NGC 2099

Messier 37 (NGC 2099) is the richest, brightest, and most impressive of the three Auriga open clusters, sitting alongside M36 and M38 in the winter constellation Auriga. It lies approximately 4,600 light-years away, spans about 27 light-years, and contains around 200 confirmed stellar members — twice the membership of its neighbors. M37 may have been recorded by Giovanni Battista Hodierna before 1654; Charles Messier catalogued it in 1764. At apparent magnitude 5.6, it is a naked-eye object from a dark site, and through any telescope it is the most spectacular of the three clusters by a considerable margin.

NGC 2099 is estimated to be around 300–350 million years old — significantly more evolved than M36 — and its age shows in its stellar population. Scattered among the blue-white majority are numerous red and orange giant stars, bright evolved members that have exhausted their core hydrogen and expanded enormously. These warm-colored stars stand out vividly in color images against the cooler blue-white cluster background, making M37 particularly beautiful photographically. The cluster's rich, compact appearance is enhanced by a relatively concentrated central region surrounded by a more diffuse halo of fainter members.

Through binoculars M37 appears as the brightest and most condensed of the three Auriga clusters; a small telescope at low power delivers a stunning, fully resolved view of stars of many colors scattered across the field, with the rich reddish giants immediately catching the eye. This color composite was made from CCD images taken in December 1994 at the Kitt Peak National Observatory 0.9-meter telescope.