M37 - Open Cluster in Auriga
| Polish version is here |
Messier 37, also cataloged as NGC 2099, stands out as the richest in stars and brightest open cluster in the constellation Auriga. The object was first recorded by Giovanni Battista Hodierna prior to 1654, although it was later overlooked by subsequent observers, including Guillaume Le Gentil, until it was independently rediscovered by Charles Messier in 1764. In the Trumpler classification system, the cluster is designated I,1,r or I,2,r, indicating a strong central concentration, a wide luminosity range among its member stars, and a high overall stellar population.
With an estimated mass of approximately 1,500 M⊙, the cluster contains more than 500 confirmed member stars, about 150 of which are brighter than 12.5m. Age determinations range from roughly 347 to 550 million years, placing it at a relatively advanced stage of stellar evolution. This evolutionary status is reflected in the presence of at least a dozen red giants and in the fact that the hottest star still on the main sequence is of spectral type B9 V. Its metallicity - defined as the abundance of elements heavier than helium - is comparable to, or slightly higher than, that of the Sun.
Observations
February 02, 2026, about 10:00 p.m. - Katowice, Poland
urban conditions, very high level of light pollution
Although the sky remained largely overcast and far from ideal for observing, brief clearings allowed my camera to collect enough photons (Photo 1).
A significant astrophysical feature of M37 is the presence of a small planetary nebula physically associated with the cluster, although it is not visible in the image above. Such occurrences are rare; as of 2022, this is only the third confirmed instance of a planetary nebula being gravitationally bound within an open cluster. Far more common are apparent alignments caused by line-of-sight projection, such as NGC 2438 in M46. Given a distance of approximately 4,500 light-years, the cluster’s angular diameter of 24 arcminutes corresponds to a real size of roughly 20 to 25 light-years. Estimated at 46 to 59 light-years, the tidal radius marks the boundary where external gravitational perturbations begin to dominate over the cluster’s internal stellar dynamics.
Photo 1 Parameters:
- total exposure time: 30 minutes (stack of 60 RAW frames at 30s each, using an appropriate number of dark, bias, and flat frames)
- ISO: 1600
- Maksutov-Cassegrain telescope (100/1400), prime focus exposure
- A filter was used to reduce the effects of artificial light pollution and atmospheric glow
- Mount: equatorial mount with tracking, aligned using the drift method and controlled by a custom-built system.
Further readings:
- Fragkou V., Parker Q. A., Zijlstra A. A., Vázquez R., Sabin L., Rechy-Garcia J. S., The Planetary Nebula in the 500 Myr Old Open Cluster M37, The Astrophysical Journal Letters, 935 (2), 2022, p. L35
- Pancino E., Carrera R., Rossetti E., Gallart C., Chemical abundance analysis of the open clusters Cr 110, NGC 2099 (M 37), NGC 2420, NGC 7789, and M 67 (NGC 2682), Astronomy and Astrophysics, 511, 2010, p. A56
- Piskunov A. E., et al., Tidal radii and masses of open clusters, Astronomy and Astrophysics, 477 (1), 2008, pp. 165-172
Marek Ples