M101
Messier 101 (M101), also known as the Pinwheel Galaxy, is a face-on spiral galaxy approximately 25 million light-years away in the constellation Ursa Major. Spanning about 170,000 light-years, it is nearly twice the size of the Milky Way and is characterized by its grand design spiral structure.
Source: science.nasa.gov
APODs including "M101"
M101: The Pinwheel Galaxy
14/04/2009
Why do many galaxies appear as spirals? A striking example is M101, shown above, whose relatively close distance of about 27 million light years allows it to be studied in some detail. Recent evidence indicates that a close gravitational interaction with a neighboring galaxy created waves of high mass and condensed gas which continue to orbit the galaxy center. These waves compress existing gas and cause star formation. One result is that M101, also called the Pinwheel Galaxy, has several extremely bright star-forming regions (called HII regions) spread across its spiral arms. M101 is so large that its immense gravity distorts smaller nearby galaxies.