In the past, astronomers thought globular star clusters were made up of stars that have similar ages and similar chemical abundances. However, recent studies suggest that it might not be that simple. It seems as though many globular clusters contain stars with different chemical abundances, suggesting that they host multiple populations of stars born at different times.

This image of the globular cluster Caldwell 107, or NGC 6101, combines observations taken in visible, infrared, and ultraviolet light by Hubble’s Wide Field Camera 3 and its Advanced Camera for Surveys. The stars in Caldwell 107 are more loosely packed than those in other globular clusters, which usually have dense, compact cores. Astronomers observed Caldwell 107 with Hubble to study and characterize the multiple populations within the cluster.

Caldwell 107 was first observed in 1826 by the Scottish astronomer James Dunlop, who lived in Australia for many years. Astronomer John Herschel, who observed the cluster in the 1830s, described the cluster as being large, faint, round, and only a little brighter in the middle. The cluster is easiest to spot during the winter from the Southern Hemisphere. Caldwell 107 has an apparent magnitude of 10.6 and is located roughly 50,000 light-years from Earth in the constellation Apus. A mid-sized telescope is needed to resolve some of the cluster’s individual stars.

Credit: NASA, ESA, G. Piotto (Università degli Studi di Padova), and A. Sarajedini (Florida Atlantic University); Processing: Gladys Kober (NASA/Catholic University of America)