M 101 is a beautiful example of
a grand design spiral galaxy, thus warranting its name as the Pinwheel
Galaxy. Its arms are distorted due to tidal forces from interaction
with companion galaxies. This, in turn, created many H II regions
in which star formation is taking place.
M 101 is approximately 25 million
light years away in the constellation of Ursa Major. It is about
70% larger than our Milky Way (175,000 light years in diameter)
and contains approximately 100 billion solar masses. It was first
discovered by Pierre Mechain in 1781. Charles Messier then included
this galaxy in his catalogue.