When astronomer John Herschel completed his survey of the heavens in the 1840s, he had catalogued thousands of beautiful but hazy objects scattered everywhere across the sky amid the innumerable stars. Different from planetary nebulae, which often have distinct structures surrounding specific stars and are generally clustered along the plane of the Milky Way, these "nongalactic nebulae" generally looked like fuzzy footballs, spirals, or whirlpools. For the next hundred years, astronomers puzzled over their origin. Not only did no one know what they were, no one was even sure if they were local objects inside our galaxy or distant island universes comparable to the Milky Way. [The Hubble Space Telescope at 25: A Photo Anniversary]
In the 1920s, the American astronomer Edwin Hubble finally had a tool for finding out. Using the 100-inch Hooker telescope at Mount Wilson, California in 1924, he was able to resolve a large number of individual stars in several of the larger nongalactic nebulae, proving that their fuzziness was cau...
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