What’s Up - M 41, Open Cluster in Canis Major

By Steve Coe

M 41 has been a favorite of mine for many years.  If you are making a table of the best open clusters in the sky and M 41 is not on that list I believe you have made some poor choices.  But, that is just my opinion.

This big and bright cluster can be seen with the naked eye from a dark site.  It is easy to locate to the south of Sirius.  Aristotle mentioned it in 325 B.C.; Flamsteed and Le Gentile wrote of it before Messier added it to his famous list in 1765.

The cluster is about the size of the full moon at 30 arc minutes.  It has had about 100 stars assigned to it by studying the motion of the stars in this part of the sky.  The average of the distance estimates is 2300 light years.

A prominent feature of M 41 is its lucinda, or brightest star.  This star has a variety of designations, I will provide BD -20 1555.  It is a 6.91 magnitude star with a B-V value of +1.51.  That color index means the star will appear yellow or orange to most observers.  This star is almost centered in the cluster.

Observing M 41 with 8X42 binoculars shows 8 pretty bright stars and 14 stars total.  Averted vision shows about 20 stars and a fuzzy background.  I see the cluster as very well detached and triangular in shape.

Moving up in aperture to a 6" f/8 refractor really shows off this excellent open cluster.  On a night I rated the seeing at 7 out of 10 and the transparency at 8 out of 10 it is very bright, large, pretty compressed and rich in stars.  I counted 50 stars including the orange star in the middle, it has good color.  That observation was made using a 27mm Panoptic for about 40X.  Raising the power with a 14mm UWA provides great contrast, the stars are like diamonds on velvet.  The cluster is about 75% of the field, 60 stars are resolved; this includes several faint and very faint members.  There are two matched pairs on the northern edge, one is fainter than the other, and both are delicate pairs.  There is a long curved chain of 8th and 9th magnitude stars near the core that I have called the "Nautilus" for years, it is a winding spiral shape that stands out best in modest apertures. 

Using a 13" f/5.6 Newtonian on a night I rated 8/10 for both seeing and transparency this cluster is truly fascinating.  It was easy in the 11X80 finder scope, with 11 stars resolved in a rich Milky Way field.  Using 60X in the 13" this cluster is very large, very bright, pretty rich and little compressed, with stars of 7th mag on down.  I counted 47 stars at this low power against a faint glow of unresolved members.  There are several lovely chains of stars and a beautiful bright orange member near the center of the cluster.  Going to 100X provides the best view; the cluster is about 60% of the field of view.  I count 83 stars at this power, including many faint members.  There is a delicate chain of stars near the orange bright one in the middle.  At the edges of the cluster it is difficult to tell where the cluster ends and the Milky Way begins.  At 150X it seems that you are seeing all the stars there are to see in this grouping. 

Sirius and M 41  135mm lens f/4.5
90 seconds exposure

M 41  Vixen ED 80 f/6 refractor  3 minutes exp.
Canon T2i DSLR camera  set at ISO 800