Etymology
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nebula (n.)

mid-15c., nebule "a cloud, mist," from Latin nebula, plural nebulae, "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 "luminous cloud-like patch in the heavens" is from c. 1730. As early as Hershel (1802) astronomers realized that some nebulae were star clusters, but the certain distinction of relatively nearby cosmic gas clouds from distant galaxies (as these are now properly called) was not made until the 1920s, when the latter were resolved into individual stars (and nebulae) using the new 100-inch Mt. Wilson telescope.

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nebular (adj.)
1821, "pertaining to an (astronomical) nebula or nebulae," from nebula + -ar.
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nebulizer (n.)

"instrument for reducing a liquid to spray" (for inhalation, etc.), 1865, agent noun from verb nebulize "to reduce to a mist or spray" (1865), from Latin nebula "mist" (see nebula) + -ize. Related: Nebulization.

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*nebh- 
Proto-Indo-European root meaning "cloud."

It forms all or part of: nebula; nebular; nebulosity; nebulous; Neptune; Nibelungenlied; Niflheim; nimbus.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nabhas- "vapor, cloud, mists, fog, sky;" Greek nephele, nephos "cloud;" Latin nebula "mist, vapor, fog, smoke, exhalation;" German Nebel "fog;" Old English nifol "dark, gloomy;" Welsh niwl "cloud, fog;" Slavic nebo.
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ombro- 

word-forming element meaning "rain, rainfall; excessive moisture," from Greek ombros "shower of rain," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Latin imber "rain, heavy rain; rainwater"), from root *nebh- "moist; water" (see nebula). 

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blur (n.)
1540s, "a moral stain;" c. 1600, "a smear on the surface of writing;" perhaps akin to blear. Extended sense of "a confused dimness" is from 1860 [Emerson, in reference to the Orion nebula].
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nebulosity (n.)

1754, "cloudiness, haziness," from French nébulosité, from Late Latin nebulositatem (nominative nebulositas), from Latin nebulosus, from nebula "mist, vapor" (from PIE root *nebh- "cloud"). From 1761 as "faint, misty appearance surrounding certain stars."

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planetary (adj.)

1590s, "of or pertaining to a planet;" see planet + -ary. Perhaps from or based on Late Latin planetarius "pertaining to a planet or planets," but according to OED this is attested only as a noun meaning "an astrologer." Planetary nebula, so called for its shape as seen through a telescope, attested from 1785.

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Messier 

by 1801 in reference to a catalogue of about 100 nebulae, star clusters and galaxies begun in 1758 by French astronomer and comet-hunter Charles Messier (1730-1817), who was deceived in his telescopic searches by fuzzy objects that resembled distant comets but turned out to be fixed.

What caused me to undertake the catalog was the nebula I discovered above the southern horn of Taurus on September 12, 1758, whilst observing the comet of that year. This nebula had such a resemblance to a comet in its form and brightness that I endeavored to find others, so that astronomers would no more confuse these same nebulae with comets just beginning to appear. [Messier, 1800]

The first version of the catalogue was published 1771, and the fuller version in 1781.

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imbrication (n.)
"an overlapping of edges" (as of roof tiles, etc.), 1640s, from French imbrication, noun of action from stem of Latin imbricare "to cover with tiles," from imbricem (nominative imbrex) "curved roof tile used to draw off rain," from imber (genitive imbris) "rain, heavy rain; rainwater," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Greek ombros "rain, a shower"), from root *nebh- "moist; water" (see nebula).
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