Messier Objects

M102: Lenticular Galaxy (Draco) RA: 15h 06.5m / DEC: +55° 45'.8
Instrument: 10-inch Starfinder

NGC 5866 is a lenticular galaxy in Draco. It is identified here as M102, although this is fairly controversial. Charles Messier did include a 102nd object in his final catalog. But the observation was not made by Messier. It was made by his assistant, Pierre Mechain. Years later, Mechain recanted this as a duplicate observation of M101. Normally, this is where M102's story would have ended. But there were problems.

Mechain's notes describe M102 as "a very faint nebula situated between Omicron Bootis and Iota Draconis: near to it is a 6 mag. star." The first problem arises from the stars Mechain uses to describe the object's position. Omicron Bootis and Iota Draconis are separated by more than 40 degrees. It is odd that he chose two stars with such a large separation to describe the location of a nebula. The second problem is that his description is so different from that he wrote for M101: "nebulae without star, very obscure and pretty large, 6' or 7' diameter between the left hand of Bootes and the tail of the Great Bear. Difficult to distinguish when graticule lit."

It has been suggested that Mechain misidentified Theta Bootis as Omicron Bootis in his notes. Although a bit of a stretch, this would bring several issues into the clear light of day. If he did make this surprising error, the possibility looms that he did observe something other than M101. Could NGC 5866 be M102? NGC 5866 does reside along a line between Iota Draconis and Theta Bootis, two stars separated by about 22 degrees. Also, note that Mechain mentioned no bright star near to M101. Perhaps, this is because the closest 6th magnitude star to M101 almost a degree to the southwest, 6.9 magnitude HD 122007. The closest 6th magnitude star to NGC 5866 is 6.9 magnitude HD 133666, about 31' to the northwest, close enough that inclusion a description for this object would make sense.

Neither solution to the mystery of M102 is completely satisfactory to me. However, I chose to err on the side of inclusion rather than exclusion. Perhaps if Messier's and Mechain's original notes on this object are ever found, we will have a definitive solution. Until then, I will continue to include NGC 5866 as M102.

The sketch at left presents a 129X view in my 10-inch Newtonian. M102 is relatively bright at 9.8 magnitude and displays an unmistakable lenticular shape across a 4'x1' area. It's sandwiched between 11th and 12th magnitude GSC stars. The three brighter stars in the triangle grouping range from 8th to 10th magnitude. The brightest nearby star to M102 is 5.2 magnitude SAO 29407, 1.2 degrees due south. The wonderful edge-on galaxy, NGC 5907, is just 1.4 degrees to the northeast.

M101 M103-Christmas Tree Cluster


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Revised: February 10, 2002 [WDF]