The Pinwheel Galaxy is a spiral galaxy in Ursa Major. It was discovered in 1780 and, as often happened, catalogued as a nebula: only in 1850 was it recognised for what it is.
It lies 21 million light years away and is almost twice the size of our own galaxy — a full 170,000 light years across — with an estimated mass of between 100 and 1,000 billion solar masses.
The unusual shape of its spiral arms comes from gravitational interaction with its five companion galaxies, themselves of respectable size: NGC 5204, NGC 5474 (top left in the photograph), NGC 5477 (top centre), NGC 5585 and UGC 8837. This is not a cosmetic detail: they are the ones tugging at the arms and making the pinwheel so lopsided.
Another point of interest: four supernovae have been identified in this galaxy so far — explosions caused by hydrogen piling up on the surface of white dwarfs, producing a flash brighter than the galaxy itself.
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