Open Star Cluster M37. Although M37 is the brightest of the 3 open clusters in southern Auriga, this cluster was missed by Le Gentil when he rediscovered M36 and M38 in 1749, so that it was to Charles Messier to find this one independently on September 2, 1764. Generally unknown until 1984, all three clusters had been previously recorded by Hodierna before 1654.
M37 is the richest of the 3, containing about 150 stars brighter than mag 12.5, and perhaps a total of over 500 stars. As indicated by the fact that it has at least a dozen red giants, and that the hottest main sequence star is of spectral type B9V, this cluster is a more evolved group with an estimated age of about 300 million years. Its distance is given discordantly: Kenneth Glyn Jones gives 3,600 light years, the Sky Catalog 2000 has 4,400 while Götz gives about 4,100, Mallas 4,600, and Burnham 4,700 light years. Its apparent diameter of 24' corresponds to a linear extension of about 20 to 25 light years, according to which distance is taken.