M35 and NGC2158

This image shows two open stars clusters in the constellation Gemini.  M35 (also catalogued as NGC2168) occupies the most real estate in the image and is seen to the left.  The more compact NGC2158 is seen to the lower right.

An open cluster is a gravitationally bound group of stars that were formed from a large molecular cloud.  There are over 1,000 known open clusters in the Milky Way Galaxy.

M35 occupies a region 25 to 28 arcminutes in angular diameter, which is about the size of the full moon.  Its actual size is about 22 to 25 light years.  It lies at a distance of about 2800 light years from Earth.  M35 contains over 400 stars, and many of these are young, hot blue stars as the cluster is a mere 175 million years old.

The companion cluster, NGC2158, is only a companion in the sense of visual proximity in the field.  In fact, this cluster is much further away than M35, and a recent 2021 estimate (Sariya et al,  A Comprehensive Analysis of NGC2158 in the Gaia ERA: Photometric Parameters, Apex, and Orbit, The Astronomical Journal, Volume 161, Number 3) places NGC2158 a bit further away than previous estimates at 15,290 light years.  It occupies 5 arcminutes angular diameter, which would translate to about 22.25 light years in actual diameter based on the new distance estimate, a bit larger than the 16 light years diameter that is quoted in older sources.  Sariya et al count 800 stars with greater than 90% probability of being true members of the cluster, which is consistent with, if a bit lower, than estimates given in older sources.  NGC2158 is estimated to be about 2 billion years old and is composed of more mature yellow stars than its younger companion cluster.  NGC2158 was once classified as a globular cluster, but it is now known to be an open cluster.

M35 is a popular visual target and is easily seen in almost any telescope or set of binoculars.  NGC2158 is much fainter, and through moderate apertures appears as a small fuzzy glow at the edge of M35.  My first view of NGC2158 was in 1988 using an 8" Meade SCT; I noticed the nearby faint glow while enjoying M35 and did not know what it was until I looked it up in the venerable Burnham's Celestial Handbook.  In 2010 I had a nice view of NGC2158 at high power using my 20" Obsession, and with this equipment I could resolve dozens of stars amidst an unresolved glow that hinted at many more stars.  NGC2158 is one of my favorite open clusters.

Data for this image were acquired in February of 2024.  Seeing and transparency were only so-so, but having a clear sky in February in this region is a rare gift.  I used short 30-second exposures in order to avoid overexposing the bright stars and washing out color.  In retrospect I could have tried even shorter exposures, but hundreds of 30-second frames was already a massive burden on available hard drive storage capacity.  The main image shows both clusters.  I uploaded a cropped version, zooming in on NGC2158, which can be found under revisions as image B.