Messier 102
An ambiguously identified galaxy in the Messier Catalogue.
Messier 102 (M102) is an entry in the Messier Catalogue whose exact identity remains uncertain. The object was first observed by Pierre Méchain in early 1781 and added to the catalog by Charles Messier later that year, though Messier omitted its coordinates, creating confusion. Méchain later retracted the discovery in a 1783 letter, stating that M102 was simply a mistaken duplicate of M101, caused by an error in sky charts. Despite this, later historical analysis suggests that M102 is most likely the galaxy NGC 5866, a lenticular galaxy in Draco that closely matches Méchain’s description and Messier’s handwritten position notes. NASA officially treats M102 as identical to NGC 5866. Other candidates have been proposed, including NGC 5879, NGC 5907, NGC 5905, and NGC 5908—all near NGC 5866’s position—but these are fainter and less likely. NGC 5928 has also been suggested, based on a possible misidentification of a star, but it is too dim to have been observed by Messier or Méchain.
- Catalog designation
- Messier 102 (M102)
- Object type
- galaxy
- Discoverer
- Pierre Méchain
- Discovery date
- late March or early April 1781
- Catalog publication year
- 1781
- Retraction year
- 1783
- Primary candidate
- NGC 5866
Lore & Background
M102 was observed by Pierre Méchain in late March or early April 1781 and was added by Charles Messier to the final version of his catalogue published in 1781. However, Messier did not include the coordinates of M102 on his catalogue, leading to confusion about the exact object they observed. His description of the object was: 'Nebula between the stars Omicron Boötis and Iota Draconis: it is very faint; near it is a star of 6th magnitude.'
In 1783, Méchain retracted his discovery in a letter written to J. Bernoulli and claimed that M102 was actually an accidental duplication of M101 in the catalog. This letter was later published twice: first in original French in the Memoirs of the Berlin Academy for 1782, and second in German translation and somewhat rearranged by Johann Elert Bode in the Berliner Astronomisches Jahrbuch for 1786. Méchain wrote: '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.'
Since the publication of the Messier Catalogue, a number of galaxies have been identified as corresponding to M102. The two most likely candidates are Messier 101 (the Pinwheel Galaxy) and NGC 5866 (the Spindle Galaxy). NGC 5866 appears to closely match both the object description in the printed version of the Messier Catalog of 1781 and the object position given by Messier in hand-written notes on his personal list. Other possible objects include NGC 5879, NGC 5907, NGC 5905, NGC 5908, and NGC 5928, though these are considered less likely.
Reader's Guide
The significance of Messier 102 lies in its unresolved identity, which has sparked historical debate among astronomers. The entry illustrates the challenges of early astronomical cataloging, where errors in sky charts and ambiguous descriptions led to confusion. Méchain's retraction and claim of duplication highlight the collaborative yet imperfect process of compiling the Messier Catalogue. Later historical evidence favors NGC 5866 as the true object, and NASA officially considers M102 to be the same as NGC 5866. However, the lack of coordinates in the original catalog and the existence of multiple candidate galaxies—such as M101, NGC 5879, NGC 5907, NGC 5905, NGC 5908, and NGC 5928—mean that M102 remains a controversial entry. Its legacy is a reminder of the human element in scientific discovery, where interpretation and error can persist for centuries. For amateur astronomers, M102 represents a puzzle that connects them to the history of astronomy, encouraging investigation of the night sky and the stories behind the objects.
Did You Know?
- Méchain retracted his discovery in 1783, claiming M102 was a duplicate observation of M101.
- The National Aeronautics and Space Administration (NASA) considers M102 to be the same as NGC 5866.
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