Meanings
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1
noun
an interstellar cloud of gas and dust, often associated with the formation or death of stars
Ultraviolet radiation from young stars causes the Orion Nebula to glow.
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2
noun
historically, any diffuse celestial object seen as a cloudy patch; the term once included objects now known to be galaxies
Early astronomers called Andromeda a spiral nebula before establishing that it is a separate galaxy.
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3
noun
in ophthalmology, a faint opacity or scar of the cornea
The ophthalmologist described the faint corneal opacity as a nebula.
Thesaurus
Etymology
Borrowed from Latin nebula ("mist, cloud"). Early telescopic observers applied the cloud image to unresolved fuzzy objects in the sky. Modern astronomy reserves nebula mainly for interstellar gas and dust, while former "spiral nebulae" such as Andromeda are recognized as galaxies.