M46 - Open Cluster in Puppis
| Polish version is here |
Messier 46, or NGC 2437, is one of the more interesting open clusters in the night sky. Charles Messier, a French astronomer, first listed it on January 19, 1771. It is in the star-rich constellation Puppis. M46 may not look like much on paper, but in practice it is very rewarding. Its appeal lies less in sheer visual spectacle and more in the wealth of physical and observational detail packed into a single field of view.
The cluster is located at a distance of approximately 5,000 to 5,400 light-years from Earth and is receding from us at a radial velocity of about 41.4 km/s. Its stellar population spans a region roughly 30 light-years in diameter, with an age estimated at 250 to 300 million years. On astronomical timescales, this makes M46 a relatively young object. It is composed of stars that formed from the same molecular cloud at roughly the same epoch. The most massive members have long since evolved off the main sequence, and the cluster itself (like many open clusters) is gradually dispersing into the Galactic disk.
With an estimated 500 members, M46 boasts a rich stellar population. This includes a distinct group of about 150 stars with apparent magnitudes ranging from 10m to 13m. The brightest members are of spectral type A0, with an apparent magnitude of approximately 8.7m, each shining with a luminosity about 100 times that of the Sun.
Observations
January 24, 2026, about 10:00 p.m. - Katowice, Poland
urban conditions, very high level of light pollution
From Poland, M46 is a relatively challenging target, as it lies far to the south. In practical terms, this means that even at the optimal point of the observing season the cluster never climbs high above the horizon. This significantly enhances the effects of atmospheric extinction as well as light pollution on the resulting view. Nevertheless, under favorable conditions the cluster can still be successfully observed.
The most captivating feature of Messier 46 is the planetary nebula NGC 2438, which appears to be nestled deep within the cluster's stars. However, this striking arrangement is a mere cosmic illusion. Precise radial-velocity data, coupled with stellar evolution models, have confirmed that the nebula is not physically tied to M46. Differences in the ages of the two objects, as well as their relative motions, demonstrate that the nebula lies at a different distance and merely happens to fall along the same line of sight, creating one of the most beautiful 'cosmic collages' accessible to amateur telescopes. The physical presence of planetary nebulae inside star clusters is a fairly rare occurrence, yet they do exist, with M37 being a prime example.
Photo 1 Parameters:
- Total exposure time: 80 minutes (stack of 40 RAW frames at 120s each, using an appropriate number of dark, bias, and flat frames)
- Canon EOS 60D
- ISO: 1600
- Achromatic refractor Messier AR-152S (152/760), 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:
- Wu Z.-Y., et al., The orbits of open clusters in the Galaxy, Monthly Notices of the Royal Astronomical Society, 2009, 399(4), pp. 2146-2164
- Majaess D. J., Turner D., Lane D., In Search of Possible Associations between Planetary Nebulae and Open Clusters, Publications of the Astronomical Society of the Pacific, 2007, 119(862), p. 1349
- Davidge T. J., The Open Cluster NGC 2437 (Messier 46), Publications of the Astronomical Society of the Pacific, 2013, 125(924), pp. 115-125
- Mermilliod J.-C., Clariá J. J., Andersen J., Piatti A. E., et al., Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259, Astronomy & Astrophysics, 2001, 375, pp. 30-39
- Öttl S., et al., Ionization structure of multiple-shell planetary nebulae. I. NGC 2438, Astronomy & Astrophysics, 2014, 565, p. A13
- Dalnodar S., The ionization state of the halo planetary nebula NGC 2438, Planetary Nebulae: An Eye to the Future, Proceedings of the IAU Symposium, 2012, 283, pp. 338-339
Marek Ples