Astronomy pics - Al Alicea Photography
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M101 Pinwheel Galaxy

The Pinwheel Galaxy (also known as Messier 101, is a face-on spiral galaxy distanced 21 million light-years away in the constellation Ursa Major, first discovered by Pierre Méchain on March 27, 1781. M101 is a large galaxy comparable i size to the Milky Way. With a diameter of 170,000 light-years it is roughly equal the size of the Milky Way. It has a disk mass on the order of 100 billion solar mmassesmasses, along with a small central bulge of about 3 billion solar masses.