Messier 102 has been considered a "missing" Messier Object for a long time. In semi-recent publications, this entry in Messier's Catalog has been frequently taken for a duplication of the preceding one, M101. However, historical evidence tends to favor the view that M102 is identical to the galaxy NGC 5866.
French astronomer Charles Messier compiled his famous "Catalog of Nebulae and Star Clusters" during the years 1758 to 1781 (Messier 1781) - or 1782 if counting the last additions discovered by his colleague Pierre Méchain, which are contained in most modern versions of the catalog. Besides Messier's own discoveries, this catalog also contains objects known previously, of which Messier had checked the positions, and in particular also the contributions by Pierre Méchain, who had been collaborating with Messier in the years before 1781.
Contrary to prior and contemporary observers who had a large number of errors (nonexistent objects) in their lists, the entries of his catalog correspond to actual astronomical objects in all cases, perhaps with one exception, his entry number 102 (there are positional errors for 3 other objects, M47, M48, and M91, which could be figured out by the time). In the following, some light will be brought into the case of M102.
In spring 1781, Messier and Méchain had taken great effort to discover and catalog new nebulous objects. Up to April 13, 1781 (a Friday btw), Messier had checked the positions of 100 objects. At that time, the deadline was approaching for contributions to the French yearbook, the "Connoissance des temps" for the year 1784, where the catalog was to be published. Therefore, Messier added the last three objects reported to him by Méchain without further verification at the end of the catalog, as No. 101, 102, and 103, with the remark that these were communicated to him "Through M[onsieur] Méchain, which M. Messier has not yet observed." The printed version of the catalog contains a position only for M101, while for M102 and M103, only descriptions are provided. However, Messier has added positions for both these objects to the personal copy of his catalog (Messier 1781a), see below. It remains to remark that there was always consent about the identity of M101 and M103 because of their descriptions, despite an error of 12m or 3deg in RA in Méchain's position for M101, but only M102 became subject of dispute.
Messier's description for M102 reads as follows (Messier 1781, p. 267):
102. Nebula between the stars Omicron Bootis and Iota Draconis: it is very faint, near it is a star of 6th magnitude.
About two years after the entry was made and published, Pierre Méchain retracted his discovery and claimed that the observation was an error, a duplicate observation of M101, and a star chart error. On May 6, 1783, he wrote a letter to Bernoulli of the Prussian Royal Academy in Berlin, which was published twice, first in small circulation in original French, in the Memoirs of the Berlin Academy (Méchain 1783), and second in some wider circulation and in German translation by Johann Elert Bode in the Astronomisches Jahrbuch for 1786 (Bode and Méchain 1783). Together with other contributions, including observations of the newly discovered planet Uranus, Méchain describes his observations of "nebulae," as far as they were not already included in Messier's catalog of 1781. Based on this letter, Helen Sawyer Hogg in 1947 (Sawyer Hogg 1947, Sawyer 1948) and Owen Gingerich in 1953 (Gingerich 1953) have added the objects M104 to M109 to the modern version of the Messier Catalog. In a short side remark following his nebula discoveries, Méchain retracts his discovery of M102 as follows:
I will add only that No. 101 & 102 on the p. 267 of the Connoissance des tems [for] 1784 are nothing but the same nebula, which has been taken for two, by an error in the [sky] charts.For the publication in his Astronomisches Jahrbuch for 1786, Bode has rearranged the contents of this letter, put the remark on M102 in a more prominent place near the beginning of Méchain's nebula descriptions, and has enhanced it as follows:
On page 267 of the Connoissance des tems [sic] f. 1784, Mr. Messier lists under No. 102 a nebula, which I have discovered between Omicron Bootis and Iota Draconis: this is a mistake. This nebula is the same as the preceding No. 101. Mr. Messier, caused by an error in the sky charts, has confused this one in the list of my nebulous stars communicated to him.Note that Méchain, in contrast to Bode, doesn't go into detail about the nature of the chart error, nor assign the error to either Messier or himself.
