Black holes are not necessarily as big – or as small – as astronomers thought, a team of international researchers has discovered.
Evidence has emerged of a so-called "middleweight" black hole, the first of its mass known to science, about 13,000 light years from Earth.
The research, published in the journal Nature on Thursday, showed a black hole with 2200 times the mass of Earth's sun, at the centre of the 47 Tucanae globular star cluster.
Globular clusters form spherically, with the stars tightly bound by gravity as they orbit a galactic core en masse.
Previous attempts to find a central black hole in 47 Tucanae had proven unsuccessful.
University of Queensland researcher Holger Baumgardt, from the university's School of Mathematics and Physics, said all previously known black holes had fallen into two categories.
Those were the small, stellar-mass black holes that weighed a few times the mass Earth's sun; and supermassive black holes that could weight billions of times more.
"Astronomers expect that intermediate-mass black holes, weighing 100 to 10,000 times the mass of our sun, also exist," Associate Professor Baumgardt said.
"But until now, no conclusive proof of such middleweights has been found."
As part of the research, the UQ researcher did computer simulations in partnership with project leader Bulent Kiziltan, from the Harvard-Smithsonian Centre for Astrophysics.
Dr Kiziltan said the intermediate-mass "middleweight" black holes were "missing links" between stellar-mass and supermassive black holes.
"They may be the primordial seeds that grew into the monsters we see in the centres of galaxies today," he said.
Most black holes, Dr Kiziltan said, were found by looking for the X-rays that were emitted from the hot disks of material that swirled around the event horizon.
But that was a method that only worked if the black hole was actively feeding on nearby gas, which was problematic in 47 Tucanae.
"The centre of 47 Tucanae is gas-free, effectively starving any black hole that might lurk there," Dr Kiziltan said.
"A supermassive black hole at the centre of the Milky Way gives clues to its presence by its influence on nearby stars.
"Years of infrared observations have shown a handful of stars at our galactic centre whipping around an invisible object with a strong gravitational tug.
"But the crowded centre of 47 Tucanae makes it impossible to watch the motions of individual stars."
That meant there needed to be a rethink on what to look for to prove there was a black hole in 47 Tucanae.
Black holes possess such strong gravity that nothing – not even light and time – can escape their effect.
The 47 Tucanae cluster contains about 3 millions stars in a sphere with a diameter of about 120 light years.
Also within the cluster were about 25 pulsars – rotating neutron stars that emit powerful beams of electromagnetic radiation – that were important targets of the researchers' investigation.
Dr Baumgardt said those pulsars, with their easily detected radio signals, were "flung about" due to the middleweight black hole's gravitational influence.
The other line of evidence was the overall motions of the stars throughout the cluster.
Dr Baumgardt said globular clusters' environments were very dense, so the stars with more mass tended to sink to the centre.
"An intermediate-mass black hole at the cluster's centre acts like a cosmic 'spoon' that stirs the pot, causing the stars near it to slingshot to higher speeds and greater distances, imparting a subtle signal that astronomers can measure," he said.
"By employing computer simulations of stellar motions and distances, and comparing them with visible-light observations, the team found evidence for just this sort of gravitational stirring.
"That caused them to be found at greater distances from the cluster's centre than would have been expected if no black hole was present."
Because the black hole had eluded detection for so long, the researchers said that suggested similarly sized black holes could exist in other globular clusters.
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