"In the solar neighborhood and in most stellar clusters, the least massive stars are the most common by far," explains Guido de Marchi, an astronomer with the European Space Agency and lead author of the paper presenting the findings in yesterday's Astronomy & Astrophysics. "Our observations with [the Very Large Telescope in Chile] show this is not the case for Messier 12."
The astronomers first turned to this particular cluster in 1999, hoping it would be representative of typical clusters that had not been disturbed by the gravitational pull of our galaxy. Previous models had predicted that Messier 12 would show little or no effects of such gravity. But when they surveyed more than 16,000 of the cluster's roughly 200,000 stars, the researchers found that it lacked the smallest and most common stars. Considering the typical ratio of small stars to large ones in a cluster, the team estimates that Messier 12 has lost roughly a million stars to our greedy galaxy, where they may reside in its halo.
This makes Messier 12 an anomaly among the 200 known globular clusters in the Milky Way. Such clusters are like a class of schoolchildren: they consist of a large aggregation of individuals born at roughly the same time and in the same place but varying in size. Either Messier 12 was born without any small stars--unlike any other known cluster--or its orbit takes it closer to the dense galactic core than previously thought. Two other anomalous clusters that pass close to the core have been stripped of their smaller stars as well. (The illustration above right depicts a possible star-stripping orbit for a globular cluster.)
Such gravity-based stripping ultimately leads to the dissolution of the cluster and it appears that Messier 12 may be particularly short-lived compared to its peers. New calculations predict that it will disappear in the next 4.5 billion years, compared to 17 billion for many other, less perturbed clusters. But although Messier 12 may not provide a normal example of cluster evolution as the team initially hoped, it does provide an opportunity for astronomers to study how the Milky Way created a halo of stars stolen from its attendant clusters.