M101, aka NGC 5457, aka the “Pinwheel Galaxy” is about 27 million lightyears distant in the constellation Ursa Major. It spans 170,000 lightyears in diameter, making it a huge spiral, albeit somewhat asymmetric in appearance. In fact, Arp included M101 in his “Atlas of Peculiar Galaxies”. This image is fairly deep, showing the arms quite clearly, including the numerous HII regions that indicate areas of active star formation.

Technical details:

Imaged from New Mexico Skies in 2007 using a 16″ RCOS at f/8.7 with an SBIG STL11K on a Paramount ME. Image scale 0.53″/pixel. Assembled as an (Ha+L)(Ha+R)GB with 7.5 hours Lum, 2.75 hours/channel RGB, and 3.5 hours Ha. Calibration in Maxim, alignment in Registar, integration in Sigma Combine, and everything else in Photoshop.