Messier 101 – The Pinwheel Galaxy

One of the most famous, and most photographed, galaxies in the northern sky, Messier 101 (M101) lies approximately 25 million light years away in the Ursa Major constellation just to the northeast of an even more famous night-sky sight—the Big Dipper.

While, although not symmetrical, it’s a great example of a grand-design spiral galaxy, Halton Arp characterized it as a “Spiral with One Heavy Arm” in his Atlas of Peculiar Galaxies.

It’s spiral arms are littered with glowing pink star-forming regions, which I’ve been able to reveal in this image using a specialized photographic filters that allows only light from ionized hydrogen gas to pass through. With an estimated diameter of around 170,000 light years and a membership of as many as a trillion stars, it’s twice as large as our Milky Way galaxy.