M50 is an appealing and relatively bright open cluster located in the rich star fields of Monoceros, close to the Canis Major border. With an apparent magnitude of +5.9, it's just about visible to the naked eye, appearing as a faint patch of nebulosity. Telescopically the cluster is moderately dense, contains a number of bright stars and spans about half the apparent diameter of the full Moon. It's a nice object for owners of small to medium size scopes.
The constellation Monoceros straddles the equatorial equator, east of majestic Orion and north of bright Canis Major. Despite lying in rich Milky Way star fields, Monoceros is a faint constellation with no stars brighter than 4th magnitude. Consequently, tracing the outline of this dim grouping requires some patience. However, it does contains a number of bright and interesting deep sky objects. In addition to M50, it's home to the Rosette Nebula (NGC 2237, 2238, 2239, and 2246), the Christmas Tree Cluster (NGC 2264) and Hubble's Variable Nebula (NGC 2261). All of these are best seen during the months of December, January and February
M50 was discovered by Charles Messier on April 5, 1772. However, it's possibly G.D. Cassini had already seen it before 1711, according to a report by his son in his book, "Elements of Astronomy".
Finding M50 is relatively easy. Start by aiming your sights on Sirius, the brightest star in the night-time sky. Located 9.5 degrees north-northeast of Sirius is M50, with θ CMa (mag. +4.1) sandwiched halfway between them. Through binoculars, M50 appears as a bright and relatively large patch of nebulosity with at least 2 or 3 stars resolvable. An 80mm (3.1-inch) scope shows a heart-shaped figure containing a sprinkling of mostly 7th and 8th magnitude blue-white stars, set against a background haze. Towards the southern edge of the cluster is a distinct orange star. The cluster is an attractive grouping when viewed at low to medium powers through 150mm (6-inch) or 200mm (8-inch) instruments. At least 40 mainly blue-white stars, but also the orange star and some yellow stars, are resolvable in this somewhat irregular shaped grouping. At about 100x magnification, the stars begin to overflow the eyepiece field of view with many fainter background members visible, bringing the total number to more than 80.
M50 covers 16 arc minutes of apparent sky. It lies at a distance of 3,200 light-years from Earth and has a spatial diameter of 20 light-years. In total, it contains over 100 stars and has an estimated age of 78 Million years.
M50 Data Table
|Object Type||Open cluster|
|RA (J2000)||07h 02m 42s|
|DEC (J2000)||-08d 23m 26s|
|Apparent Size (arc mins)||16 x 16|
|Age (years)||78 Million|
|Number of Stars||>100|
|Other Name||Collinder 124|