Messier 37, NGC 2099

Messier 37 is the brightest open cluster in Auriga, and is considered to be one of the finest in the sky.

This cluster was first discovered by Giovanni Batista Hodierna some time before 1654, but Hodierna's observations were generally unknown until 1984. Although M 37 is the brightest of the three open clusters in southern Auriga, it was missed by Le Gentil, who discovered M 36 and M 38 in 1749. Thus M 37 was an independent discovery of Charles Messier, who found it in 1764.

M 37 has a total magnitude of 6.2, and is also the richest of the three Messier clusters in Auriga. It contains about 150 stars from magnitude 9 to 12.5, and perhaps a total membership over 500 stars. With a diameter of 24', it covers an area nearly as large as the full moon. A 9th magnitude star lies at its center, and the cluster has a prominent chain of ten 9th to 10th magnitude stars aligned N-S. The field is very rich, and includes an orange star south of the cluster.

M 37 is about 4,600 light-years away, and has a diameter of about 25 light-years. Its stars have a total luminosity of about 2,500 suns. M 37 is older and more evolved than M 36, with an estimated age of about 300 million years. The hottest main sequence star in M 37 is of spectral type B9 V; the majority of its bright members are main sequence type A stars of about absolute magnitude -1 (whereas the brightest members of M 36 are B2 and B3 stars with absolute magnitudes near -3). M 37 also contains at least a dozen red giant stars, while M 36 has none.