October 18, 2004I
mages taken by the Spitzer Space Telescope and the Wyoming Infrared Observatory reveal a previously undiscovered globular star cluster passing through our galaxy's disk.
"It's like finding a long-lost cousin," says team leader Chip Kobulnicky, a professor of physics and astronomy at the University of Wyoming in Laramie.
Globular clusters are spherical collections of old stars whose orbits, centered on each galaxy's core, carry them into distant so-called halo regions. Globulars are much different than the other type of star group, called open clusters, which are smaller collections of recently formed stars. Our Milky Way has more than 150 globular clusters associated with it.
The newly discovered cluster lies in the direction of the constellation Aquila, near that star group's border with Serpens. It is between 10,000 and 17,000 light-years from Earth, with the closer distance being more probable. This places the cluster closer to us than most other globulars.
If the new cluster lies at that distance, its true size is between 3 and 6 light-years across. It has a mass estimated at 300,000 times the Sun's and an age estimated at 10 billion years. This fits nicely with the ages of most globulars: between 10 and 13 billion years old.
Spitzer snapped the images as part of work on the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire, or GLIMPSE. The discovery team has designated this object GLIMPSE-C01, although, as Brent Archinal, a geodecist at the U.S. Geological Survey in Flagstaff, Arizona, points out, the object has a name: IRAS 18462-0133. "This object already was identified — as an object of unknown type in the IRAS survey," he says. IRAS, or the Infrared Astronomical Satellite, scanned the sky for 10 months in 1983 and discovered 500,000 infrared sources.
Whatever name astronomers finally assign to the object, many of its features hint it's a globular cluster:
- There is no radio emission (a characteristic of star-forming regions in the Milky Way), which indicates the cluster is an old object with no massive stars.
- The cluster contains many giant stars but no bright supergiants.
- The object is much brighter and more massive than known open clusters. Because it's closer to Earth, it's also brighter than most known globulars.
- The density of stars in the object is consistent with known globulars.
- Its position in the galaxy — only 20,000 light-years from the Milky Way's center — rules out the possibility that it's an open cluster; open clusters have been found no closer than 25,000 light-years from our galaxy's center.
Astronomers think this discovery is the tip of the iceberg. Archinal notes that the cluster is obvious in all three wavelengths of the 2-Micron All-Sky Survey. "The only reason it wasn't found before is because no one is looking," he says. "If a cluster this bright hadn't yet been found, you can bet there are plenty more."