The mystery of Messier 102

Deep within the popular Messier Catalogue of deep-sky objects lies an enigma. Mark Hardaker investigates the curious case of M102

- Mark Hardaker

The Messier Catalogue is one of the most well-known collections of astronomical objects.

Compiled in the 18th century by French astronomer and avid comet hunter Charles Messier as a quick reference of non-cometary bodies he could ignore, today it serves as a comprehensive list of some of the deep sky's most alluring objects to observe.

Among its entries are various nebulae, star clusters and galaxies, but one entry in particular, Messier 102 (or M102), has sparked considerable controversy and debate within the astronomical community.

M102 was originally recorded by Messier's assistant Pierre Méchain in spring 1781. Méchain made many of the discoveries in the catalogue, which Messier usually confirmed before recording. However, under pressure to publish in the annual French almanac Connaissance des Temps, Messier entered M102 without a position, though he later pencilled one in his personal copy.

The entry for M102 reads: "102. Nebula between the stars Omicron Boötis and Iota Draconis: it is very faint, near it is a star of sixth magnitude."

But that's where the certainty ends. In 1783, Méchain wrote to astronomer Johann Bernoulli III in Berlin, claiming that his original observation was incorrect and that he had simply made a duplicate observation of nearby M101. In short, there was no M102.

This ambiguity has prompted a long-standing debate: is M102 simply an erroneous duplicate of M101, or is it another celestial body entirely that has yet to be accurately identified?