M 73 (NGC 6994) is an asterism in
the constellation of Cepheus. It was discovered by Charles Messier
in 1780. He identified It as a star cluster with some nebulosity.
John Herschell did not observe any nebulosity and questioned its
designation as a star cluster. However, he added this to his General
Catalog. The debate about the true nature of M 73 continued into
the 21st century. In 2000, a group of astronomers examined the color-luminosity
relationship of the core stars and concluded that they were typical
of an open star cluster. However, in 2002, M Odenkirchen and C.
Soubiran noted that the distance of the stars were very different
and were moving away from each other. The conclusion was that M
73 is an asterism vs. an open cluster.
M 73 is one of only three Messier
objects that are not bonafide deep-sky objects. The others are M
40, a double star, and M 24, a patch of the Milky Way.