Collinder 121 -- Open Cluster in Canis Major
Cluster Upon Cluster
When I read Matt Wedel’s Binocular Highlight section in the March 2020 issue of Sky &
telescope magazine about Collinder 121, I was hooked. Nor was I disappointed once I
viewed it through the telescope. In my 2.4-degree field-of-view drawing, using the 155mm
refractor, Collinder 121 covers a full degree at the center of the drawing. Don’t see it? Well
neither do I. The Open Cluster Collinder 121 resides at a distance of 3,200 light years;
2,000 light years further than the Canis Major OB2 association which superimposes Collinder
121 and the entire drawing -- extending far beyond the eyepiece view. One cannot separate
the stars between these two Open Clusters without careful scientific measurements. For us
casual observers the two cluster appear as one.
The Canis Major OB2 association consists mainly of young, bright, white or blue stars. The
Red Supergiant Omicron-1 is a pleasing exception. It is so massive that it has already burned
itself into old age and now shines as a reddish-orange star.
Whether you view Collinder 121/Canis Major OB2 association with binoculars, as
recommended by Matt Wedel, or in a wide-field telescope, this, cluster-upon-cluster, will not