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The giant spiral disk of stars, dust, and gas is 170,000 light-years across or nearly twice the diameter of our galaxy, the Milky Way. M101 is estimated to contain at least one trillion stars. Approximately 100 billion of these stars could be like our Sun in terms of temperature and lifetime. The galaxy's spiral arms are sprinkled with large regions of star-forming nebulae. These nebulae are areas of intense star formation within giant molecular hydrogen clouds. Brilliant young clusters of hot, blue, newborn stars trace out the spiral arms. The disk of M101 is so thin that Hubble easily sees many more distant galaxies lying behind the galaxy. M101 (also nicknamed the Pinwheel Galaxy) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of 25 million light-years from Earth. Therefore, we are seeing the galaxy as it looked 25 million years ago — when the light we're receiving from it now was emitted by its stars — at the beginning of Earth's Miocene Period, when mammals flourished and the Mastodon first appeared on Earth. The galaxy fills a region in the sky equal to one-fifth the area of the full moon.