Enrichment by supernovae in globular clusters with multiple populations

Globular clusters a mixed bag The globular star clusters that orbit the Milky Way are regarded as the best approximations we have of stellar populations of uniform age and identical composition, recording stellar evolution since the birth of our Galaxy. The most luminous of these clusters though, ω Centauri, has long been recognized as an exception to this trend, containing multiple stellar populations with a significant spread in iron abundance and ages. Two groups report the discovery of further global clusters with mixed populations. Lee et al. confirm the suspicion that the massive global cluster M22 contains distinct multiple populations with different calcium abundances, as do several other clusters in their sample. Ferraro et al. report that Terzan 5, a globular cluster-like system in the Galactic bulge, has two populations with different iron content and ages. These findings suggest that ω Cen, M22, Terzan 5 and other similar clusters are the relics of dwarf galaxies and other primordial bodies that merged to eventually form the Milky Way.