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In long-exposure photographs M33, another member of our Local Group of galaxies, looks like an enormous spiral with innumerable suns clinging to wildly spinning arms. But with a diameter of 50,000 light years, you could easily fit three M33s in the disk of M31. In fact, M33 may be a satellite galaxy of M31, orbiting it just as the moon does the earth. The Andromeda Galaxy is also about 15 times more massive than M33, which is about two times smaller and seven times less massive than our Milky Way. As seen from an imaginary planet in the Pinwheel Galaxy, M31 would be an impressive sight - an oblique swarm of faintly glittering suns stretching 6° in that hypothetical sky. Regardless of its true size, the Pinwheel is a great sight from earth. It has long been a naked-eye challenge for amateur astronomers. While some find it easily visible to the naked eye or in binoculars, others cannot see it at all. The problem lies in the galaxy's low surface brightness.
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