Messier 73 (M73, also known as NGC 6994) is an asterism of four stars in the constellation Aquarius. It lies several arcminutes east of globular cluster M72. According to Gaia EDR3, the stars are 1030±9, 1249±10, 2170±22, and 2290±24 light-years from the Sun, with the second being a binary star.[3]
M73 was once treated as a potential sparsely populated open cluster, which consists of stars that are physically associated in space as well as on the sky. The question of whether the stars were an asterism or an open cluster was a matter of debate in the early 2000s.
In 2000, L. P. Bassino, S. Waldhausen, and R. E. Martinez published an analysis of the colors and luminosities of the stars in and around M73. They concluded that the four bright central stars and some other nearby stars followed the color-luminosity relation that is also followed by stars in open clusters (as seen in a
′ of the centre point. They demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions. Therefore, they concluded that the stars were only an asterism.[8]
Although M73 was determined to be only a chance alignment of stars, further analysis of asterisms is still important for the identification of sparsely populated open clusters. A full study of very many such clusters would demonstrate how, how often, and to what degree open clusters are ripped apart by the
gravitational forces in the Milky Way and reveal more of the sources of these forces
.
Location
M73 is in Aquarius, just to the east (left) of M72.
See also
List of Messier objects
Messier catalogue
that was also mistakenly identified as having nebulosity
20h 58m 54s, −12° 38′ 00″
