M104 was discovered by Pierre Méchain on May 11, 1781, and included by hand in Messier's personal copy of his catalog in the Connoissance des Temps for 1784. Camille Flammarion added it to the Messier catalog in 1921 (Flammarion 1921). M104 is the only additional Messier Object which Burnham accepts in his Celestial Handbook.
It is probably the decision of Camille Flammarion of 1921 to add M104 that any of the additional objects were appended to Messier's catalog. Messier's own handwriting indicates that this would very probably have been in the sense of Messier.
M105, M106, and M107 were discovered by Pierre Méchain and mentioned in his letter to Bernoulli of May 6, 1783, together with M104, in which he disclaimed his M102 discovery (but see the discussion of this subject). Méchain discovered M105 on March 24, 1781, "near M95 and M96, ... four or five days after the other two." M106 was found by him in July 1781, M107 in April 1782. Herewith, globular cluster M107 was the last Messier object ever discovered. All three have been added to the Messier catalog by Helen Sawyer Hogg in 1947 (Sawyer Hogg 1947, Sawyer Hogg 1948), and apparently popularized as Messier (or "Méchain") objects by Owen Gingerich in 1953 (Gingerich 1953).
M108 and M109 were first seen by Pierre Méchain and are mentioned in Messier's original description of M97 in his final printed version of the catalog, as well as in Méchain's letter. According to Messier's manuscript notes, Méchain saw these two objects when observing M97: M108 on February 18 or 19, 1781, and M109 on March 12, 1781. Messier seems to have seen them also on March 24, 1781 when he measured M97 and created the description - according to the text of the description, and his manuscript version where he has them preliminarily numbered "98" (M108) and "99" (M109) without positions. Nevertheless, Kenneth Glyn Jones states inacurately that M108 and M109 were discovered "in 1781 or 1782". These two objects were included into the catalog in 1953 by Owen Gingerich (Gingerich 1953). M108 (NGC 3556) is generally accepted, while Gingerich's M109 (NGC 3992) has been found to lack coincidence with Méchain's discovery, which was found (by Henk Bril) to be NGC 3953 (M109B) - Gingerich's M109 may be an observation (and discovery) of Charles Messier.
M110 was finally added by Kenneth Glyn Jones in 1966 (Glyn Jones 1966). This one was discovered by Charles Messier on August 10, 1773, as he published only in 1798 in a report printed in the Connoissance des Temps for 1801, and painted in his drawing of the Andromeda galaxy M31 (which also includes the other companion galaxy, M32), published in 1807.
It may be of interest that Pierre Méchain, in his letter to Bernoulli of May 6, 1783, writes that in the region of the Virgo Cluster of Galaxies, "there are, however, in that region some others also, which he [Messier] has not seen and the positions of which I intend to determine, as soon as I [Méchain] will have a comfortable observing place, .." - It seems that there are no records if Méchain ever carried out this observing project, so we don't know the additional objects he had found in this region of the sky.
Since the first addition of M104, it is disputed if such a procedure makes sense, as historically, Messier has only numbered 103 of the objects. Burnham, e.g., only accepts M104 and disregards the others. Besides the reasons given above for each object, the general argument holds that they were known to Messier and Méchain. Also, Messier's list together with these objects summarizes most deepsky objects north enough to view them from the latitude of Paris, and known before Herschel's survey. Moreover, these objects are now wellknown by their "M" numbers, especially among amateurs, so that it seems appropriate to accept these additions.
Last Modification: January 5, 2021