The Pinwheel Galaxy (M101) is a face-on spiral galaxy 25 million light-years away from Earth in the constellation Ursa Major. The giant spiral disk of stars, dust and gas is 170,000 light-years across — nearly twice the diameter of our galaxy, the Milky Way. M101 is estimated to contain at least one trillion stars. The galaxy’s spiral arms are sprinkled with large regions of star-forming nebulas. These nebulas are areas of intense star formation within giant molecular hydrogen clouds. Brilliant, young clusters of hot, blue, newborn stars trace out the spiral arms.
Discovered by the German astronomer Johann Elert Bode in 1774, M81 is one of the brightest galaxies in the night sky. It is located 11.6 million light-years from Earth in the constellation Ursa Major and has an apparent magnitude of 6.9. The galaxy’s spiral arms, which wind all the way down into its nucleus, are made up of young, bluish, hot stars formed in the past few million years. They also host a population of stars formed in an episode of star formation that started about 600 million years ago. Ultraviolet light from hot, young stars is fluorescing the surrounding clouds of hydrogen gas. A number of sinuous dust lanes also wind all the way into the nucleus of M81. The galaxy’s central bulge contains much older, redder stars. It is significantly larger than the Milky Way’s bulge. A black hole of 70 million solar masses resides at the center of M81 and is about 15 times the mass of the Milky Way’s central black hole.
