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The curious case of NGC 6397

Can a globular cluster create an open cluster? NGC 6397 may have done just that.
NGC 6397's collision course
Diving through the Milky Way's disk about 4.5 million years ago, globular cluster NGC 6397 appears to have left an open cluster, NGC 6231, in its wake.
Astronomy: Roen Kelly
Finder chart for NGC 6397 and 6231
Try looking for both clusters. NGC 6231 lies near the bright star Zeta Scorpii, while NGC 6397 resides in Ara a few degrees farther south.
Astronomy: Roen Kelly
Globular cluster NGC 6397
NGC 6397, located 7,200 light-years away in the southern constellation Ara, is one of the nearest globular star clusters. Globular clusters, so named for their spheroidal appearance, hold clues about the onset of star formation in our galaxy. NGC 6397 contains about 400,000 stars and has undergone a "core collapse," which makes its central area very dense. Recent work suggests that NGC 6397 was born just a few hundred million years after the Milky Way's first stars formed.
ESO
In a 1996 paper, a team led by John Wallin of George Mason University in Fairfax, Virginia, noted that a globular cluster passes through the Milky Way's disk every million years or so. The astronomers predicted that if such a passage happened near molecular clouds teetering on the brink of collapse, the globular's gravity would tip the balance, trigger the cloud's collapse, and spark an episode of star formation. The paper attracted little attention at the time.

In 2004, Richard Rees of Westfield State College in Westfield, Massachusetts, and Kyle Cudworth of the University of Chicago's Yerkes Observatory in Williams Bay, Wisconsin, announced they'd found just such an occurrence in our own Milky Way. The culprit is NGC 6397, a 12-billion-year-old globular cluster about 7,200 light-years away in the constellation Ara. The globular has a mass of about 250,000 Suns.

Rees, Cudworth, and colleagues studied photographs of NGC 6397 taken between 1893 and 1990 to determine how the cluster moved across the sky. Together with spectroscopic measurements, which revealed the cluster's line-of-sight motion via the Doppler effect, the astronomers calculated where the cluster is headed — and where it had been. NGC 6397 plunged through the Milky Way's disk about 4.5 million years ago. It now lies about 1,500 light-years beneath the galaxy's disk and is moving into the halo.

Rees and Cudworth calculated where NGC 6397 hit the disk and tracked the impact point through 4.5 million years of our galaxy's rotation. "When I looked at a map of that region of the sky, NGC 6231 jumped out at me," explains Rees. NGC 6231 is an open star cluster less than 5 million years old and located between 5,200 and 6,500 light-years away in Scorpius. "I'm surprised we found something that easily," he muses. The astronomers first presented their results at the January 2004 meeting of the American Astronomical Society in Atlanta, Georgia.

Using published motions of NGC 6231, Rees and Cudworth calculated its three-dimensional space velocity and extrapolated backward to see where the open cluster was when NGC 6397 struck the disk. "The biggest uncertainty is that we don't know the distance to NGC 6231 very well," says Rees. Its position at the time NGC 6397 hit the disk matches better if NGC 6231 is now at the far end of current distance estimates. "But even if it's at the near end, they match tolerably well." Rees plans to look for additional cases of star formation triggered by passing globular clusters.

Hearing about this study made John Wallin's day. "It's really nice when a crazy theory turns out to be true," he says.
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