DSO of the Month :
Messier 67
AKA: NGC 2682; Collinder 204
Position: 08 hr 51 min 23 sec +11 degrees 49 min
Due south at 21:18 (GMT) on 15 March 2022
Messier 67
Image: Martin Gill, HAS member. Used with permission.
Messier 44, the Beehive Cluster, in Cancer is both well-known and easily seen. But the other open cluster in Cancer is much less known and not so often observed by amateurs; although it is one of the open clusters most studied by professional astronomers. So why is it the underdog in Cancer? The explanation is simple – it is both smaller and dimmer than the Beehive. It is only 30 arcminutes in width, about the same as the full Moon, whereas M44 is 1.5 degrees wide. While M44 is bright with a magnitude of 3.7, M67 is a much dimmer 6.7. The main differences between the two, however, are not so obvious. M44 is a fairly young cluster with an age of 600 million years whereas M67 is one of the oldest open clusters in the Milky Way, with an age of 4 billion years; or roughly the age of the Earth. The Beehive is only 610 light years away, whereas M44 almost five times as distant as it is 2,880 light years from us. This great distance partly explains why it is dimmer and smaller in our sky. As M44 is such an old cluster, It has no stars above spectral class F. All the hotter, more massive stars have disappeared or left the main sequence to become red giants. With about 500 stars, it has a large number of G type stars like our Sun, red giants and white dwarfs. It was once thought that M67 could be the birthplace of our Sun, but this has recently been ruled out by computer simulations.
To find M67 in the sky consider the rather dim constellation of Cancer as a stick man with two raised arms and two legs. M67 is to the right (or west) of the left (eastern) foot, which is magnitude 4.2 Acubens or Alpha Cancri. It is also just above the halfway mark on a line drawn between Regulus and Procyon. Despite its age, it is a fairly concentrated cluster, roughly circular in shape. It was discovered and catalogued by Charles Messier in April 1780, although it was probably glimpsed earlier by the German astronomer Johann Gottfried Koehler before he discovered M59 and M60 in 1779.
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