The constellation Canis Major, the Big Dog, contains a handful of splendid star clusters for observers with a small telescope. Messier 41 is the most famous and easiest to see. The lesser known NGC 2362 is fainter, but prettier. And the lovely star cluster NGC 2360 is always worth a careful gaze as it occupies a particularly rich section of the Milky Way. This open star cluster was first discovered by Caroline Herschel on February 26, 1783 using a small refractor. Her famous brother William Herschel included the cluster in a later catalog of 1000 deep-sky objects that he and his sister discovered. In her honor, the cluster is sometimes called “Caroline’s Cluster”.
Caroline’s Cluster is easy to find about 3.5º east of the blue-white star star γ Canis Majoris, sometimes called Muliphein, one of the three stars in the head of the Big Dog (see image below). At magnitude 7.2, the cluster is bright enough to spot in binoculars. It’s also compact, which means it has high surface brightness. So it’s easy to pick out even in moderately light-polluted skies.
In binoculars, NGC 2360 appears diffuse and unresolved and spans about half the size of the full Moon. The cluster lies embedded in a rich field of stars along the plane of the Milky Way. In a telescope, the cluster is cracked open into dozens of 9th to 12th magnitude stars that are centered around an elliptical core, which curved branches poking out to the east and west. About half a degree to the west lies a bright 5th-magnitude star which is not a member of the cluster.
The cluster is old for an open cluster, with an estimated age of about 2.2 billion years. So you see very few bright blue-white stars here. Such stars, which burn fast and die young, have long ago exploded or faded away.
NGC 2360 lies about 3,700 light years away.