Messier 73

Discovered 1780 by Charles Messier.

The object Messier 73 (M73, NGC 6994)) is a little conspicuous assortment of four stars of 10th to 12th magnitude, situated in the very western part of constellation Aquarius; its nature as a physical group is doubtful. It lies only about 1.5 deg east of globular cluster M72.

Although M73 is apparently consisted of 4 stars, 3 of them being of about 10th to 11th magnitude (Burnham and Kenneth Glyn Jones give A:10.5, B:10.5, and C:11.0), the fourth (D) being of mag 12.0, it is obviously a true Messier object. Charles Messier found it on October 4, 1780, described it as follows:

"Cluster of three or four small stars, which resembles a nebula at first glance, containing very little nebulosity; this cluster is located on the parallel [of declination] of the preceding [M 72]; its position has been determined from the same star [Nu Aquarii]."

Consequently, this object has received little research interest. Collinder (1931), who cataloged it as Cr 426, estimated its distance at 12,000 light-years, and from its 2.8' angular diameter, speculated if this was an open or a globular cluster. It was also cataloged as C 2056-128 and OCL 89 in more recent catalogs of open clusters. Ruprecht (1966) classified it as of Trumpler type IV 1 p, i.e. a very sparse and poor open cluster which is not very well detached from the surrounding star field. Wielen (1971) considered it as doubtful, but classified it as an old and nearby cluster.

What remains to clear up to now, at least to the knowledge of the present author, is the check if the 4 stars in M73, or at least some of them, are physically related. There was always a great fraction of astronomers who believed that M73 is an asterism, a chance alignment of 4 stars at different distances, but it would certainly be interesting to know if all or some of the 4 stars form a physical system of some kind. As Kenneth Glyn Jones states: "This issue is perhaps a minor one, but every student of the Messier catalog would be much interested in the outcome."

P. Murdin, D. Allen, and D. Malin, in their Catalog of the Universe, summarize the problem and give the following estimate for the probability of M73 being an asterism or a physical multiple:

"[The authors] suspect in fact that M 73 might be a real little cluster, for the following reason. On average there are 60 stars per square degree which are brighter than magnitude 12, as are the four stars of M 73. The probability of finding four such stars by chance in a given area of sky one arc minute across (like M 73) is about two chances in a billion. However, there are 150 million such little areas on the sky, so the chances are only one in four that such random asterism exists on the sky. M 73 could be it, but we would gamble that it is a genuine multiple star of some kind."

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