The globular cluster NGC 2005, featured in this Hubble Picture of the Week, is not unusual in and of itself; but it is a peculiarity in relation to its surroundings. NGC 2005 is located about 750 light-years from the heart of the Large Magellanic Cloud (LMC), which is the Milky Way’s largest satellite galaxy and which itself lies about 162 000 light-years from Earth. Globular clusters are densely-packed clusters that can constitute tens of thousands or millions of stars. Their density means that they are tightly gravitationally bound and are therefore very stable. This stability contributes to their longevity: globular clusters can be billions of years old, and as such often comprise very old stars. Thus, studying globular clusters in space can be a little like studying fossils on Earth: where fossils give insights into the characteristics of ancient plants and animals, globular clusters illuminate the characteristics of ancient stars.
Current theories of galaxy evolution predict that galaxies merge with one another. It is widely thought that the relatively large galaxies that we observe in the modern Universe were formed via the merging of smaller galaxies. If this is correct, then astronomers would expect to see evidence that the most ancient stars in nearby galaxies originated in different galactic environments. As globular clusters are known to contain ancient stars, and because of their stability, they are an excellent laboratory to test this hypothesis.
NGC 2005 is such a globular cluster, and its very existence has provided evidence to support the theory of galaxy evolution via mergers. Indeed, the stars in NGC 2005 have a chemical composition that is distinct from the stars in the LMC around it. This suggests that the LMC underwent a merger with another galaxy somewhere in its history. That other galaxy has long-since merged and otherwise dispersed, but NGC 2005 remains behind as an ancient witness to the long-past merger.
[Image Description: A globular cluster, appearing as a highly dense and numerous collection of shining stars. Some appear a bit larger and brighter than others, with the brightest having cross-shaped spikes around them. They are scattered mostly uniformly, but in the centre they crowd together more and more densely, and merge into a strong glow at the cluster’s core.]