Messier 73, also known as NGC 6994, is an asterism of four stars in Aquarius. An asterism is composed of physically unconnected stars that appear close together in the sky. M 73 is one of the best-known asterisms in the sky, and has been carefully studied.
M 73 was discovered by Charles Messier in 1780; he originally described it as a "cluster of four stars with some nebulosity". Subsequent observations by John Herschel failed to reveal any nebulosity, and Herschel noted that the designation of M 73 as a cluster was questionable. Nonetheless, Herschel included M 73 in his General Catalogue of clusters, nebulae, and galaxies; and John Dreyer included M 73 as NGC 6994 when he compiled the New General Catalogue.
This "Y"-shaped group of stars is easily visible in a 4-inch telescope. It is best found from the globular cluster M 72, which is almost exactly 1.5° to the west. Three of M 73's stars are of 10th to 11th magnitude (A: 10.5, B: 10.5, and C: 11.0), the fourth (D: 12.0) is notably fainter. Although modern telescopes reveal no nebulosity around the group, this is clearly the object Messier described.
M 73 was once treated as a sparsely-populated open cluster, which consists of stars that are physically related. The question of whether the stars in M 73 are an asterism or an open cluster has generated some debate.
In 2000, astronomers published an analysis of the colors and luminosities of the stars in and around M 73. They concluded that the four bright central stars, and some others nearby, exhibited a color-luminosity relation typical of open clusters. However, the same year, another group of astronomers published exactly contradictory results.
Adding to the controversy, other astronomers calculated that the chance alignment of the four bright stars seen in M 73 was highly unlikely, so M 73 probably was a sparse open cluster.
The controversy was resolved in 2002, when high resolution spectra showed that M 73's stars were at very different distances from Earth, and moving in different directions. Therefore, the current conclusion is that the stars in M 73 are only an asterism - and that Messier's description of the nebulosity around them was simply a mistake.