Messier's devotion was looking for, discovering, and observing comets. He was
considerably successful in this endeavour, scoring with 13 first discoveries
and 7 more independent co-discoveries.
According to Gary Kronk and Messier's notes,
we can add one more independent co-discovery to the 6 listed by Don Machholz,
namely of the comet De La Nux 1758 (often quoted as anonymous); this was
actually Messier's first comet discovery.
During his professional life, Charles Messier observed and carefully recorded
46 comets.
See also his personal "Notes on My Comets".
Messier's younger colleague and close friend,
Pierre Méchain, who had contributed
a lot to Messier's catalog in the later times, was also a successful hunter of
comets and became Messier's "most serious" rival in this field in the 1780s and
1790s; he originally discovered eight comets, and co-discovered three
(including above-listed comets 1785 I Messier-Méchain and 1801 Pons):