Received astronomical wisdom holds that the Milky Way formed from a slowly twirling, homogeneous body of gas that collapsed into a star-studded pancake. According to this so-called monolithic model, stellar aggregations known as globular clusters were the first objects to take shape, arising from condensing gas in the discs "spherical halo" in the final moments of the collapse. Consequently, most of the ancient clusters reside in the halo, and their stars contain less metal than younger ones do because they formed when the interstellar medium was metal-poor. The monolithic model predicts that the clusters should vary continuously in properties such as age and metallicity according to their distance from the galactic center. Strangely, however, observations of the star clumps indicate that they instead fall into two distinct types. New findings may explain this dichotomy. Korean astronomers writing in the current issue of the journal Science have found evidence that some members of one kind of cluster may actually hail from a galaxy other than our own.

The results fit neatly with a growing body of evidence that stellar strangers abound in the Milky Way. Indeed, previous work has suggested that a number of globular clusters originated in other galaxies and were eventually absorbed by ours. But there has been no evidence that the most metal-poor clusters (and therefore supposedly the oldest) share that history. Suk-Jin Yoon and Young-Wook Lee of Seouls Yonsei University thus focused on these lowest metallicity aggregates, otherwise known as group II-b. With the help of computer models, the team used the colors of certain stars to determine the ages of clusters in this group and those in a group characterized by intermediate metal abundance. Surprisingly, the group II-b clusters turned out to in fact be younger than their more metal-rich counterparts.

Furthermore, the researchers found that most II-b clusters lie in the same plane as two of our galactic neighbors: the Large Magellanic Cloud (LMC) and the Draco dwarf galaxy. This unusual arrangement, along with hints that the II-b assemblages have an orbit similar to that of the LMC, suggests that they were forged in one of the nearby galaxies and only later acquired by the Milky Way. (Exactly when this celestial kidnapping might have occurred remains unknown.) Only more data will enable astronomers to pinpoint which of the Milky Ways resident globular clusters are natives and which are aliens. But for now, as Christine Clement of the University of Toronto says in an accompanying commentary, "evidence is mounting that the formation of our galaxy was less straightforward than [the monolithic model] suggests."