Black holes are antisocial: Not only do they gobble gas from the stars that orbit them, but according to theory, they also kick one another out of their homes.

Now, however, astronomers say they have spotted two separate black holes that inhabit the same cluster of stars, suggesting more black holes await discovery - both in this cluster and in others.

Never before have astronomers found a black hole in a Milky Way star cluster.

The star cluster in question is a so-called globular cluster, a tightly packed conglomeration containing hundreds of thousands of stars.

All Milky Way globular clusters formed long ago, so their short-lived massive stars have died and become black holes.

Each black hole weighs more than any of the many cluster stars that still shine and so should sink to the cluster's centre.

There, their large gravity ejects other black holes into the galaxy at large, leaving behind at most one black hole system.

At least, that's how the theory went. Now, astronomer Jay Strader of Michigan State University in East Lansing and colleagues report online today in Nature that they have found two separate black holes in M22, a globular cluster 10,000 light-years from Earth in the constellation Sagittarius.

Observations with the Very Large Array near Socorro, New Mexico, revealed two objects whose radio spectra resembled those of other black hole systems located outside of star clusters.

The discovery came as a surprise.

"We were pretty excited," said astronomer and team member Thomas Maccarone of the University of Southampton in the United Kingdom.

"But we wanted to be cautious and make sure that we could rule out all the other possibilities."

For example, the scientists said that white dwarfs or neutron stars wouldn't emit so many radio waves.

"It's a pretty significant discovery," said Stefan Umbreit, an astrophysicist at Northwestern University in Evanston, Illinois, who was not part of the team.

"This is an indication that the [black hole] ejection process is not as efficient as we thought."

However, astronomer Shrinivas Kulkarni of the California Institute of Technology in Pasadena is skeptical - and was even willing to bet $100 that neither object was a black hole.

"Great results need great proof," he said.

"This doesn't convince me."

Given M22's plethora of stars, Kulkarni said the cluster could easily have some odd star that mimics a black hole system's peculiar radio spectrum.

If the discoveries hold up though, they represent two firsts.

They're the first black holes ever found in a Milky Way star cluster.

They're also the first stellar-mass black holes discovered by means of radio waves.

Other stellar-mass black holes - such as the one in Cygnus X-1, the first black hole found - came to attention because hot gas swirling into the black holes emits x-rays.

In fact, Strader's team detected no x-rays from the black hole systems, indicating they give off far more radio waves than x-rays.

Strader said that's a characteristic of black holes that are 10 times to 20 times as massive as the sun, on the heavy side for stellar-mass black holes, making them comparable to the black hole in Cygnus X-1, which is 15 solar masses.

Furthermore, both of M22's black holes lie within just a couple of light-years of the cluster's centre, a location consistent with high masses, because heavy objects should sink into the cluster's core.

But Kulkarni counters that such massive objects should sink even closer to the centre and said the lack of x-rays meant they weren't black holes at all.

If the objects were black holes, however, each presumably had a star that orbited it and dumped material onto it in order to generate the observed radio waves.

In one case, observations with the Hubble Space Telescope reveal that the putative black hole lied near a red dwarf, a star much smaller, cooler, and fainter than the Sun.

In the other case, the companion may be an orange dwarf, whose properties lie between those of a red dwarf and the Sun.

But the astronomers weren't sure, and they hoped to use Hubble to make additional observations.

The discovery suggested that M22 hosts yet more black holes, because black holes without stellar partners wouldn't emit radio waves.

"I think it's pretty likely," Strader said.

If so, black holes may not be as antisocial as thought - although you probably wouldn't want to invite them to a party.

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