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M 73
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M 73

February 19, 2005
Group or Asterism of 4 Stars M73 (NGC 6994) in Aquarius

Right Ascension: 20 : 58.9 (h:m)
Declination: -12 : 38 (deg:m)
Distance: 2.5 (kly)
Visual Brightness: 9.0 (mag)
Apparent Dimension: 2.8 (arc min)

Discovered 1780 by Charles Messier.

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, as Charles Messier, who found it on October 4, 1780, described it as

"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]."

Apparently, this group found its way into Messier's catalog because he had determined its position at the same time when measuring M72, which is 1.5 degrees to the west. It may have been included because of its "first-glance nebulous" appearance in Messier's instruments. Although it is clear from this description that this group was what Messier had observed and measured, some versions of Messier's catalog omit it as an "obscure" object. However, John Herschel has included it in his General Catalogue as GC 4617, and J.L.E. Dreyer included it in the NGC catalog as entry number 6994.

Consequently, this object 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."

ESA's Hipparcos satellite has obtained measurements for the parallaxes of two and for the proper motion of all four stars. Giovanni Carraro of the University of Padua, Italy has tried to determine distances from these data, and gives values of 137 and 440 and light-years for the brightest star GSC 05778-00802 and second-brightest HD 358033, respectively (Carraro 2000, see also Sky & Telescope of July 2000, p. 26). Unfortunately, these results are of limited value only, as the famous and otherwise extremely useful Hipparcos database has turned out to contain systematical errors like nonsensial negative parallaxes, particularly for closely neighbored stars, and therefore does not help very much in issues like this (a note in S&T of October 2000, p. 20 coincides with our notion).

This "Y"-shaped group of stars is well visible in 4-inch telescopes; the fourth star is notably fainter and difficult in these instruments. It is best found from M72 which is almost at the same declination (very slightly North) and 1.5 deg West. The 4.5-mag star Nu Aquarii, mentioned by Messier, is about 2deg North and 1.5 deg to the West. East of this star (and not far from M73), the Saturn Nebula (NGC 7009) can be found.


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