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M 73

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

Right Ascension 20 : 58.9 (h:m)
Declination -12 : 38 (deg:m)
Distance 2.0 (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 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) estimated its distance at 12,000 ly, and from its 2.8' angular diameter, speculated if this was an open or a globular cluster. 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. 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."
This view was recently revived by a suggestion of Argentine astronomers led by Lilia Bassino of the National University of La Plata. The four stars which form Messier 73 were photometrically determined as follows:
Star RA (2000.0) Dec (2000.0) V     B-V
      h  m  s    deg  '  "

1 20:58:56.8 -12:38:29 10.355 1.002 2 20:58:57.8 -12:37:45 11.269 0.452 3 20:58:54.8 -12:38:04 11.675 0.575 5 20:58:53.5 -12:37:54 12.322 0.870
where the data are: Their star number, position in Right Ascension (RA) and Declination (Dec) for 2000.0, apparent V magnitude, and B-V color index. These colors show up conspicuously in color images obtained with larger telescopes, like this KPNO 0.9-meter telescope image. The authors obtain an estimated distance of this presumable "cluster remnant" of about 2,000 light-years; this would place these stars as bright, evolved giants or subgiants, above the main sequence in the Hertzsprung-Russell diagram (HRD). Assuming them to be main sequence stars would place them somewhat nearer to us.

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. Unfortunately, the famous and otherwise extremely useful Hipparcos database does not help very much in this issue, because it contains systematical errors like nonsensial negative parallaxes particularly for closely neighbored stars; this fact falsifies a claim by Giovanni Carraro of the Unicersity of Padua, Italy, based on these data (see e.g. Sky & Telescope of July 2000, p. 26), whose spectroscopic studies would nevertheless be of value here (a note in S&T of October 2000, p. 20 coincides with our notion).

So there is still an obvious need for more and newer data; 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. As the information belongs here, please email me any additional information (including pointers to it) you find !

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.

  • Historical Observations and Descriptions of M73
  • More images of M73
  • Amateur images of M73

  • SIMBAD Data of M73
  • NED Data of M73
  • Observing Reports for M73 (IAAC Netastrocatalog)


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

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    Last Modification: 20 Jun 2001, 13:30 MET