One of the world’s most famous telescopes went searching for a single black hole, and unexpectedly found a bunch of smaller black holes instead.
The strange find is producing a “gravitational pinball game” inside a collection of stars known as NGC 6397, roughly 8,000 light-years from Earth. And what we see now with the Hubble Space Telescope is not permanent, as scientists expect the black holes in the center will eventually collide, combine and send out gravitational waves across space.
This discovery is yet another example of how complicated space is, and how much more we still have to learn about the evolution of black holes, stars and well, the universe at large.
The paths of the stars in this cluster are moving almost at random due to the distribution of several black holes in the center, NASA’s Hubble website noted in a release. So how did those black holes get there in the first place? It’s a story with a few plot twists that may change as we track more of the strange gravity interactions that take place in tightly packed clusters of stars, but here’s what researchers have come up with so far.
The goal of the research at first was to search out an intermediate-sized black hole, an unconfirmed, theorized kind of black hole that would fall somewhere in between a star-sized black hole (produced when a ginormous star explodes) and a galactic-class supermassive black hole that forms a crucial part of most galaxy histories — including the one in our own galaxy, the Milky Way.
Black holes, as the name implies, cannot be seen. But we can track them by the radiation they emit and the motions of objects nearby. The star motions in NGC 6397, however, were diffuse and showed that they were being affected by several black holes bunching in the center of the cluster. (Researchers backed up the observations with Hubble using the European Gaia mission, which tracks star motions through space.)
Plotting the motions of the stars gave a sense of just how massive these black holes are, and the simple answer is — they are stellar-sized. Researchers suggest that a few larger stars exploded and, across cosmic timescales, gradually migrated to the center of the star cluster after their individual gravity wells interacted with nearby, less massive stars.
Essentially, what we’re seeing is physics writ large in the star cluster, where momentum puts the more massive gravitational objects (the black holes) towards the center and the smaller gravitational objects (the smaller stars) around the perimeter.
While two other, unrelated studies recently suggested that black holes could congregate in the center of star clusters, this is the first study that shows the particular mass and extent of black holes in the center of a cluster, the researchers said. The hope is that we can find even more of these collections of black holes and by extension, to figure out more about gravitational waves as they ripple across the universe.
A study based on the research was recently published in Astronomy and Astrophysics.
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