Usually, several hundred thousand stars are densely packed in globular clusters, but there are exceptions: for example, there are also bloated globular clusters with only a few tens of thousands of stars, often with additional tails. New computer simulations are now providing a possible explanation for this previously puzzling star cluster. Presumably, they were destroyed from within by a large number of black holes. This is the fate that most globular clusters face, according to scientists in the journal Nature Astronomy.
Astronomers know about 150 globular clusters in the halo – the spherical space around our Milky Way. About a tenth of them swelled enormously, like Palomar 5. The spherical cluster contained only ten thousand stars and had two tails that extended more than 20 degrees across the sky. Until recently, astronomers suspected that a very strong path through the galactic center Tidal power work on globular clusters. This will then tear most of its stars from the globular cluster, which will have the effect of distributing some of them in front of and behind the tail cluster.
But this statement can no longer hold up, say Mark Gieles of the University of Barcelona and colleagues. “In the case of several globular clusters that have passed through the strong tidal field from the galactic center, only minor perturbations and no tails can be detected,” the astrophysicists wrote. So there must be another reason. To track this, Gieles and his colleagues performed extensive simulations of the movement of stars in globular clusters, using Palomar 5 as a model.
In addition to dynamics, the astronomers also took into account in their simulations – and here’s what’s new – the development of stars into supernovas and black holes. It turns out that black holes can play a very important role in the evolution of star clusters. Most black holes that form after supernovae are “kicked” due to the asymmetry of the previous explosion, which pushed them out of the globular cluster. However, this is not the case for very massive stars. Because here the impact is not enough, so the black hole remains in the star cluster and, in turn, constantly ejects stars from the cluster due to its high gravity.
In this way, the star cluster loses most of its stars over billions of years, swells due to the reduced mass, and subsequently forms ejected star tails. In about a billion years, Palomar 5, according to the researchers, could be completely destroyed as a result. All that’s left is a cluster of a few thousand black holes — and a stream of stars along the orbit of the former star cluster through the galactic halo. Gieles and his colleagues estimate that the decay of the globular cluster must have created about 20 such stellar streams. And in fact, astronomers already know about thirty streams of stars in the galactic halo, the origins of which are still unknown – therefore they could be remnants of globular clusters.
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