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The Discovery of Galaxies

The Discovery of Galaxies - The Discovery of Galaxies...

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The Discovery of Galaxies Galaxies were not recognized as a distinct kind of nebular object until the late 19th century, when visual spectroscopy (Huggins) of the Andromeda spiral (M31) showed a continuous spectrum. Though a clear naked-eye object, M31 had only rarely appeared in pre-telescopic depictions (except a description by one Al-Sufi in the tenth century). Distinct structure was reported by William Parsons, 3rd Earl of Rosse, whose 72-inch speculum-metal reflector showed clear spiral features in some bright nebulae such as M33, M51, and M101. This had previously been unseen because (1) speculum mirrors are less reflective than aluminum or silver on glass, so very large mirrors were required, (2) it's much easier to see something you already know is there. Photography turned up myriads of such spiral nebulae, particularly with work around 1900 by Keeler and Curtis with the 36" Crossley reflector at Lick Observatory . Early (visual) spectroscopy showed only that their spectra lack emission lines, and therefore must be continuous (giving the name "white nebulae). Speculation centered around the possibilities either
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