Astronomers have found evidence in a dozen nearby galaxies for "super star clusters" that look to be young hotbeds of star formation.

The clusters are thought to be precursers to aged groupings of stars called globular clusters, which are common in our own Milky Way Galaxy. But their discovery creates a mystery.

Globular clusters in the Milky Way can be more than 10 billion years old. None are known to be forming now.

The young clusters spotted in other galaxies are not actually visible. Instead, they're hidden inside nebulous clouds of material. The glowing nebulas indicate the hot young clusters inside.

"The star clusters are young and still hidden within their birth clouds. Although we cannot see the clusters directly, we can clearly detect the infrared and radio emissions from the hot gas surrounding them," said Chao-Wei Tsai, a UCLA graduate student and leader of the team.

"This is star formation on steroids," said Jean Turner, a UCLA professor of physics and astronomy.

"The super star clusters hidden within these super nebulae are probably a lot like globular clusters in our own Milky Way, only younger, and they can contain up to a million young stars," Turner said. "The mystery is why our own Milky Way no longer forms globular star clusters and hasn't for 10 billion years. These galaxies still can. We want to know why."

Hot, blue stars are packed into the clusters at up to a million times greater density than stars in the vicinity of our Sun. Their collective emissions can be up to a billion times the sun's wattage, in a region only a few light years across.

The radio and infrared images were made using the Very Large Array of the National Radio Astronomy Observatory, a facility of the National Science Foundation, and the W. M. Keck Observatory in Hawaii.

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