This star cluster hides an 'anthill' of more than 100 black holes traveling through the Milky Way

Over a century ago, astronomers began cataloging compact groups of ancient stars swirling around our galaxy. These clusters were once seen as serene pockets of the Milky Way, but fresh data has exposed unexpected movement within one such group.

In a star-filled area located about 80,000 light-years (roughly 4.7 × 10^17 miles) from Earth, scientists uncovered signs of more than 100 stellar-mass black holes lurking among the stars.

The news arrives with insights from astrophysicist Mark Gieles from the University of Barcelona in Spain and astrophysicist Fabio Antonini of Cardiff University in the UK, who offer explanations for how these strange visitors got there.

Understanding star clusters – the basics

Star clusters are like cosmic neighborhoods – groups of stars that were born from the same giant cloud of gas and dust. These stars stick together because of gravity, kind of like how a family might live in the same town.

Some clusters are small and loose, called open clusters, and you’ll find them scattered across the arms of spiral galaxies like our Milky Way.

Others are massive and tightly packed, called globular clusters, and they hang out more toward the edges of galaxies, like ancient time capsules full of old stars.

By studying star clusters, astronomers get a peek into how stars live and die. Since all the stars in a cluster are about the same age but have different sizes, scientists can compare them like they’re in a natural science experiment.

It’s a bit like watching a group of people grow up together – some burn bright and fade fast, while others hang around for billions of years.

Peering into a star cluster’s heart

Globular clusters are tightly packed spheres of stars containing hundreds of thousands of suns. They date back to the dawn of the Milky Way, offering clues about early galaxy assembly.

When astronomers spotted a wide and loosely scattered band of stars around one old cluster, they suspected something odd.

The presence of so many scattered stars hinted at a push from heavy objects, and that push often comes from black holes.

Black holes hidden in star clusters

“We do not know how these streams form, but one idea is that they are disrupted star clusters” explained Gieles. Stars can get flung out of their cluster whenever they pass too close to a heavier partner.

His team’s work pointed to a scenario where a substantial fraction of the cluster’s mass is locked in black holes.

These stellar-mass black holes, each around 20 times the mass of the Sun, likely formed when huge stars exploded as supernovas. Their gravitational influence gradually scattered other stars into a long, flowing band.

Surprising streams beyond the core

Over time, these loosely bound stars stretch across tens of thousands of light-years. Once they drift far from the cluster’s center, they form a tidal stream that arches around the Milky Way.

Astronomers suspect there could be many more of these stellar highways linked to other clusters.

The swarming black holes remain in the cluster’s interior and continue kicking out stars, increasing the chance that the cluster itself will dissolve. Before it fades, the final core may end up as a dense pocket of black holes all orbiting together.

Evidence for future collisions

“A big unknown in this scenario is how many black holes there are in clusters, which is hard to constrain observationally because we can not see black holes.

Our method gives us a way to learn how many black holes there are in a star cluster by looking at the stars they eject” said Antonini. Black holes trapped inside dense clusters can eventually collide and merge. 

Such collisions release ripples in spacetime, called gravitational waves, detectable by observatories on Earth. The fresh findings add weight to the idea that many future detections might originate from these elderly star families.

Shifting perspectives on galactic history

Because globular clusters are ancient and hold clues from the Milky Way’s formative years, unusual findings like a mass of black holes could provide new insights into how galaxies evolve.

Researchers think some clusters may follow similar paths, losing stars at their edges until only dark objects remain.

Other star groupings (tidal streams) might have begun in the same way, although none so far show a nearby core cluster. Future maps of the Milky Way may turn up more clusters in the process of shedding streams.

Preparing for deeper searches

Astronomers plan to watch closely as more data becomes available.

The detailed measurements of star motions and compositions help pinpoint suspected black holes. They also highlight features like tidal tails, which can be teased out of extensive survey images.

New instruments and surveys may reveal whether similar black hole swarms lurk in other star clusters. The patterns seen so far suggest this might be more common than once imagined.

Glancing at the bigger picture

From massive star explosions to merging black holes, the life and death of these clusters affect how our galaxy changes over time.

Their outer layers spill out, mixing with other stars, and possibly bringing black holes deeper into the Milky Way.

Meanwhile, the race to find more of these dark wanderers continues. Some might end up in gravitational dance pairs, lighting up gravitational wave detectors in the future.

Others may just roam quietly, leaving behind star streams as cosmic footprints.

The study is published in Nature Astronomy.

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