Milky Way Main article: Milky Way Galactic Centerof the Milky Way The Greekphilosopher Democritus(450–370 BC) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. [ 19 ] Aristotle(384–322 BC), however, believed the Milky Way to be caused by the ignition of the fiery exhalation of some stars that were large, numerous and close together and that the ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions. [ 20 ]The Neoplatonistphilosopher Olympiodorus the Younger(c. 495–570 AD) was scientifically critical of this view, arguing that if the Milky Way were sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world. [ 21 ] According to Mohani Mohamed, the Arabianastronomer Alhazen(965–1037) made the first attempt at observing and measuring the Milky Ways parallax, [ 22 ]and he thus determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere. [ 23 ]The Persianastronomer al-Bīrūnī(973–1048) proposed the Milky Way galaxy to be a collection of countless fragments of the nature of nebulous stars. [ 24 ] [ 25 ]The Andalusianastronomer Ibn Bajjah(Avempace, d. 1138) proposed that the Milky Way was made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refractionfrom sublunary material, [ 20 ] [ 26 ]citing his observation of the conjunctionof Jupiter and Mars as evidence of this occurring when two objects are near. [ 20 ]In the 14th century, the Syrian-born Ibn Qayyimproposed the Milky Way galaxy to be a myriad of tiny stars packed together in the sphere of the fixed stars. [ 27 ] Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galileiused a telescopeto study the Milky Way and discovered that it is composed of a huge number of faint stars. [ 28 ]In 1750 the English astronomer Thomas Wright, in hisAn original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. [ 29 ]In a treatise in 1755, Immanuel Kantelaborated on Wrights idea about the structure of the Milky Way. The shape of the Milky Way as deduced from star counts by William Herschel in 1785; the solar system was assumed to be near the center. The first attempt to describe the shape of the Milky Way and the position of the Sunin it was carried out by William Herschelin 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the solar system close to the center. [ 30 ]Using a refined approach, Kapteynin 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapleybased on the cataloguing of globular clustersled to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. [ 29 ]Both analyses failed to take into account the absorption of lightby interstellar dustpresent in the galactic plane, but after Robert Julius Trumplerquantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged. [ 31
Posted on: Sat, 15 Mar 2014 08:50:37 +0000
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