Caldwell 71, or NGC 2477, is an open star cluster. Open clusters are loosely bound collections of stars. However, Caldwell 71 is relatively compact and strikingly spherical, so it is easily mistaken for a globular cluster. It is located roughly 4,000 light-years from Earth in the constellation Puppis. With an apparent magnitude of 5.8, it appears fairly bright in the sky and can be spotted easily with a pair of binoculars. The loose open cluster NGC 2451 appears near it, and both clusters can fit in the same field of view with binoculars or a low-power telescope. Discovered by the French astronomer Nicolas-Louis de Lacaille in 1751, Caldwell 71 is best observed in the Southern Hemisphere during the summer and in the lower latitudes of the Northern Hemisphere during the winter.

This image, showing just a portion of Caldwell 71, is a composite of visible and infrared observations by Hubble’s Wide Field and Planetary Camera 2. The observations were made to study the evolution of cooling white dwarfs (the dense cores left behind after a star has exhausted all of its nuclear fuel) and to investigate the cluster’s Initial Mass Function (IMF) — a tool astronomers use to describe how mass is initially distributed within a population of stars. Using Hubble’s observations, astronomers found that the IMF for Caldwell 71 was the same as the other star clusters they studied.

Credit: NASA, ESA, and T. von Hippel (Embry-Riddle Aeronautical University); Processing: Gladys Kober (NASA/Catholic University of America)