M73 - NGC 6994

Messier 73 (NGC 6994) is one of the most controversial and unusual entries in the Messier catalog. Located approximately in the constellation Aquarius, it was cataloged by Charles Messier in 1780, who described it as a cluster of three or four small stars resembling a nebula. For over two centuries, M73 was listed in catalogs as an open star cluster, but modern research has cast serious doubt on this classification. Studies using precise stellar distances from the Hipparcos satellite and subsequent analyses have shown that the four stars making up M73 are at very different distances from Earth, ranging from roughly 1,000 to 2,500 light-years, meaning they are not gravitationally bound and do not constitute a true physical cluster. Instead, M73 appears to be an asterism, a chance alignment of unrelated stars that happen to appear close together on the sky. This finding has been supported by multiple studies examining the stars' proper motions, radial velocities, and distances, all of which confirm they are unrelated. Some catalogs have reclassified M73 as an asterism rather than a cluster, while others retain the historical cluster designation. M73 is the smallest and least impressive object in the Messier catalog, consisting of just four stars of 10th to 12th magnitude arranged in a Y or triangular pattern spanning only about 2.8 arcminutes. Despite its underwhelming nature, M73 holds historical interest as an example of how visual appearances can be deceiving in astronomy, and it serves as a cautionary tale about assuming physical association based on apparent proximity. Through a telescope of any size, M73 simply appears as four faint stars close together.

M73 consists of four unrelated stars of 10th to 12th magnitude spanning about 2.8 arcminutes, at distances ranging from 1,000 to 2,500 light-years.