Skip to main content

Hubble finds an unexpected collection of black holes

Hubble's view of dazzling globular cluster NGC 6397
Hubble’s view of dazzling globular cluster NGC 6397. NASA, ESA, and T. Brown and S. Casertano (STScI) Acknowledgement: NASA, ESA, and J. Anderson (STScI)

When scientists using the Hubble Space Telescope turned their attention to a globular cluster called NGC 6397, they were expecting to find one, medium-sized black hole at its center. But instead, they found something rather more strange. They found evidence of a collection of smaller black holes, in a cosmic oddity that could teach us about how black holes evolve.

There’s a “missing link” in the world of black holes, as we see regular-sized black holes caused by the collapse of a star and very large black holes at the heart of galaxies called supermassive black holes. But we almost never see black holes in between these two sizes. This means we don’t really understand how black holes merge or grow.

The hunt for medium-sized black holes, or intermediate-mass black holes (IMBH) as they are known, is what drew the researchers’ attention to NGC 6397. They hoped to find one of these missing link black holes at the center of the cluster. But that’s not what they found.

Artist’s Impression of the Black Hole Concentration in NGC 6397
This is an artist’s impression created to visualize the concentration of black holes at the center of NGC 6397. In reality, the small black holes here are far too small for the direct observing capacities of any existing or planned future telescope, including Hubble. It is predicted that this core-collapsed globular cluster could be host to more than 20 black holes. ESA/Hubble, N. Bartmann

The orbits of the stars in the cluster indicated that there wasn’t a single point of mass around which they orbited. They had seemingly random orbits that were best explained by the presence of multiple points of mass.

“We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster,” explained lead researcher Eduardo Vitral in a statement.

That made them think that what they were looking at was a collection of smaller black holes, each too small to be imaged directly. But their presence can be inferred by looking at the movements of the stars.

This unusual finding fits in with other recent work suggesting that the central regions of globular clusters could be full of smaller black holes. “Our study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster,” Vitral said.

Scientists believe these black holes formed from the remnants of the massive stars which once populated the cluster before they ran out of fuel and collapsed in on themselves. These stars sank to the center of the cluster due to their mass, forming the concentration of smaller black holes.

Editors' Recommendations

Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
Researchers want to use gravitational waves to learn about dark matter
Artist's conception shows two merging black holes similar to those detected by LIGO.

When two sufficiently massive objects collide -- such as when two black holes merge -- the forces can actually bend space-time, creating ripples called gravitational waves. These gravitational waves can be detected even from millions of light-years away, making them a way to learn about distant, dramatic events in far-off parts of the universe. And now, a team of astronomers has come up with a method for using gravitational waves to study the mysterious phenomenon of dark matter.

The idea of the research was to create different computer models of what gravitational waves from black hole mergers would look like in universes with different types of dark matter. By comparing the models to what is seen in the real world, we can learn more about what type of dark matter is most likely.

Read more
One galaxy, two views: see a comparison of images from Hubble and Webb
The peculiar galaxy NGC 3256 takes centre stage in this image from the NASA/ESA Hubble Space Telescope. This distorted galaxy is the wreckage of a head-on collision between two spiral galaxies which likely occurred 500 million years ago, and it is studded with clumps of young stars which were formed as gas and dust from the two galaxies collided.

It might not seem obvious why astronomers need multiple different powerful space telescopes. Surely a more powerful telescope is better than a less powerful one? So why are there multiple different telescopes in orbit, either around Earth or around the sun?

The answer is to do with two main factors. One is the telescope's field of view, meaning how much of the sky it looks at. Some telescopes are useful for looking at large areas of the sky in less detail, working as survey telescopes to identify objects for further research or to look at the universe on a large scale -- like the recently launched Euclid mission. While others, like the Hubble Space Telescope, look at small areas of the sky in great detail, which is useful for studying particular objects.

Read more
James Webb spots the most distant active supermassive black hole ever discovered
Crop of Webb's CEERS Survey image.

As well as observing specific objects like distant galaxies and planets here in our solar system, the James Webb Space Telescope is also being used to perform wide-scale surveys of parts of the sky. These surveys observe large chunks of the sky to identify important targets like very distant, very early galaxies, as well as observe intriguing objects like black holes. And one such survey has recently identified the most distant active supermassive black hole seen so far.

While a typical black hole might have a mass up to around 10 times that of the sun, supermassive black holes are much more massive, with a mass that can be millions or even billions of times the mass of the sun. These monsters are found at the heart of galaxies and are thought to play important roles in the formation and merging of galaxies.

Read more