Messier 35 is a beautiful cluster in Gemini which can be seen with the naked eye.
The discovery of M35 is usually assigned to Philippe Loys de Cheseaux, who observed and cataloged it in 1745. It is also printed in John Bevis' Uranographia Britannica, completed in 1750, so Bevis must have discovered it independently. Charles Messier, who cataloged M 35 in 1764, acknowledged Bevis' discovery.
Observing Messier 35
The naked eye finds this cluster near the 3 "foot stars" of Gemini under fairly good observing conditions. M 35 is a splendid sight in a small telescope, best seen at low powers. The cluster contains over 120 stars from 6th to 13th magnitude in a 30' area, and may contain more than 500 stars in total.
The outer edge is highly irregular, blending imperceptibly into the surrounding star field. Outliers up to half a degree from the cluster center double the star count. An E-W star-poor lane divides the cluster. The northern portion is box-shaped, with a prominent curved star chain on its NW side. The southern portion is highly extended ESE-WNW. A star chain runs through the center of the cluster along the star-poor lane.
Properties and Companions
At distance of 2,800 light years, M 35 has a linear diameter of about 24 light years; its central density is about 6.21 stars per cubic parsec. The mass within the central 3.75 parsecs of M 3 5has been computed at 1600 to 3200 Suns. M 35 is of intermediate age, about 100 million years old, and contains some post-main-sequence yellow and orange giants of spectral type G and K. Its hottest main sequence star is of spectral class B3. M 35 is approaching us at 5 km/sec.
The much smaller open cluster NGC 2158 lies 26' away, toward the south west. It is smaller (5' in diameter) and fainter (magnitude 8.6) because it is much further away - 16,500 light years as opposed to 2,800 for M 35. It is over ten times older than M 35, and because of this, its light is dominated by yellower stars; the hottest is of spectral type F0.
About 50' to the west of M35, the faint open cluster IC 2157 can be found. With a total visual magnitude of 8.4 and an apparent diameter of 8', it is similar to NGC 2158, but contains many fewer stars. IC 2157 is a loose and poor cluster, containing some very hot young OB stars; wide-field optics may show all three clusters in one field of view.