M73 (also known as NGC 6994) is a group of four stars very close to each other in the constellation of Aquarius. It was discovered by Charles Messier on October 4, 1780.

The true nature of M73 is as yet undetermined. It may well be an asterism - a chance alignment as seen from earth of four physically unconnected stars. However, it has been classified in the past as a very sparsely populated open cluster, and even a globular cluster. Distance estimates for the object range from 2,000 to 12,000 light years.

One argument against M73 being an asterism is based on simple probability. The four stars of M73 are all brighter than 12th magnitude, and there are on average 60 such stars in any given square degree of sky. The chance of finding four such stars in an area only one arcminute across anywhere in the sky is only about 25%, if the stars are randomly distributed, and therefore there is a 75% chance that the stars are physically related.

To determine the true nature of M73, better data is needed. Knowledge of the spectral type of each star would give a clue as to whether they were born at the same time or not, as would accurate astrometry and determination of the stars' proper motions. As yet, however, observations of sufficient quality do not exist for this object.

External links

• Messier 73, SEDS Messier pages