A black hole likely hides in the middle of the brilliant globular star cluster Messier 4.

Invisible to the eye lies a powerful force at the center of a star metropolis.

The brilliant star cluster Messier 4, the closest such cluster to Earth at some 6,000 light-years away, contains hundreds of thousands of stars. It’s a sight to see. Now, NASA has employed its legendary Hubble Space Telescope to reveal what is likely a black hole, some 800 times the mass of the sun, at the center of the star cluster.

« You can’t do this kind of science without Hubble, » Eduardo Vitral of the Space Telescope Science Institute, which operates the instrument’s science missions, said in a statement.

This black hole is rare: It’s not a small black hole, the type of rogue object that roves our galaxy (scientists estimate there are a whopping 100 million of these in our Milky Way galaxy alone). And it’s not one of the monstrous « supermassive » black holes that lie at the center of galaxies — such as Sagittarius A* — weighing millions of times more than the sun (astronomers captured a rare picture of this giant Milky Way object). Rather, the new observation is a curious « intermediate mass » black hole, an oddity scientists have many questions about — like why might they be so rare?

Black holes contain unimaginable mass, with gravitational pulls so strong not even light can escape. How, then, did researchers reveal evidence of an unseeable object? They looked at Hubble’s observations of Messier 4 from a 12-year period, watching how stars moved near the cluster’s core, « like bees swarming around a hive, » NASA explained.

The star cluster Messier 4 contains hundreds of thousands of stars.
The star cluster Messier 4 contains hundreds of thousands of stars. Credit: NASA / ESA / Hubble

The research, recently published in the Monthly Notices of the Royal Astronomical Society, determined the motions of stars were likely influenced by a powerful center of gravity. They couldn’t realistically explain the stars’ behavior from other forces, like dense stars near the core. The evidence points to a singular black hole, something relatively small amid the grander Messier 4.

« It’s too tiny for us to be able to explain other than it being a single black hole, » noted Vitral.

You can see the dance of stars around the core of the star cluster in the NASA video below, at 50 seconds in:

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The case of the curious force at the center of Messier 4, however, is still ongoing. Though the evidence for a black hole is compelling, there’s still a chance that other forces might be at play, like previously unknown star activity and physics.

Hubble will be watching.