M67: The Ultimate Survivor

Text by Ken Croswell

Published in Astronomy (March 2004, page 71).

An open star cluster like the Beehive in Cancer sprinkles space with glowing gems, but the cluster itself faces a harsh future. Giant blobs of gas and dust called molecular clouds, some wielding more than a hundred thousand times more mass than the Sun, roam the Milky Way. Their gravity can tear a cluster asunder. Chances are, a billion years from now, the Beehive won't exist.

Yet the same constellation features an open star cluster that has survived about as long as the 4.6-billion-year-old Sun. The cluster, called M67, is 2,800 light-years from Earth, so the light you see from its stars tonight was already en route to us when the Greek philosopher Plato was born. Because of M67's age, astronomers often observe its stars to study stars as old as the Sun.

They don't have many other choices. Astronomers have catalogued some 1,200 open star clusters in the Milky Way, but those older than a billion years number just a few dozen. Those older than the Sun number even fewer. That's how relentlessly giant molecular clouds rip these clusters apart.

M67, however, is well positioned to survive--literally. It resides in the outer Galaxy, where giant cluster-shredding molecular clouds are rare. The Sun is 27,000 light-years from the Milky Way's center--about halfway from the center to the edge of the stellar disk--whereas M67 is 29,000 light-years away from the Galaxy's heart.

Most giant molecular clouds inhabit the inner Galaxy. Thus, star clusters like M67 are able to dodge these deadly objects. Indeed, of the twenty or so open clusters that exceed the Sun's age, 90 percent dwell in the outer Milky Way.

Furthermore, M67 hovers 1,400 light-years north of the Galaxy's plane. This distance between the cluster and the Galactic plane protects M67 because the plane harbors the giant molecular clouds. Like all the Milky Way's objects, M67 bobs up and down on its orbit around the Galactic center. It passes through the Galactic plane like a horse on a merry-go-round, but it spends most of its time above or below the danger zone. In contrast, the Sun never strays farther than 250 light-years from the Galactic plane.

Still, M67 has surely lost stars. The most vulnerable are the smallest--the red and brown dwarfs. As M67's stars orbit the cluster's center, they exchange energy with one another in a way that equalizes each star's energy of movement, or kinetic energy. Because this energy depends on a star's mass and velocity, the more massive stars slow down and sink toward M67's center. The less massive ones can speed up so much they escape from the cluster.

Most of M67's mass lies within a dozen light-years of its center. Any star that strays more than 30 light-years away from the cluster's heart may never return, and indeed, a star leaves the cluster once every few million years. Most of M67's original red dwarfs are probably gone.

Nevertheless, the overall cluster survives, still boasting more than a thousand stars. Blessed by its birth in the outer Milky Way and its perch far from the Galactic plane, M67 must have witnessed the destruction of countless other star clusters. Chances are, a billion years hence, M67 will be shining still, occasionally shedding a small red star into the Galaxy at large.

Ken Croswell earned his doctorate in astronomy at Harvard University and is the author of Magnificent Universe and Magnificent Mars.

"Elegant and eloquent"--Washington Post. See all reviews of Magnificent Universe here.