star, so we can’t be sure. So, to understand how these streams formed, we had to study one that was associated with a star system. Palomar 5 is the only case, and it is the ‘key’ to understanding stream formation. That’s why we studied it in detail.”
It is located in the "galactic halo," which is made up of old stars surrounding our Milky Way. It is one of the rarest clusters in the "halo."
It is also known for its two long "tails," made up of extensive streams of stars. These "tails" extend across the night sky, allowing scientists to use them as a key way to understand how stars form throughout the universe.
In the new study, scientists simulated the “lives” of the cluster’s stars individually. The researchers can’t study the black holes themselves – as their name suggests, they’re impossible to see – so the study relies on a method that relies on how many there might be.
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The simulations showed that eventually – in about a billion years – the cluster will begin to disintegrate completely – the stars will “explode”, leaving only the black holes behind.
Fabio Antonini, an astrophysicist at Cardiff University, who also co-authored the study, said: “A large fraction of binary black hole mergers are thought to form in star clusters. A big unknown X in this scenario is how many black holes there are in clusters, which is difficult to answer because we can’t see the black holes. The new method gives us a way to find out how many black holes there are in a star cluster, simply by looking at the stars they are ejecting.”
He showed that these two defining features – its sparse structure and these tails – could be the result of 100 black holes that together make up 20% of its mass.
There are so many of these black holes, and they are so gigantic, that stars will be lost faster than them. So in the simulation the cluster swelled and its tail grew.
Eventually, the black holes will have so much gravitational force within the cluster that all the stars will be thrown out of it, and it will consist only of them.
