by Stephen Jones
Constellation – Ursa Major
Type: Spiral Galaxy
Discoverer: Pierre Méchain, 1781
Equipment necessary: For most observers, at least a 6-inch telescope.
Spiral structure in galaxies was first noted by Lord Rosse when he turned his massive telescopes on M51, the Whirlpool Galaxy; not long after this, he saw the same in M101. Face-on spirals like M51 and M101 can be among the most interesting galaxies to observe with medium-to-large sized amateur telescopes under a dark sky, as it is easiest to see the spiral arms when the galaxy has its face toward us.
M101 is a large spiral galaxy of relatively low mass for its size, located about 27 million light years from Earth. It is the primary member of group of about 9 galaxies, some of which are also relatively easy to observe with amateur telescopes (though they are much smaller). M101 contains a lot of star-forming HII regions, many of which are relatively easily visible. Quite a few HII regions in M101 even ended up with their own NGC numbers, as the true nature of the object was not yet known when the NGC was published.
M101 is relatively easy to locate, as it makes something close to an equilateral triangle with Mizar (ζ Ursae Majoris) and Alkaid (η Ursae Majoris), the last two stars in the handle of the Big Dipper.
M101 is quite large as deep sky objects go, but its surface brightness is low, making it more difficult to detect than some other, brighter objects. In a small telescope or under non-optimal conditions, M101 looks like a large misty oval. Averted vision will increase your chances at detecting it. In medium-sized telescopes under dark skies, M101 shows a small round nucleus with a mottled outer halo, that in the best of conditions you may be able to trace out the spiral arms. With large telescopes and good seeing, the spiral arms are quite clear, and you may be able to identify some of the NGC-numbered HII regions.