EMBARGOED FOR RELEASE: 9:20 AM EST, January 5, 2004

ASTRONOMERS FIND FIRST EVIDENCE FOR A NEW MECHANISM FOR TRIGGERING STAR FORMATION

Astronomers are announcing today the first observational evidence for a new mechanism to the trigger the formation of new stars – an old, massive star cluster may have the sparked the formation of a younger star cluster when it plunged through the disk of the Milky Way Galaxy less than five million years ago. This announcement is being made in a presentation today at the American Astronomical Society Meeting in Atlanta, Georgia, by Dr. Richard Rees of Westfield State College in Westfield, Massachusetts, and Dr. Kyle Cudworth of the University of Chicago’s Yerkes Observatory in Williams Bay, Wisconsin. The result is of special interest because it increases our understanding of how stars (including the Sun) are formed and how different components of the Milky Way Galaxy interact with each other.

The old star cluster in question is NGC 6397, which is one of about 150 known globular clusters in the Milky Way Galaxy. Globular clusters typically contain about a few hundred thousand stars. They are found mostly in the spherical halo of stars around the Milky Way, whereas most stars, including the Sun, are found in the Milky Way’s flattened disk. NGC 6397 is one of the closest to the Earth, about 7,500 light years away. NGC 6397 is in the southern constellation Ara and is about 12 billion years old.

The discovery is the result of a follow-up to a study of the motion of NGC 6397 done by Rees and Cudworth in collaboration with Jeff Benensohn, formerly of the University of Chicago, and Steven Majewski, now at the University of Virginia. In that study, photographs of the cluster taken between 1893 and 1990 with four different telescopes – the former Harvard 0.3-meter (13-inch) and 0.6-meter (24-inch) telescopes in Peru, the former Harvard 1.5-meter (60-inch) telescope in South Africa, and the DuPont 2.5-meter (100-inch) Telescope at the Las Campanas Observatory in Chile – were compared to measure the small changes in position of nearly six hundred stars in the vicinity of NGC 6397. The changes are so small that they can be only accurately measured by using photographs covering such a long span of time.

These small changes in position – called proper motions – enable astronomers to determine how fast the cluster is moving perpendicular to our line of sight, or its tangential velocity. The cluster’s velocity along the line of sight – its radial velocity – can be measured by use of the Doppler Effect, the shift in the color of the light from an object moving toward or away from the Earth. Combining the cluster’s tangential velocity and radial velocity tells astronomers how it is moving in three-dimensional space. They can then calculate where the cluster is going – and where it has been.

NGC 6397 is currently about 1,500 light years below the Milky Way’s disk and is moving away from it. “We extrapolated its motion backwards to see how long it has been since it went through the Milky Way’s disk,” said Rees. “We found that it went through the disk less than five million years ago. That’s a long time for a human, but it’s the blink of an eye to NGC 6397.

“Since NGC 6397 is about 250,000 times the mass of the Sun, it seemed reasonable that it may have had a noticeable effect on the Milky Way’s disk at the point of impact,” Rees continued.

Rees and Cudworth carefully calculated where NGC 6397 hit the Milky Way’s disk. The disk of the Milky Way rotates, so they calculated where five million years of that rotation would have brought the impact point to today. This gave them a region on the sky to search for anything that might be connected.

“When I looked at a map of that region of the sky, NGC 6231 jumped out at me. I’m surprised we found something that easily,” Rees said.

NGC 6231 is a very young star cluster, less than five million years old, in the disk of the Milky Way in the constellation Scorpius. Its distance from the Earth is not precisely known, but it is between about 5,200 and 6,500 light years away.

Fortunately, several determinations of the motions of NGC 6231 had already been published. Using these published motions of NGC 6231, Rees and Cudworth calculated its three-dimensional space velocity and extrapolated backwards in time to see where it was when NGC 6397 plunged through the disk. The two clusters were indeed close to each other at the time.

“The biggest uncertainty is that we don’t know the distance to NGC 6231 very well,” Rees said. “Their positions at the time NGC 6397 hit the disk match better if NGC 6231 is now at the far end of its current distance estimates, but even if it’s at the near end they match tolerably well.”

It seems that NGC 6231 formed at about the same time (or shortly thereafter) as the NGC 6397 disk impact, and in about the same place, but can a globular cluster plunging through the disk trigger star formation? Astronomers believe that stars form from giant clouds of interstellar gas when the clouds are compressed, leading to gravitational collapse. This compression can be caused by supernova explosions, winds from luminous stars, collisions between clouds, in collisions between galaxies, and now it seems by globular clusters.

In a 1996 paper in The Astrophysical Journal, John Wallin of George Mason University and collaborators predicted that the gravitational field of a globular cluster plunging through a galaxy’s disk would provide the necessary compression of a marginally stable interstellar gas cloud to trigger star formation. That paper attracted little notice until now.

“This makes my day,” said Wallin. “It’s really nice when a crazy theory turns out to be true.”

Wallin and his collaborators noted that a globular cluster passes through the disk of the Milky Way roughly every million years.

“We’ll be reexamining other globular clusters we’ve studied,” said Rees. “We may find that one or more of them has also recently triggered star formation in the disk.”

This research has been partially supported by the National Science Foundation and the Westfield State College Continuing Scholarship Fund.

Electronic versions of this release, the technical paper presented at the American Astronomical Society Meeting, the accompanying figures, and links to telescopic images of NGC 6397 and NGC 6231 can be found at http://www.physci.wsc.ma.edu/dept/aas.html.

Contact: Richard Rees, Department of Physical Science, Westfield State College, Westfield, MA 01086, 413-572-5370, rrees@wsc.ma.edu

The Milky Way Galaxy as it would appear from outside. The bold yellow arrow shows the direction of the motion of NGC 6397 (the distance traveled by NGC 6397 in the last five million years is much less than this; the size of the motion has been exaggerated for clarity). Figure by Richard Powell and Richard Rees. (A downloadable version of this image can be found here.)

View of the Milky Way Galaxy from above. The box is shown in detail below. Figure by Richard Powell and Richard Rees. (A downloadable version of this image can be found here.)

View of the Milky Way Galaxy from above (detail). The Sun is located at the yellow dot, NGC 6231 is at the green dot, and NGC 6397 is about 1,500 light years below the red dot. Less than five million years ago, NGC 6397 passed through the Milky Way’s disk in the red rectangle (the size of which represents the measurement uncertainties); NGC 6231 formed at about the same time in the green rectangle. The yellow shows the significant overlap between the two regions, indicating that NGC 6231 formed where NGC 6397 passed through the disk. This picture is about 20,000 light years across. Figure by Richard Powell and Richard Rees. (A downloadable version of this image can be found here.)