C73

An imaginary line traced directly southward from Orion the Hunter, through Lepus the Hare, comes to the constellation of Columba the Dove. In the southwestern parts of Columba lies the globular cluster C73 or NGC 1851. With a magnitude of +7.1, the cluster cannot be seen by the naked eye, but can be detected with a modest telescope. C73 pursues a highly elliptical orbit around the Milky Way Galaxy, and is currently some distance outside the Galactic plane. The cluster presently lies at a distance of some 39,500 light years from the Solar System.

C73 has several unusual properties. It is among the most densely packed globular clusters, with a busy core surrounded by a thinning halo that extends out for some eight hundred light years from the centre. Investigation of the stars that make up the cluster shows that it contains at least two, and possibly three, different populations, implying that it has a complex history. It has been proposed that C73 in fact represents two separate clusters that collided and merged in the distant past, though there are difficulties with this view. Alternatively, it may represent the core of a dwarf galaxy, whose surrounding material has been stripped away and absorbed by the much more massive Milky Way Galaxy.