May evenings provide likely the best opportunities to visit targets in Ursa Major. Among them is the grand spiral galaxy Messier 101, also catalogued at NGC 5457 and known as the Pinwheel Galaxy. An easy star hop off the handle of the Big Dipper, you can observe this large target in darker skies as a challenge in binoculars, smaller refracting telescopes as an obvious diffuse glow, and in greater detail showing spiral arm structure in larger instruments. Here is where to look:
Discovered in 1781 by French astronomer Pierre Méchain, a protégé of Charles Messier, it was described as "a nebula without star, very obscure and pretty large, 6' to 7' in diameter." In your telescope you should see an object 28' diameter, rather faint but belying its listed magnitude of 7.9, due to its large size ? it has a dim surface brightness of 14.9.
The galaxy itself is a terrific visual target in larger instruments, showing several star forming HII regions peppering the spiral arms. Here is an observation from expert visual astronomer Steve Gottlieb through a 13.1" telescope: "fairly bright, very large, round, about 20' diameter, bright core. Fairly low surface brightness but beautifully resolved into several distinct arms and sections of arms with a pinwheel design. Obvious mottling along the spiral arms which appear clumpy with two or more easily recognized HII regions. At least six stars are superimposed."
To help you navigate this target, here is an early sketch by Lord Rosse though his 72" Leviathan, the largest telescope in the world in 1845:
A good modern sketch with a scientific photo side-by-side is this, from Jeremy Perez, who typically used an 8" telescope when drawing at the eyepiece:
Note the glowing regions in the spirals. They are nicely defined in this sketch by Michael Vlasov using a 12" telescope: Try using a narrow-band filter like the Orion Ultrablock, after centering M101, to see if you can isolate the glowing HII star formation areas.
And a final image for your enjoyment, this photo from NASA:
Messier 101's characteristics are similar to those of our Local Group's largest galaxy, Andromeda. And like Andromeda, M101 is the anchor of its own group of galaxies, including but not limited to NGC 5204, NGC 5474, NGC 5477, NGC 5585, UGC 8837 and UGC 9405.
No matter if you use a small instrument to pick out some of the spiral structure, a larger instrument for the NGC star forming regions, or to log the M101 group of galaxies, there is plenty to see. Any telescope will do!
