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M 35
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M 35

February 18, 2005
Open Cluster M35 (NGC 2168), type 'e', in Gemini

Right Ascension: 06 : 08.9 (h:m)
Declination: +24 : 20 (deg:m)
Distance: 2.8 (kly)
Visual Brightness: 5.3 (mag)
Apparent Dimension: 28.0 (arc min)

Discovered by Philippe Loys de Chéseaux 1745-46. Independently discovered by John Bevis before 1750.

Open star cluster M35 is consisted of several hundred stars (of which Wallenquist has counted 120 brighter than mag 13) scattered over the area covered by the full Moon (30'); the Sky Catalogue 2000.0 and the first edition of Uranometria 2000.0 give 200 members, the second edition of Uranometria 2000.0 gives 434, while Cudworth (1971) counted 513 probable member stars. At its distance of 2,700 (WEBDA) or 2,800 light years (Sky Catalogue 2000.0), this corresponds to a linear diameter of about 24 light years; its central density is about 6.21 stars per cubic parsec. Some authors have estimated a larger diameter of up to 46' (H. Shapley in 1930). With about 100 million years (WEBDA gives a value 95, the Sky Catalogue 2000.0 of 110 million years), it is of intermediate age, and contains some post-main sequence stars (including several yellow and orange giants of spectral type late G to early K). Its hottest main sequence star is given as of spectral class B3 (Sky Catalogue 2000.0), and its Trumpler classification as III,3,r by all sources. It is approaching us at 5 km/sec.

The discovery of M35 is usually assigned to Philippe Loys de Chéseaux who observed and cataloged it in 1745 or 1746. It is also printed in John Bevis' Uranographia Britannica which was completed in 1750, so that this astronomer must have discovered it independently before this time, maybe or maybe not before De Chéseaux. Charles Messier, who cataloged it on August 30, 1764, acknowledges Bevis' discovery.

Even the naked eye finds this cluster easily near the 3 "foot stars" of Gemini under fairly good observing conditions. The slightest optical instrument will resolve the brighter stars and make it a splendid view at low magnifications, a nearly circular cluster with rather uniform stellar distribution. In telescopes, low powers and wide-field eye pieces show M35 at its best.

Amateurs with more powerful telescopes can view its fainter neighbor, NGC 2158 (at the upper left in our image); it is situated just about 15 arc minutes southwest of M35. NGC 2158, of about mag 8.6 and about 5 arc minutes angular diameter, contains many more stars, is much more compact, over 10 times older and over five times more remote than M35 (the Sky Catalogue 200.0 gives about 16,000 light years), and because it consists of older stars, its light is dominated by yellower stars; the fottest star is of spectral type F0. Because of these properties, NGC 2158 was once even taken for a globular cluster candidate.

Somewhat more to the west, about 50 arc minutes from M35, faint open cluster IC 2157 can be found. With a total visual magnitude 8.4 and an apparent diameter of 8 arc minutes, it is similar to NGC 2158 in size and brightness, but contains much 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 of about 1.5 deg.


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