Evaporating clusters
Twenty years ago, there was no observational evidence that globulars actually dissolve. In 2000, a team led by Michael Odenkirchen of the Max Planck Institute for Astronomy in Heidelberg, Germany, found twin tails of stellar debris arcing away from the faint globular cluster Palomar (Pal) 5 (see “Cluster’s last stand” at left). Subsequent studies found tidal tails issuing from 20 other globulars, including NGC 288 and Pal 12 and 13. Globular clusters do evaporate, and astronomers today observe only the most massive remnants of the Milky Way’s original globular population.
Further evidence supporting a unified view of clusters arrived in 2003. A team led by Richard de Grijs, then at the University of Cambridge in England, examined a billion-year-old “fossil starburst” at the center of M82, the nearest and best-studied starburst galaxy. Tidal interactions with neighbors M81 or NGC 3077 probably triggered M82’s fierce pulses of star formation.
The astronomers looked at a region where they had previously identified more than 100 gravitationally bound clusters. If the unified view is correct, then many low-mass clusters that formed in this episode of star formation have already evaporated. If de Grijs and his colleagues could see sufficiently faint clusters, they could determine if those in M82 follow a bell-curve distribution like globular clusters.
In fact, de Grijs and his team did just that. They produced conclusive evidence that young clusters show characteristic luminosities and distributions nearly identical to globulars orbiting M31, M87, old elliptical galaxies, and our own Milky Way. Young globular clusters are, indeed, forming today, and the distinctions we make regarding the Milky Way’s clusters are artificial at best. Some astronomers, like Whitmore, suggest it’s time to create an objective classification system for star clusters, one that provides more meaningful terminology for clusters near and far, young and old.
Still, there remain many caveats with the “continuum view” of star clusters. “Clearly, things become more complicated once you start looking at the details,” says Larsen.
As early as the 1950s, astronomers realized the Milky Way’s globular clusters contain different populations distinguished by metal content, location, and orbital motion.
“The origin of the different subpopulations is currently a matter of much debate, and we are still very far from having a clear picture,” explains Larsen. “Generally, globular clusters residing in the halo appear to fall into two groups, based on their ages and other properties, and the younger ones may have been accreted.”
The Sagittarius Dwarf Spheroidal Galaxy, which is now passing through the Milky Way’s disk, may have contributed a handful of globulars. They include Terzan 7 and 8, Arp 2, Pal 12, and M54 — the last an object some astronomers point to as the disrupted galaxy’s nucleus. Omega Centauri, the Milky Way’s brightest and most massive globular, likewise may be the stripped nucleus of another dwarf galaxy.
In a 2004 study of globular subpopulations, Dougal Mackey and Gerry Gilmore, then of Cambridge University’s Institute of Astronomy, estimated that our galaxy has experienced at least seven merger events. They argue that as much as half of the stellar mass in the Milky Way’s halo originated in cluster-bearing dwarf galaxies swallowed by our own. If so, young globulars are the remnants of our most recent acquisitions.
For every cluster that survives to an age of 10 billion years, a thousand were created and have been destroyed, their stars dispersed throughout the galaxy. To put an astronomical twist on Mark Twain’s famous quip about thunder and lightning, galaxies are impressive, but it’s star clusters that do the work.