نظام الإحداثيات المجرية: الفرق بين النسختين
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سطر 43:
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== دوران المجرة==
[[ملف:Rotation curve (Milky Way).JPG|thumb |300px |[[Galaxy rotation curve]] for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to [[dark matter]] or perhaps a modification of the [[MOND|law of gravity]].<ref name=Schneider>{{cite book |title=Extragalactic Astronomy and Cosmology |url=http://books.google.com/?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+Milky+way |author=Peter Schneider |publisher=Springer |isbn=3540331743 |year=2006 |page= 4, Figure 1.4}}</ref><ref name=Koupelis>{{cite book |title=In Quest of the Universe |author=Theo Koupelis, Karl F Kuhn |page=492; Figure 16-13 |url=http://books.google.com/?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22 |isbn=0763743879 |year=2007 |publisher=Jones & Bartlett Publishers}}</ref><ref name=Jones>{{cite book |title=An Introduction to Galaxies and Cosmology |author=Mark H. Jones, Robert J. Lambourne, David John Adams |page = 21; Figure 1.13 |url=http://books.google.com/?id=36K1PfetZegC&pg=PA20&dq=Milky+Way+%22rotation+curve%22 |isbn=0521546230 |year=2004 |publisher=Cambridge University Press}}</ref> ]]
[[ملف:Milky Way infrared.jpg|thumb|250px|The [[anisotropy]] of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars at low galactic latitudes, and those which require a low density of stars at high galactic latitudes. For this image the [[Mollweide projection]] has been applied, typical in maps using galactic coordinates.]]
{{seealso|منحنى دوران المجرة}}
يعتمد نظام احداثيات المجرة على اعتبار الشمس هي نقطة الصفر . وتدور [[الشمس]] في فلك دائري حول مركز المجرة يسمى "دائرة الشمس " بعكس دوران الساعة (مع اعتبار الرؤية من القطب الشمالي للمجرة) وهي تبعد عن المركز نحو 8 [[فرسخ فلكي]] وتدور بسرعة 220 [[كيلومتر في الثانية]] . <ref name=Combes>{{cite book |page= 19 |chapter=Mapping the Milky Way and the Local Group |title=Mapping the Galaxy and Nearby Galaxies |author=F. Combes, Keiichi Wada |isbn=0387727671 |year=2008 |publisher=Springer}}</ref> which gives an approximate galactic rate of rotation (here at the location of our solar system) of 200 million years/cycle. At other locations the galaxy rotates at a different rate, depending primarily upon the distance from the galactic center. The predicted rate of rotation based upon known [[mass]] disagrees with the observed rate, as shown in the [[galaxy rotation curve]] and this difference is attributed to [[dark matter]], although other explanations are continually sought, such as [[modified Newtonian dynamics|changes in the law of gravitation]]. The differing rates of rotation contribute to the [[proper motion]]s of the stars.
== اقرأ أيضا ==
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