Despite Bode's publication, this reference got virtually forgotten for a long time. The Jahrbuch publication is mentioned only twice in more than 170 years: Edward S. Holden mentions it as source in his "Index Catalogue of Books and Memoirs Relating to Nebulae and Clusters" (Holden 1877), and it was cited again in 1907 in a low-circulation publication by Guilliaume Bigourdan (Bigourdan 1907). It was only recovered by Helen Sawyer Hogg in 1947/48 (Sawyer Hogg 1947, Sawyer 1948). This view, that M102 is a duplication of M101, was adopted by a number of consequent publications including Owen Gingerich (Gingerich 1960), Kenneth Glyn Jones (Glyn Jones 1968 and 1991), and John Mallas and Evered Kreimer's Messier Album (Mallas & Kreimer 1978).
However, there remain some doubts at Méchain's retraction: First of all, both Méchain and Messier were very careful observers, indicated by the fact that M102 is the only possible "non-object" left in the catalog. Also, the descriptions for M101 and M102 differ notably; that for M101 reads:
It appears not necessarily obvious that this is the same object as M102 described above. In addition, Méchain has only published his retraction in a letter for the Berlin Jahrbuch, written over two years after the publication of the "discovery," a fact giving room for speculations anyway. In particular, it was never published in the Connoissance des Temps, where Messier's catalog and supplements were first printed, nor in the volumes of the Paris Academy, despite the fact that Méchain himself has served as editor in chief of the Connoissance from 1785 to 1792 (issues for the years 1788 to 1794). Eventually, the Messier Catalog has been reprinted unchanged well after 1783, in the Connoissance for 1787, including the entry M102.
March 27, 1781. 101. 13h 43m 28s, +55deg 24' 25". Diam. 7'. Nebula without stars, very obscure and pretty large, between 6' and 7' in diameter, between the left hand of Bootes and the tail of Ursa Major. Difficult to distinguish when graticule lit.
Moreover, there is a celestial object which perfectly matches Méchain's description (Machholz 1994, Frommert 1995). However, first note that this description contains an obvious error: Omicron Bootis is about 40 degrees away and south of Iota Draconis! Therefore, it is impossible to locate any particular object from it. Very probably, (at least) one of the stars is misprinted due to a writing or printing error. Thus, J.L.E. Dreyer, in Notes and Corrections to the NGC (appendix to the IC catalog, Dreyer 1895), has speculated that Iota Draconis was mistaken for Iota Serpentis; then M102 could be situated near the position of the faint galaxy NGC 5928 at RA=15:25.9, Dec=+18:05 (2000.0). However, this proposition can be waived with great certainty because of its faintness of only 14th magnitude; the faintest objects in Messier's catalog are of about 10th magnitude.
More probably, in particular from the similarity of the Greek characters, appears the proposition that 'Omicron' is a misprint and should read 'Theta' Bootis, as first suggested by Admiral William Smyth in his 'Bedford Catalogue' (Smyth 1844). In this case, M102 could be a member of a small group of galaxies located in that area, about 3 degrees SW of Iota Draconis. The brightest of these galaxies is NGC 5866, a lenticular galaxy of 10th magnitude, and the only one bright enough for the instruments of the two Frenchmen. It is brighter than the Messier Objects M76, M91, M98, and M108, and about equal to M97 and M99. However, Symth has proposed the fainter NGC 5879. The galaxy group, called the NGC 5866 group, also contains the famous edge-on galaxy NGC 5907 and a number of fainter galaxies:
Moreover, another fact makes NGC 5866 a good candidate for M102: Situated close to it, about 1 degree distant and almost exactly to the south, is the 5.25-mag star HR 5635; this star is perfectly suited to help finding the object. Therefore, it is a good candidate for being the "star of 6th magnitude," mentioned in Méchain's and Messier's description, as it appears probable that Méchain wanted to describe a route to his newly 'discovered' object. There is no such helpful star in the neighborhood of M101, so that Méchain's description better matches with NGC 5866, suggesting that this may have been the object he had seen in his discovery observation.
Even more important for the identification of M102 is historical evidence that Charles Messier has probably observed NGC 5866 when attempting to verify the object No. 102:
In his personal copy of the catalog (Messier 1781a), Messier has added approximate positions by handwriting for both objects M102 and M103, probably shortly after its publication in 1781. This position was only published in low circulation by Camille Flammarion, who had acquired Messier's personal copy by a happy coincidence (Flammarion 1917, perhaps also Flammarion 1921). The present author has been pointed to these positions by Dr. Don Greeley (Greeley 1995), who has also generously provided him with a copy of Messier's personal copy of the catalog.
