Messier 101, the Northern Pinwheel

Messier 101 is a large galaxy in the constellation Ursa Major. It is about 25 million light years distant, and about 170,000 light years across. Despite being fairly large in our sky (about 2/3 the size of the full moon), it is extremely faint. Small telescopes only reveal the brighter, central portion in dark skies. Either a telescope with a large aperture or a long-exposure image is required to discern the spiral structure.

Messier 101, ST80 on Vixen SP, 31x180

Also called the Northern Pinwheel, M101 is a grand design spiral, which means that it has clearly defined spiral arms radiating from its center.

The galaxy's distorted shape is likely due to gravitational interaction with its companion galaxies, particularly NGC 5474, seen to the lower right in this image. NGC 5474 is a dwarf spiral galaxy, which is a relatively rare galaxy type. Its core is offset in the direction of M101.