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M 73

Group or Asterism of 4 Stars M73 (NGC 6994) in aquarius

Rektascenzija 20 : 58.9 (u:m)
Deklinacija -12 : 38 (sto:m)
Razdalja 0 (*1000 sv.l.)
Vizual. magnituda 9.0 (mag)
Zorni kot 2.8 (loc min)

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 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 its position was auxiliary for measuring M72 which is 1.5 degrees to the west. 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 Dreyer has included it in the NGC catalog.

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. The present author, however, tends to join the opinion of P. Murdin, D. Allen, and D. Malin, expressed in their Catalog of the Universe:

"[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."
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. So if anybody knows newer data of the star, or could check a database like that of the Hipparcos data, please do ! It would even be of advantage to get some basic data, as e.g. the spectral classes, of these four stars, as they are not given in the references known to the present author. And as the information belongs here, please email me!

Povzeto po:
Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

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