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Messier 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 2.5 (kly)
Visual Brightness 9.0 (mag)
Apparent Dimension 2.8 (arc min)

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]."
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 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."
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).

The view of Murdin et.al. was recently revived by a suggestion of Argentine astronomers led by Lilia Bassino of the National University of La Plata (Bassino et.al. 2000). 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 investigated another 140 stars in the region of M73, and identified a total of 24 member candidates, including the four stars of M73. These candidates can be fit into a color-magnitude diagram of an old cluster of age 2 or 3 billion years. They obtain an estimated distance of this presumable "cluster remnant" at about 2,000 light-years; this would place the four M73 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. Based on this work, Bica et.al. (2001) list M73 as a "Possible Open Cluster Remnant (POCR)."

The most recent major investigation of the nature of M73 to now was published by Odenkirchen and Soubiran (2002), who used spectroscopic data obtained with the Elodie spectrograph on the 1.93-meter telescope at the observatory at Haute Provence, France, together with the proper motion data from the Tycho 2 Catalog, based on above-mentioned Hipparcos data. The latter, the proper motions given in the Tycho 2 Catalog, are considerably discordant for these four stars, enough to make a common proper motion improbable. The spectriscopic investigation provided radial velocity data as well as, by special software, atmospherical data including effective temperature, gravity and chemical composition. The software provided estimates for absolute magnitudes, which in turn were used to estimate the distance of each star. The authors conclude that M73 may rather be an asterism of four independent stars at different distances, and give the following data:

Star                      RA (2000.0) Dec (2000.0) V     T_eff   RV   M_V   Dist

GCS 5778-0802 BD 135809 20:58:56.77 -12:38:30.4 10.48 4730 -26.8 0.86 2.61 GCS 5778-0509 HD 358033 20:58:57.59 -12:37:45.8 11.32 5938 -54.0 3.62 1.11 GCS 5778-0594 BD 135808 20:58:53.32 -12:37:54.5 11.90 4965 -21.5 2.30 2.48 GCS 5778-0492 20:58:54.79 -12:38:04.2 11.94 5735 - 8.9 4.70 0.88
(T_eff: Effective temperature in K, RV: Radial velocity in km/s, V and M_V: Apparent and absolute visual brightness in mag, Dist: Distance in kly)

Looking at these data, only the values for radial velocity and distance of the first and the third star are close enough together that there remains a dim chance that they may form a physical pair, a possibility that would be made even dimmer, should their Tycho 2 proper motions be confirmed, which differ by about 10 mas/yr (milli arc seconds per year), corresponding to about 30-40 km/s at the estimated approximate distance of 2,500 light-years.

The present author [hf] tends to feel that it would still be hasty to finally close this issue. Reasons are, first, the limited usefulness of the Hipparcos data used to make the Tycho 2 Catalog. Second, Odenkirchen's and Soubiran's valuable spectroscopic work was evaluated by kind of black box software to estimate absolute magnitudes, and thus to derive distance estimates. Third and perhaps most important, the observational data in the different investigations are still spread considerably; consequently, e.g. the derived distance values for the four stars are correspondingly uncertain.

Therefore, there is still an obvious need for more and hopefully even better data; even the basic data of these four stars, e.g. their spectral types, might be improved (or confirmed). As the information belongs here, please send us 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. To the southeast of M73 is a collection of faint galaxies, the brightest of which is IC 1344 at visual mag 13.7; about 1.5deg E and little N is equally faint (13.7 mag) NGC 7010.

  • Historical Observations and Descriptions of M73
  • Data of the stars of M73
  • More images of M73
  • Amateur images of M73

  • WEBDA cluster page for M73
  • SIMBAD Data of M73
  • NED Data of M73
  • Publications on M73 (NASA ADS)
  • Observing Reports for M73 (IAAC Netastrocatalog)
  • NGC Online data for M73


    Hartmut Frommert
    Christine Kronberg

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    Last Modification: November 14, 2007