early 15c., nebule "a cloud, mist," from Latin nebula "mist, vapor, fog, smoke, exhalation," figuratively "darkness, obscurity," from PIE root *nebh- "cloud."
Re-borrowed from Latin 1660s in sense of "cataracts in the eye;" astronomical meaning "cloud-like patch in the night sky" first recorded c. 1730. As early as Hershel (1802) astronomers realized that some nebulae were star clusters, but certain distinction of relatively nearby cosmic gas clouds from distant galaxies was not made until 1920s, using the new 100-inch Mt. Wilson telescope.
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