The
Pinwheel Galaxy (also known as
Messier 101,
M101 or
NGC 5457) is a face-on
spiral galaxy 21 million
light-years (6.4
megaparsecs) away from Earth in the
constellationUrsa 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 between 100,000 and 120,000=10.5px light-years. It has around a trillion stars.=10.5px It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.=10.5px Its characteristics can be compared to those of
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 are capable of creating hot
superbubbles.
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