galaxy (n.)youdaoicibaDictYouDict[galaxy 词源字典]
late 14c., from French galaxie or directly from Late Latin galaxias "the Milky Way" as a feature in the night sky (in classical Latin via lactea or circulus lacteus)from Greek galaxias (adj.), in galaxias kyklos, literally "milky circle," from gala (genitive galaktos) "milk" (see lactation). The technical astronomical sense in reference to the discrete stellar aggregate including the sun and all visible stars emerged by 1848. Figurative sense of "brilliant assembly of persons" is from 1580s. Milky Way is a translation of Latin via lactea.
See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. [Chaucer, "House of Fame"]
Originally ours was the only one known. Astronomers began to speculate by mid-19c. that some of the spiral nebulae they could see in telescopes were actually immense and immensely distant structures the size and shape of the Milky Way. But the matter was not settled in the affirmative until the 1920s.[galaxy etymology, galaxy origin, 英语词源]
MessieryoudaoicibaDictYouDict
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 found his telescopic searches deceived 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.
nebula (n.)youdaoicibaDictYouDict
early 15c., nebule "a cloud, mist," from Latin nebula "mist, vapor, fog, smoke, exhalation," figuratively "darkness, obscurity," from PIE *nebh- "cloud" (cognates: Sanskrit nabhas- "vapor, cloud, mists, fog, sky;" Greek nephele, nephos "cloud;" German Nebel "fog;" Old English nifol "dark, gloomy;" Welsh niwl "cloud, fog;" Slavic nebo).

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