Black holes are the gateway to the abyss of the universe. They usually come in two sizes. The first type are stars massive enough to collapse into a singularity five to 10 times the mass of our Sun when they collapse, disintegrating into something smaller than the period at the end of this sentence.

The second known type are monster black holes, with masses millions or even billions of times greater than that of our Sun. They live in the dark hearts of galaxies, gobbling up stars that pass nearby. They must be closely associated with the early days of galaxy formation.

So-called intermediate-mass black holes, weighing hundreds or thousands of solar masses, are elusive. Are they the “missing link” in the evolution of black holes?

One of the best candidates lurks at the center of the massive globular star cluster Omega Centauri. Because black holes can’t be observed directly, astronomers must use “stellar forensics” by measuring how the black hole’s gravitational pull affects the motion of nearby stars. The speeds of stars can be used to estimate a black hole’s mass. The faster the star is moving, the stronger the black hole’s gravitational pull.

It’s a painstaking process that only the Hubble Space Telescope can do for something as distant as Omega Centauri. Astronomers used more than 500 Hubble images spanning two decades of observations to track the motions of seven fast-moving stars in the deepest part of the star cluster.

The black hole at Omega Centauri is estimated to be 8,200 times the size of our Sun. If confirmed, it would be closer to Earth than the 4.3 million solar-mass black hole at the center of the Milky Way, which is 26,000 light-years away, while Omega Centauri is 17,700 light-years away.

Hubble Space Telescope finds compelling evidence for existence of intermediate-mass black hole in Omega Centauri

Most known black holes are either extremely massive, such as the supermassive black holes found at the nuclei of large galaxies, or relatively lightweight, less than 100 times the mass of the Sun. Intermediate-mass black holes (IMBHs), however, are rare and are considered rare “missing links” in the evolution of black holes.

Now, an international team of astronomers has used more than 500 images taken over two decades of observations by NASA’s Hubble Space Telescope to find evidence of an intermediate-mass black hole by tracking the movement of seven fast-moving stars toward the Earth’s innermost region, the globular star cluster Omega Centauri.

These stars provide compelling new evidence for the existence of the gravitational pull of an intermediate-mass black hole pulling on them. To date, only a few other IMBH candidates have been found.

Omega Centauri consists of about 10 million stars bound together by gravity. It is about 10 times larger than other large globular clusters, almost as large as a small galaxy.

Research and Discovery at IMBH

Among the many questions scientists want to answer are: Do IMBHs exist, and if so, how common are they? Do supermassive black holes grow from the IMBH? How do IMBHs themselves form? Are dense star clusters their preferred home?

Astronomers have now built a massive catalogue of the motions of 1.4 million stars by measuring their velocities from Hubble images of the cluster. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they have proven to be an ideal database for the team’s research efforts.

“We discovered seven stars that shouldn’t be there,” said Maximilian Heberle of the Max Planck Institute for Astronomy in Germany, who led the study. “They are moving so fast that they escape the crowd and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that could be that big is a black hole with a mass of at least 8,200 times that of our Sun.”

Consequences of the discovery and its historical significance

Several studies have shown the existence of an IMBH in Omega Centauri. However, other studies have suggested that a central cluster of stellar-mass black holes could be producing the mass, and that the lack of stars moving faster than the required escape velocity makes IMBHs less likely in comparison.

“This discovery is the most direct evidence yet for the existence of an IMBH in Omega Centauri,” added team leader Nadine Neumeier of the Max Planck Institute for Astronomy in Germany, who initiated the study with Anil Seth of the University of Utah, Salt Lake City. “This is exciting because very few other black holes with similar masses are known. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”

If confirmed at 17,700 light-years away, the candidate black hole would be closer to Earth than the 4.3-million-solar-mass black hole at the center of the Milky Way, which is 26,000 light-years away.

Omega Centauri is visible to the naked eye from Earth and is a favorite of stargazers in the southern hemisphere. Located just above the plane of the Milky Way, the cluster appears nearly as large as the full Moon when viewed from the dark countryside. It was first listed as a single star in Ptolemy’s catalogue almost 2,000 years ago. Edmond Halley reported it as a nebula in 1677. In the 1830s, English astronomer John Herschel was the first to recognize it as a globular cluster.

A paper on the discoveries led by Heberle and colleagues was published online July 10 in the journal Nature.