Messier Objects

M101: Spiral Galaxy (Ursa Major) RA: 14h 03.2m / DEC: +54° 20'.9
Instrument: 10-inch Starfinder

M101 is a relatively easy star hop from Mizar and Alcor, the naked eye double star at the bend of the Big Dipper's handle. The galaxy is about 6.5 degrees due east from the pair. A string of 4th through 8th magnitude stars leads the way. However, getting to the right field is only half the challenge. M101 is deceptively hard to see. It has an integrated visual magnitude of 7.9, which might lead one to conclude this is an easy object to detect. However, that glow is spread over an area almost as large as the Moon. M101 is a low surface brightness object, just 23.8 magnitude per square arc second. M101 will be hard to detect if your skies are even moderately light-polluted.

The sketch at left was made at 63X under the dark, transparent skies of northern Arizona. It shows M101 as a 26'x22' misty patch of light. Closer inspection reveals a nearly stellar core embedded within a 7'x4' core region. This central core is aligned roughly east-northeast to west-southwest. Averted vision gathers up sections of the loosely bound spiral arms. One section begins at the east end of the core, winding south, then east and northeast before terminating. Another arm begins at the west end of the core. This arm curves to the northeast, then to the south. Fainter fragments of spiral structure are visible to the southwest and northeast, respectively. The field is dotted by 36 point-sources, many of which are stars associated with the Milky Way. But some are HII regions of star formation in delicate, face-on M101.

M100 M102


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Revised: May 6, 2002 [WDF]