Star packed: super cluster is first to be detected in Milky Way.

Imagine cramming half-a-million bright young stars into the solar system. Jammed that tightly, they would blast each other with radiation, and some might even coalesce into a black hole.

Known as super star clusters, such compact groupings had been detected only in remote galaxies. Now, astronomers have found a super star cluster in the Milky Way. The discovery will enable researchers to study in detail how huge gatherings of massive stars live and die.

The cluster, just 10,000 light-years from Earth, promises to be a Rosetta stone for deciphering star formation in an ultradense, extreme environment. Such conditions may have been common in the early universe. Packed into a region only 6 light-years across and estimated to be at least as heavy as 100,000 suns, the cluster is the first extremely massive, compact grouping of young stars ever found in the Milky Way, says Ignacio Negueruela of the University of Alicante in Spain.

He and his colleagues, including J. Simon Clark of University College London, describe their findings in two upcoming articles in Astronomy & Astrophysics.

Astronomers have known since 1961 that some sort of young-star grouping lies in the constellation Ara. But because the cluster, dubbed Westerlund 1, hides behind a large cloud of gas and dust researchers had no idea that it harbored such a high density of massive stars.

Using sensitive detectors on telescope at the European Southern Observatory in La Silla, Chile, Negueruela and his collaborators identified more than 200 massive stars in the cluster. Spectroscopy has revealed that each of these stars weighs 30 to 40 times as much as the sun. Some are a million times brighter than the sun, and a few are so bloated that if placed at the sun's location, they would bump up against the orbit of Saturn. The cluster is only about 5 million years old, the team estimates.

Measurements elsewhere in the galaxy have shown that there are about 100 sunlike stars for every star weighing 10 time that much. If the relation holds true it Westerhind 1, then the cluster contains at least half-a-million stars, most of them too lightweight to send visible light through the cloud of gas and dust. Observers will need a powerful infrared telescope to peer through the dust and document the enormity of the population, the team notes.

Observations of distant galaxies during the past few years have spotlighted other super star clusters, notes Cole Miller of the University of Maryland at College Park. When the universe was young, many galaxies tended to produce their stars within clusters, studies indicate. Miller says that Westerlund 1 could therefore reveal the star-forming processes dominant in the universe several billion years ago.

Collisions among massive stars in the Milky Way's crowded cluster could lead to, or may already have spawned, an intermediate-mass black hole, the discovery team notes. Such black holes, which weigh between several hundred and several thousand times the mass of the sun, have been proposed but never directly detected Strong X-ray emissions from the region could provide evidence of a black hole, Miller says.