Pinwheel Galaxy (M101, NGC 5457) at 8th magnitude. This is a large, bright, spiral galaxy some 20 million light years away in the constellation of Ursa Major. M101 has a mass of three thousand million Suns and a diameter of approximately 200, 000 light years, around twice that of our own galaxy, the Milky Way. Its extremely narrow, far- flung spiral arms are dominated by huge clouds of glowing hydrogen, some of which are 3000 light years across.