GREEN RIVER - What has a half-million stars, weighs as much as 300,000 suns and seems from Earth to be about as big as a grain of rice held at arm's length?

Scientists say it's the newly discovered Cowboy Cluster in the Milky Way.

University of Wyoming astronomers have discovered a never-before-seen globular cluster within the dusty confines of the Milky Way galaxy, according to a Tuesday release from the UW News Service.

New infrared images taken from NASA's Spitzer Space Telescope and the University of Wyoming's Infrared Observatory revealed the cluster of hundreds of thousands of stars earlier this year. The release said the new finding will be reported in a forthcoming issue of the Astronomical Journal.

"It's like finding a long-lost cousin… We thought all the galaxy's globular clusters had already been found," said Chip Kobulnicky, a professor in UW's Department of Physics and Astronomy and the lead author of the new scientific paper.

Andrew Monson, a UW graduate student, first spotted the unnamed cluster.

"I couldn't believe what I was seeing," he said. "I certainly wasn't expecting to find such a cluster."

Kobulnicky said globular clusters are ancient bundles of stars that date back to the birth of the Milky Way about 13 billion years ago. The tightly packed knots of stars are among the oldest objects in the galaxy.

Astronomers use clusters as laboratories for studying the age and formation of the Milky Way. The clusters contain hundreds of thousands of stars, most of which are older and less massive than our sun.

Infrared astronomy allows scientists to see stars and other celestial bodies being born and glimpse what the universe looked like when it first formed.

The new cluster is located in the constellation Aquila and is one of about 150 star clusters known to orbit the center of the Milky Way, Kobulnicky said.

Monson said he first noticed the new cluster while scanning data from the Spitzer Space Telescope's infrared survey of the galaxy's mid-plane region. He then searched for archival data for a match and found only one undocumented image of the cluster from previous NASA surveys of the same region of sky.

Dust blocks most visible light in the Milky Way's mid-plane, and stars are completely hidden in visible light.

"The cluster was there in the data, but nobody found it," Monson said.

Follow-up observations with the infrared observatory helped set the distance of the new cluster at about 9,000 light years from Earth, relatively close as far as clusters go, according to the astronomers. The mass of the cluster is equivalent to 300,000 suns.

Monson said the cluster's apparent size, if viewed from Earth, is comparable to a grain of rice held at arm's length.

UW's the largest infrared observatory in the continental United States and ranks as one of the premier observatories in the world. The telescope can map the rotational velocities of individual galaxies.

The 2.3-meter telescope operates by collecting light in the infrared range of the spectrum. Infrared detection allows astronomers to see through clouds of cosmic gas and dust.

Since its completion in 1977, the telescope has been located at a former U.S. Forest Service fire station on Jelm Mountain, a 9,662-foot peak located about 25 miles west of Laramie.

Southwest Wyoming bureau reporter Jeff Gearino can be reached at 307-875-5359 or at gearino@trib.com.