This image from the NASA/ESA Hubble Space Telescope shows the globular star cluster NGC 2419. Globular clusters are both beautiful and fascinating. They are globular clusters of stars orbiting the center of a galaxy, and in the case of NGC 2419, that galaxy is our own Milky Way. NGC 2419 is about 300,000 light-years from the Solar System, in the constellation Lyxus.

The stars that reside in globular clusters are quite similar, having formed around the same time. Astronomers can determine a star’s relative age from its chemical composition, a property known as its metallicity. Because the stars in a globular cluster all formed at about the same time, they tend to display similar properties. Astronomers believed that this similarity included their stellar helium content. They reasoned that all stars in a globular cluster would contain similar amounts of helium.

However, Hubble’s observations of NGC 2419 revealed that this is not always the case. This globular cluster is home to two distinct populations of red giant stars, and one is unusually helium-rich. The stars of NGC 2419 contain other elements that also vary. Specifically, their nitrogen content varies. To make things even more interesting, the helium-rich stars are mostly at the center of the globular cluster and are rotating. Hubble’s observations raised questions about the formation of globular clusters. did these two drastically different groups of stars form together? Or was this globular cluster formed by an entirely different route?