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M 41
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M 41

February 18, 2005
Open Cluster M41 (NGC 2287), type 'e', in Canis Major

Right Ascension: 06 : 46.0 (h:m)
Declination: -20 : 44 (deg:m)
Distance: 2.3 (kly)
Visual Brightness: 4.5 (mag)
Apparent Dimension: 38.0 (arc min)

Discovered by Giovanni Batista Hodierna before 1654. Perhaps known to Aristotle about 325 B.C.

M41 is lying about 4 degrees nearly exactly south of Sirius, the brightest star in the sky. It contains about 100 stars, including several red (or orange) giants, the brightest being of spectral type K3 and mag 6.9, and situated near the cluster's center. This star is about 700 times more luminous than our Sun. The stars are distributed over a volume about 25 or 26 light years across, and all receding from us at 34 km/sec. As they are at a distance of 2,300 light years, they appear scattered over an area of 38 arc minutes diameter.

The age of M41 was estimated at 190 million years (Sky Catalog 2000) and 240 million years (G. Meynet's Geneva Team). The hottest star has been found to be of spectral type A0. All sources agree that it is to be typized as of Trumpler class I,3,r. This stellar swarm is receding from us at 34 km/sec.

Helfer, Wallerstein, and Greenstein have investigated M41's K-type red giant stars, and found their chemical composition very similar to that of our sun.

J.E. Gore mentions that M41 was "possibly" recorded by Aristotle about 325 B.C.; this would make it the "faintest object recorded in classical antiquity" (from Burnham). However, this identification is uncertain: A.A. Barnett presumes that Aristotle may have described the Milky Way near the star d CMa.

Hodierna was the first to catalog it before 1654, and it got generally known after John Flamsteed's independent rediscovery of February 16, 1702, who remarks (No. 965 in his catalog): "Near this star (12 CMa), there is a cluster." It was independently found again by Le Gentil in 1749, and apparently by Charles Messier, who added it to his catalog on January 16, 1765.

The relatively bright star in the upper right (Southeastern) corner of our image is the 6th-mag star 12 Canis Majoris. According to the Sky Catalog 2000 (Vol. 1), this star is a blue giant of spectral type B7 III n, which is at roughly half the distance of the cluster (1,100 light years) and thus not a member. This star is also in the lower left of the DSSM image.

Archinal and Hynes (2003) note that the distance of M41 is almost the same as that of another, less conspicuous open cluster, Collinder 121, which is situated at an angular separation of 4.6 degrees: The linear distance of these two open clusters is thus only about 60 light-years. Therefore, the authors speculate there might be some sort of physical relation between them.

Michael Ferrio reports that an error of 1 min in Right Ascension exists in many catalogs for M41 (they give RA=06:47.0 instead of 06:46.0; this can be verified e.g. with the help of the Digital Sky Survey). He discovered this when proofreading the first edition of Uranometria 2000.0. While Messier had this position correctly in his original catalog, this error occurs in the NGC, and it may be that Dreyer was the source of the error.

This cluster is easy to find, as it is nearly exactly south of Sirius, at an angular distance of 4 degrees.