The Pinwheel Galaxy, Messier 101, is a broad face-on spiral in Ursa Major, just off the handle of the Big Dipper. From Earth the disk lies almost flat to the sky, so the arms unwind in the open: blue knots of young stars, pink clouds of ionized hydrogen, and dark dust lanes wrapped around a small, bright core.
M101 lies about 21 million light-years away. Its bright spiral stretches roughly 170,000 light-years across — nearly twice the span of the Milky Way — yet on the sky it covers almost the width of the full Moon. That mix of true size and low surface brightness is why the galaxy looks faint in a telescope and rich in a long exposure. These frames follow the same spiral, from the crowded star-forming arms out to the pale outer light.
Tides from companion galaxies in the M101 Group have tugged the disk slightly out of round and stirred the arms into vigorous star formation. Pierre Méchain found the glow in 1781, and Charles Messier entered it near the end of his catalog. On the order of a trillion stars are gathered here.
Facts
- Face-on spiral in Ursa Major, also catalogued as NGC 5457
- About 21 million light-years from Earth
- Bright disk roughly 170,000 light-years across
- Apparent size about 29′ × 27′, nearly a full Moon across
- Visual magnitude 7.9, east of Mizar in the Big Dipper’s handle
- Discovered by Pierre Méchain on 27 March 1781
