Messier 47, NGC 2422

The open cluster Messier 47 (also known as NGC 2422) appears in the constellation Puppis. It is a coarse, bright cluster which can be glimpsed with the naked eye under good conditions.

Historical Observations

M 47 was first observed by Giovanni Batista Hodierna around 1654, but this fact remained unknown until Hodierna's book came to light in 1984. Thus, Charles Messier independently discovered the cluster in 1771. There is actually no cluster in the position indicated by Messier, which he expressed in terms of coordinates with respect to the star 2 Puppis. But if the signs of Messier's coordinate differences are changed, the position matches that of NGC 2422.

Despite Messier's error, Caroline Herschel observed and identified M 47 at least twice in early 1783. William Herschel also independently rediscovered M 47 in 1785, and numbered it H VIII.38. Although there was no object at Messier's erroneous position, it survived in many catalogs, including John Herschel's (GC 1594) and Dreyer's (NGC 2478). As John Herschel put it in his Notes on the GC, "This cluster has not since been observed. It is probably a very loose and poor one."

M 47 remained a missing object until 1959, when it was identified as NGC 2422 by T. F. Morris (along with the also-missing M 48). Several historians have reported that M 47 had been previously identified by Oswald Thomas in 1934, but this is apparently a confusion with that author's identification of M 48.

Appearance and Properties

Visually, M 47 is a large, bright cluster of magnitude 5.2, visible to the unaided eye as a hazy patch in the Milky Way. There are about 50 stars in the cluster, scattered over an area 30' across - the same size as the full moon. The cluster is coarse and is irregular, containing knots and chains separated by several starless voids. Its brightest star is of magnitude 5.7; it also contains the fine double Struve 1121, with components of magnitude 7.9 separated by 7.4".

M 47 lies at a distance of about 1,600 light years, and is about 15 light years in diameter. In the central portion, the stellar density is about 16 stars per cubic parsec; its overall population resembles that of the Pleiades. Its brightest star is of spectral class B2; M 47 also contains two orange K giants with luminosities of about 200 Suns. This stellar swarm has an estimated age of about 60 million years, and is receding from us at 9 km/sec.