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]."

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."

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

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 -12:38:04.2  11.90  4965 -21.5  2.30  2.48
GCS 5778-0492             20:58:54.79 -12:37:54.5  11.94  5735 - 8.9  4.70  0.88

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 of 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.