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Although it is 25 Million light-years away, the Pinwheel Galaxy – technically known as Messier 101 – fills as much of the night sky as one-fifth of the full moon. That’s because it measures 170,000 light-years in diameter – more than twice the size of our own Milky Way.
Each of the vast spiraling arms of M101 contains hundreds of star-forming regions, visible as bright blue areas in the photo. Over 3,000 such regions are visible in the photo. Hubble scientists estimate that there are over one trillion stars in M101, including 100 billion stars like our sun, in the prime of their stellar lives.