You can count on the fingers of one hand the galaxies that rival, let alone are superior in the northern sky to Messier 101 (NGC 5457), the awesome and dominant Pinwheel Galaxy. Together with Messier 51 in Canes Venatici and Messier 81, a fellow resident of Ursa Major, Messier 101 is one of ‘the big three’ spiral galaxies of the spring sky.
Messier 101 a bit of a monster; physically spanning around 170,000 light years (it lies some 20.9 million light years away) and believed to host a trillion stars, Messier 101 is the second-largest galaxy in Messier’s catalogue after the mighty Andromeda Galaxy (Messier 31). Thus, unsurprisingly, it is the third largest by appearance on the sky after Andromeda and M33, the Triangulum Galaxy, spanning a whopping 27’ x 26’. Physically, M101 matches up well with M31 and somewhat dwarfs M33.
Deep images show its spectacular spiral structure in the ‘grand-design’ manner, meaning a face-on spiral with prominent and well-defined spiral arms. Arguably, M101 is the archetypal or textbook such galaxy (Messier 51 devotees will argue not). Its sweeping spiral arms are liberally punctuated by prominent blue, intense star-forming regions, including many bright young clusters of hot, newly-formed stars, and reddish-pink HII regions ionised by those youthful stars.
Messier 101 is surely one of the best galaxies in the entire sky for imagers looking for a spectacular deep-sky portrait. However, visual observers would do well to ignore looking at such images before heading out into the crisp spring night to track down M101 and perhaps do battle with Messier 101’s spiral arms. Unfortunately, M101’s huge apparent diameter equates to a fairly low surface-brightness, which, despite a catalogued integrated +7.7 magnitude, makes it a testing target for a pair of binoculars, except under an exceptional sky.
M101’s position east of the Plough’s handle makes its not too hard to track down; it marks the apex of a rough isosceles triangle, with the distance to it from the ‘handle stars’ Mizar (delta UMa and Alkaid (eta UMa) both being about 5° 40’. At nightfall at the start of April M101 is around 40° high by 9pm BST, and transits at the zenith (overhead) at about 2.15am.
Once you’ve found M101, if you want to notice the dark lanes that mark the boundaries of M101’s spiral arms, you’ll need the light grasp and resolving power of a 250–300mm (10- to 12-inch) telescope. With lesser aspirations, an 80mm (~three-inch) telescope can reveal around the inner 25 per cent of the galaxy, including a small nucleus and hints of the numerous H-II regions as brighter spots in the overall haze.