The middling black holes were spotted using the Hubble Space Telescope in two separate globular star clusters in Earth's celestial neighborhood, astronomers said Tuesday at a briefing at NASA headquarters.
"These intermediate black holes were the missing link," said Steinn Sigurdsson of Pennsylvania State University.
While astronomers have known for years about vastly large black holes and rather small ones, Sigurdsson said, "We didn't know if we could get from one to the other or if they were completely unrelated, and this seems to be the step that takes us from one to the other."
Black holes are unimaginably dense regions in space whose gravitational pull allows nothing, not even light, to escape. For that reason, black holes are invisible but can be detected by the pattern of swirling stars and gas around their edges.
In the past several decades, black holes have gone from being rare and almost mythic phenomena whose existence was routinely questioned to being accepted by most astronomers as a feature of the cosmos.
Until now, though, black holes were thought to come in two basic sizes.
There were so-called stellar-mass black holes, created when stars about 10 times the size of our sun died in big explosions called supernovae.
Then there were supermassive black holes believed to lurk at the center of galaxies, including the Milky Way that contains Earth. Those black holes could have the mass of millions or even billions of suns.
Astronomers wondered whether there was a mid-sized version, and now they have found two of them, not in galaxies or floating free, but in tightly packed swarms of stars called globular star clusters.
Both fit the profile of what a mid-size black hole should be. The first, in cluster M15, has about 4,000 times the mass of our sun; the second, in cluster G1, has about 20,000 solar masses.
Because globular star clusters contain the oldest stars in the universe -- the smaller of the two mid-size black holes is in a cluster 13 billion years old -- information about them could help scientists figure out how the clusters form.
Mid-size black holes are in a "very important mass range," according to Karl Gebhardt of the University of Texas at Austin.
"That has implications for how you make a supermassive black hole and it is possible that these black holes can act as the seeds on which you make the supermassive black hole," Gebhardt said.
Scientists found a powerful pattern in the mid-size black holes, Gebhardt said. Their mass was related to the mass of the star cluster in just the same way the mass of supermassive black holes is related to the mass of the galaxies that contain them, he said.
"That has implications for how a globular cluster is related to a galaxy and how a galaxy is formed," he said.