Messier 67 is a very old open cluster located in the constellation of Cancer. Estimations of its age vary between 3.2 and 5 billion years, with recent valuations putting it at 4 billion. This makes it one of the oldest known open clusters. As a consequence, M67 contains a variety of stellar types including many Sun like stars, red giants and white dwarfs. It's easily the most ancient Messier open cluster. For comparison, the Praesepe (M44) is 600 million years old and the brilliant Pleiades (M45) a very youthful 100 million years.
The constellation of Cancer is a faint zodiac constellation that's bordered by much brighter Leo to the east and Gemini to the west. To the north lies Lynx, with Canis Minor and Hydra positioned on the southern side. To find M67, first locate stars Asellus Australis (δ Cnc - mag. +3.9), Asellus Borealis (γ Cnc - mag. +4.7), η Cnc (mag. +5.3) and θ Cnc (mag. +5.3). This quadrangle forms the main part of the constellation and located at its centre is M44. Next focus on another faint star, α Cnc (mag. +4.3), which is positioned 9 degrees further to the southeast. M67 is positioned 1.75 degrees west of this star.
M67 was discovered sometime before 1779 by German astronomer Johann Gottfried Koehler. He described it as being rather conspicuous and nebula like, although with his basic telescope he was unable to resolve any stars. Charles Messier then independently rediscovered M67, resolved it into stars and catalogued it on April 6, 1780. He noted it as a "cluster of small stars with nebulosity, below the southern claw of Cancer. The position determined from the star Alpha [Cancri].".
M67 is best seen from the Northern Hemisphere during the months of February, March and April.
With an apparent magnitude of +6.1, M67 is at the limit of naked eye visibility. However, it's easily seen in 10x50 binoculars, appearing as an elongated blur of light with several member stars resolvable. Through telescopes M67 is a beautiful object. An 80mm (3.1-inch) refractor reveals a sprinkling of bright stars superimposed on a large concentrated misty patch of light. When seen through 150mm (6-inch) or 200mm (8-inch) scopes, the cluster is resolved into dozens of stars with most of them concentrated towards the centre. The background appears hazy with high magnifications resolving fainter members. A larger 300mm (12-inch) scope at about 100x magnification reveals about 100 stars and at the same time the background mist that's visible in small scopes disappears.
In total, M67 contains at least 200 stars spread over an apparent diameter of 30 arc minutes, which is equivalent to that of the full Moon. The cluster lies 2,700 light-years away. Although often overshadowed by M44, it's a wonderful cluster in its own right and provides a marvellous contrast to the sprawling Beehive.
M67 Data Table
|Object Type||Open cluster|
|RA (J2000)||08h 51m 20s|
|DEC (J2000)||+11d 48m 43s|
|Apparent Size (arc mins)||30 x 30|
|Age (years)||4 Billion|
|Number of Stars||>200|
|Other Name||Collinder 204|