M73 -- Asterism in Aquarius
Tradition over Substance
Charles Messier’s famous list of deep sky objects has a few quirks. Messier 73 is one of
them. When Messier viewed the four stars (above the label in the drawing) on October
4 & 5, 1780, he believed that he saw some faint nebulosity and thus assigned it as an
object that could be confused for a comet (Messier was a comet hunter). We now know that
there is no nebulosity at this location. His error can easily be blamed on the quality of the
telescope he was using which blended the light of the close stars into a false nebula.
The four stars are not a gravitationally bound star-cluster or even a multiple-star system, just
four stars of chance alignment. The best one could call it is an Asterism, but even that is
being kind. Regardless of its true nature it is still on the Messier list and thus is viewed on a
regular basis by amateur astronomers around the world. More surprising was that these
four stars were assigned an NGC number (6994) even though by then they were aware of
Messier’s error and that no nebulosity existed. I guess M73 is worth a drawing for historical
reasons even though it is of no true astronomical importance.