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A spiral galaxy is a kind of galaxy that looks like a flat, slowly rotating disk with a bulge in the centre, and spiral patterns extending outward from the bulge. It contains stars, gas, dust, dark matter and a supermassive black hole at its centre.
Galaxies were long thought to be nebulae. The spiral nebula as a type was first described by Edwin Hubble in his 1936 work The Realm of the Nebulae. They are now listed as galaxies, and are named after their shape.
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