Messier's position for M103 is: RA "1.20," Dec "61.". Precessed to modern times, this becomes RA = 1:34.6, Dec = +62.1 (2000.0), little more than 1 degree north and very little east to the correct position of this cluster, which is RA = 1:33.1, Dec = +60.7.
Messier's position for M102, for the approximate epoch 1781, reads:
Precessed to the epoch 2000.0, Messier's position of M102 becomes:
As pointed out by Owen Gingerich aleady in 1960, there is no obvious object close to this position which Messier could have seen, and matches Méchain's desciption (Gingerich 1960). This notion is repeated by Glyn Jones (1968 and 1991). However, it is interesting that the position lies between the stars Iota Draconis and Theta Bootis, so that it is at least validated that apparently the "Omicron" in the description is a misprint for Theta. Moreover, as noted by the present author [hf] in late 1995, there is an object which is situated almost exactly 20 minutes, or 5 degrees, in Right Ascension east (following): NGC 5866, which is also little north of that position (an angular distance of almost 3 degrees):
A look in the sources suggests that Messier has normally used sky charts with grids of lines every 5 degree, as e.g. his chart showing the path of the comet of 1779. Then a deviation of exactly 5 degrees may have several simple reasons: A wrongly labelled chart, an erroneous look on the neighboring label, a wrong count to an un-numbered tick, etc. Messier had done a similar mistake in data reduction when computing the position of another missing object, M48, which was also measured about 5 degrees false (in that case, in declination).
Therefore, it appears probable that Charles Messier has observed NGC 5866 when he measured the position of M102, which he could probably locate without much difficulty because of Méchain's acurate description, but due to some reductional error, plotted it exactly 5 degrees west (preceding) of its correct position.
The object that really deserves the designation "Messier 102" should be identical to one of the two observed by Méchain and Messier, may they be identical or not. As nobody is still alive who has witnessed them during their observation and recording, we can currently not reconstruct what they actually observed. Méchain's description gives good evidence that the object M102 could be NGC 5866, which most probably everybody would believe if he had not retracted the discovery in the letter mentioned, or if this letter had stayed forgotten. It may depend on taste to speculate which was erroneous: The observation or the letter. Moreover, Messier has probably observed NGC 5866 and taken it for M102, but again made an error in data reduction. Once more, it is a question of taste if these facts entitle the lenticular galaxy NGC 5866 to bear the designation "M102".The authors of this page think that there is sufficient evidence that both Méchain and Messier have probably observed NGC 5866 in context of catalog entry No. 102. Therefore, it should be considered to take this last missing object back into the catalog.
At least, observers who want to go for sure that they observed all Messier objects should thus turn their telescopes to aim NGC 5866. They will be rewarded by quite an easy, beautiful object.
The positions in Messier's catalog were very faded and difficult to interpret. It was necesary to make a copy of that page so dark that the printing on the page behind it showed through. I made a slide of the page and when projected on a flat white wall showed that M102 was "14.40" in RA and "56." in Dec. M103 was much harder to see but is probably "1.20" in RA and "61." in Dec. Now they must be corrected for precesion for modern charts.The acuracy of Messier's values is probably indicated by the rough decimals, and in case of M103 more than one degree off in declination. This gives an approximate feeling for their acuracy, and is probably the reason why Messier had only pencilled them in his private notes.
On a closer look at the case of M102, one also fails with a sign error in a positional difference to a nearby star, as it had occured for M47, or a parallel shift due to taking a wrong reference star or object as for M91. But interestingly, the position is almost exactly at the correct declination for NGC 5866 and M101, and it is almost exactly 5 degrees (20 min) west (preceding) of NGC 5866 in right ascension, a fact recognized almost immediately by the present author [hf] when he first learned of this position in late 1995. It is also roughly 10 degrees east of M101, but much less acurately; the 2000.0 positions of NGC 5866 and M101 are RA 15:06.5, Dec +55.7, and RA 14:03.2, Dec +54.3, respectively, compared to Messier's RA 14:46.5, Dec +55.1 (2000.0).
Note: An alternative hypothesis was brought up semi-recently by Stephen O'Meara (O'Meara 2005); this will be subject to a future discussion here. In the opinion of the present authors, it does not take anything from the historical evidence presented here for M102 being NGC 5866.
Definitive Version: April 4, 2006 Last Modification: August 12, 2011