Distinction from other nebulae Sketch of Messier 51by Lord Rossein 1845, later known as the Whirlpool Galaxy In the 10th century, the Persian astronomer Al-Sufimade the earliestrecorded observation of the Andromeda Galaxy, describing it as a small cloud. [ 32 ]Al-Sufi, who published his findings in his Book of Fixed Starsin 964, also identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan; it was not seen by Europeans until Magellans voyage in the 16th century. [ 33 ] [ 34 ]The Andromeda Galaxy was independently rediscovered by Simon Mariusin 1612. [ 32 ]These are the only galaxies outside the Milky Way that are easily visible to the unaided eye, so they were the first galaxies to be observed from Earth. In 1750 Thomas Wright, in hisAn original theory or new hypothesis of the Universe, speculated (correctly) that the Milky Way was a flattened disk of stars, and that some of the nebulaevisible in the night sky might be separate Milky Ways. [ 29 ] [ 35 ]In 1755, Immanuel Kantused the term island Universe to describe these distant nebulae. Toward the end of the 18th century, Charles Messiercompiled a catalogcontaining the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel. [ 29 ]In 1845, Lord Rosseconstructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kants earlier conjecture. [ 36 ] In 1912, Vesto Sliphermade spectrographic studies of the brightest spiral nebulae to determine if they were made from chemicals that would be expected in a planetary system. However, Slipher discovered that the spiral nebulae had high red shifts, indicating that they were moving away at a rate higher than the Milky Ways escape velocity. Thus they were not gravitationally bound to the Milky Way, and were unlikely to be a part of the galaxy. [ 37 ] [ 38 ] In 1917, Heber Curtishad observed a nova S Andromedaewithin the Great AndromedaNebula (as the Andromeda Galaxy, Messier object M31, was known). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudesfainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called island universes hypothesis, which holds that spiral nebulae are actually independent galaxies. [ 39 ] Photograph of the Great Andromeda Nebula from 1899, later identified as the Andromeda Galaxy In 1920 the so-called Great Debatetook place between Harlow Shapleyand Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. [ 40 ] The matter was conclusively settled in the early 1920s. In 1922, the Estonianastronomer Ernst Öpikgave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. [ 41 ]Using the new 100 inch Mt. Wilsontelescope, Edwin Hubblewas able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. [ 42 ]In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence. [ 43 ]
Posted on: Sat, 15 Mar 2014 08:52:52 +0000
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