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What Is a Nebula?

Hubble Telescope image of the Horsehead Nebula. Credit: T.A.Rector (NOIRLab/NSF/AURA) & Hubble Heritage Team (STScI/AURA/NASA ...
Nebulae are interstellar clouds of gas and dust, initially mistaken for galaxies by early astronomers. Diffuse nebulae lack defined shapes and include reflection nebulae (reflecting starlight), ...