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A nebula (Latin for 'cloud' or 'fog'; pl. nebulae, nebulæ or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, the term was used to describe any diffused astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the "Andromeda Nebula" (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Edwin Hubble discovered that most nebulae are associated by stars and illuminated by starlight. He also helped categorize nebulae based on the type of light spectra they produced.