A black hole 20,000 times larger than the sun sounds like it would be hard to miss.
But it took researchers 20 years to find a new category of black holes that could potentially help explain the evolution of galaxies.
Black holes are deep gravitational sinks from which not even light can escape, making them difficult to see with a telescope.
Until now, scientists assumed that there were two types of black holes: stellar ones, which have about 50 times the mass of the sun, and supermassive ones, which are so large that they are located at the center of galaxies.
For two decades, researchers have been looking for evidence of medium-sized black holes to better understand how galaxies form.
Now they claim there is clear evidence for this.
It was discovered at the center of the Omega Centauri star cluster in the Milky Way, about 18,000 light-years away, and is believed to be 20,000 times the size of the Sun.
“When I saw the data, I knew immediately that we had found something special,” said Holger Baumgardt, a researcher at the University of Queensland.
Researchers examined more than 500 telescopic images of 1.4 million stars in the Omega Centauri cluster and identified seven that were moving so fast they should have left the cluster and never returned.
The most plausible explanation they could find for the rest of these stars in the cluster was a black hole.
This is due to the strong gravitational force that keeps the stars in their orbit, while scientists cannot see anything at the center.
Mr Baumgardt believes this discovery is the missing link to understanding the evolution of black hole size and galaxy formation.
He said astronomers aren’t sure how the supermassive black holes could get so big while the other type is so much smaller.
However, the existence of an “intermediate” black hole could explain how their size increases.
“This would have some consequences for our understanding of the evolution of galaxies,” he said.
The researchers now want to find out how they came into being, how common they are and whether black holes can grow.
Mr. Baumgardt believes that there are other medium-sized black holes.
The research results were published in the scientific journal Nature.