M 73 is an odd entry in Messier's list. It consists of 4 dim stars in
Aquarius. Messier's entry indicates that he included it in the list because
he saw dim nebulosity around the stars, but in fact there is none. There has
long been a debate about whether M 73 is in fact a very sparse open cluster,
formed together and travelling together, or is an "asterism", a chance
arrangement of stars when seen from Earth's perspective. A 2002 study of the
spectra of these stars seems to have solved the question: the stars have very
different distances from earth and are moving in different directions, and so
are not a cluster of stars at all.

Image data:
Camera: Canon 350 XT (modified)
Exposure: ISO 800, 1 minute x 15
Telescope: 10" Schmidt-Newtonian, Baader MPCC