Star formation can occur wherever interstellar gas becomes compressed and undergoes gravitational collapse. That compression might be caused by a supernova explosion, or the collision of galaxies. At last week's meeting of the American Astronomical Society in Atlanta, Georgia, Richard Rees and Kyle Cudworth presented the first evidence for star formation triggered by a new mechanism — a globular cluster of stars that punched through the disk of the Milky Way five million years ago.

The globular cluster NGC 6397 (pictured), 12 billion years old and containing a few hundred thousand stars, currently sits about 1,500 light years below the disk of the Milky Way. Its motion has been tracked by various telescopes since 1893, so Rees and Cudworth were able to extrapolate its path back, right through the plane of our Galaxy. And at the point at which the cluster crossed the disk five million years ago lies NGC 6231, a young cluster of stars estimated to have formed less than five million years ago.

The implication is that the passage of NGC 6397 through the Milky Way disk compressed the gas in that region and triggered the birth of NGC 6231 — a mechanism for star formation that was suggested in a little-noticed 1996 paper (J. F. Wallin et al. Astrophys. J. 459, 555–557; 1996). There it was noted that a globular cluster passes through the disk of the Milky Way roughly every million years. So it seems that the star-making talent of globular clusters should be factored into our picture of the evolution of this, and other, galaxies.