Messier 102: Spindle Galaxy

M102 is the one Messier number whose object nobody can name with certainty. Most modern lists, and this page, take it to be NGC 5866, a neat edge-on galaxy in Draco often called the Spindle Galaxy. Others argue that M102 was never a separate object at all, only a second sighting of the Pinwheel Galaxy, M101. Either way, NGC 5866 is worth a look.

What it is

NGC 5866 is a lenticular galaxy: it has a disk and a central bulge like a spiral, but little or no spiral structure. We see it almost exactly edge-on, and photographs show a sharp, thin dust lane cutting along the length of the disk. It lies about 44 million light-years away, and its 5.2′ length corresponds to roughly 65,000 light-years.

When to see it

This is a late-spring and early-summer object, highest around midnight in mid-May and well placed in the evening from May into July. From about 40°N it climbs to roughly 74° and never sets. From the Southern Hemisphere it barely clears the northern horizon from around 30°S and is out of reach farther south.

How to find it

Find Iota Draconis (Edasich), a third-magnitude star in the tail of Draco, northeast of the Big Dipper's handle. Then find Theta Boötis, a fourth-magnitude star at the top of Boötes, about 11° to the southwest. NGC 5866 lies on the line between them, about 4° southwest of Iota Draconis, or a little more than a third of the way toward Theta Boötis.

What you will see

Binoculars will not show it. In a 70–100 mm telescope it is small, but its bright core helps: you see a short, elongated glow with a brighter center, running northwest to southeast. A 200 mm or larger telescope shows a clear lens shape with a bright, almost starlike nucleus, and a 250–300 mm scope may show the dark dust lane as a thin line along the middle. Filters do not help.

History

Pierre Méchain reported the object in 1781, and Messier listed it as number 102 without measuring a position, giving only a description. In 1783 Méchain wrote to Johann Bernoulli that M102 was a mistake: a repeat observation of M101 that had been entered twice. The case for NGC 5866 comes from Messier's published description, which places the nebula between Omicron Boötis and Iota Draconis. Omicron Boötis lies far to the south and makes no sense as a guide star, but if "Omicron" is a slip for "Theta" Boötis, the line points straight to NGC 5866, the only bright galaxy there. Neither view can be proven, and you will see both in books and charts.