- Messier 50 or M 50, also known as NGC 2323 or the Heart-shaped Cluster, is an open cluster of stars in the constellation Monoceros. It was recorded by G. D. Cassini before 1711 and independently discovered by Charles Messier in 1772[b] while observing Biela’s Comet. It is sometimes described as a ‘heart-shaped’ figure or a blunt arrowhead.