The Pinwheel Galaxy (Messier 101, M101 or NGC 5457) is a face-on spiral galaxy located 21 million light-years (6.4 megaparsecs) from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781[a] and was communicated that year to Charles Messier, who verified its position for inclusion in the Messier Catalogue as one of its final entries.
M101 is a large galaxy with a diameter of 170,000 light-years. By comparison, the Milky Way has a diameter of 87,400 light-years. It has around a trillion stars.[14] It has a disk mass of 100 billion solar masses and a small central bulge of about 3 billion solar masses. Its characteristics can be compared to those of the Andromeda Galaxy.
M101 has a high population of H II regions, many of which are very large and bright. H II regions usually accompany the enormous clouds of high-density molecular hydrogen gas contracting under their own gravitational force where stars form. H II regions are ionized by large numbers of extremely bright and hot young stars; those in M101 can create hot superbubbles.
