M73 is a curious entry in the Messier catalogue. It's a small group of four faint stars shaped like a "Y" in the constellation of Aquarius. The grouping was discovered by Messier himself on October 4, 1780, the night he also catalogued M72. He described the object as a cluster of three or four stars with some nebulosity. Subsequent observations by William and John Herschel revealed no nebulosity. Nevertheless, John Herschel included M73 in his General Catalogue of clusters, nebulae, and galaxies (GC) and John Dreyer added M73 to the New General Catalogue (NGC).

It's now known that no such nebulosity exists and recent measurements suggest that M73 is not a true open cluster at all but just an asterism.

M73 is located in southern Aquarius close to the border with Capricornus. The asterism is positioned just over 1 degree east of 9th magnitude distant globular cluster M72. Finding M73 - also M72 - can be quite challenging; Aquarius and Capricornus are both relatively faint constellations with few bright stars.

One possible route to locate M73 is with the stars of Capricornus. Locate theta (θ Cap - mag. +4.1) and then image a line from this star in a northwesterly direction for 9 degrees until you reach marginally brighter star, Albali (ε Aqr - mag. +3.8). Just over halfway along this line is M72 and located 1 1/3 degrees to the east of M72 is M73.

The asterism is best seen during the months of July, August and September and ideally from tropical, Southern Hemisphere regions where it appears high in the sky.

M73 Asterism of four stars (NOAO/AURA/NSF)

Finder Chart for M73 (also shown M30, M72 and M75)

Finder Chart for M73 (also shown M30, M72 and M75) - pdf format

M73 has an apparent magnitude of +9.0 and hence is a difficult 7x50 or 10x50 binocular object. Larger 20x80 binoculars fair better, although since the four stars are between 10th and 12th magnitude in brightness, the asterism appears as a poor faint point of light. Through 100mm (4-inch) telescopes the "Y" shape of the group is visible with three of the stars blue-white in colour, the other orange. What's noticeable is that one star is fainter than the other three (component magnitudes are +10.4, +11.3, +11.7 and +12.3). The apparent diameter of M73 is only 2.8 arc minutes and hence it's possible to push up the magnification as high as the seeing conditions allow. A 150mm (6-inch) telescope or larger telescope will easily show the asterism in full detail. It's often been claimed that the telescopic view of M73 appears fuzzy suggesting of the same appearance Messier himself observed.

Scientifically there has not been a great deal of interest in M73. The cluster was previously treated as a sparsely populated open cluster, where the stars are loosely bound to each other by mutual gravitational attraction. However, evidence has always been thin on the ground and there has been a small debate on whether the stars are an asterism or an open cluster.

The current belief is that the cluster is an asterism with the four stars located at different distances and moving in different directions. A rough estimate of the average distance of M73 is 2500 light-years. Despite being only a chance alignment of stars, analysis of asterisms is useful to astronomers studying how open clusters are pulled apart by Milky Way gravitational forces.

M73 Data Table

Object TypeAsterism
Distance (kly)~2.5
Apparent Mag.9.0
RA (J2000)20h 58m 56s
DEC (J2000)-12d 38m 08s
Apparent Size (arc mins)2.8 x 2.8
Number of Stars4
Other NameCollinder 426

Sky Highlights - April 2017

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