M73

M73 is a small group of 4 stars which, although only 10th to 12th magnitude, shine together at about magnitude 9. The group was found by Messier who thought it might be a nebula at first but immediately discovered that it consisted of individual stars and today there is uncertainty regarding whether the members of the cluster are actually related physically or simply an asterism. The distance to M73 is also difficult to determine but seems to be approximately 2500 light years, although if the object is indeed an asterism, this value could be substantially different for individual stars. At the present time, it seems unlikely that M73 is an asterism due to the slim statistical chances of that being the case (about 1 in 4 that such a random group exists in the sky) and if the object is actually an open cluster it appears to be quite old, perhaps 2 or 3 billion years. This would mean that the four stars are bright, evolved giants or subgiants, above the main sequence in the HR diagram. However, some recent data collected by astronomers in France indicates that the proper motion of the stars is sufficiently discordant to indicate that they are not, in fact, physically related. And so, the controversy continues as to the real nature of this object.

M73 can be seen is telescopes of about 4-inch size, although the dimmest of the 4 stars is fainter and more difficult. It lies about 1 1/2 degrees west of the globular cluster M72 in Aquarius.

Taken October 20, 2011 - exposure is 100 seconds with the C-14 operating at F:7 and the ST-8 binned to 2 X 2