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Intermediate spiral galaxy
An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy.〔"Building Galaxies", Leslie Kipp Hunt, ''15 October 2004''〕〔"Galaxy Formation", Malcolm S. Longair, ''1998'', Springer, 〕 It is designated as SAB in the galaxy morphological classification scheme.〔〔 By definition, a galaxy is a congregation of stars held together by gravity.〔Sparke & Gallagher III 2000〕 The first intermediate spiral galaxy discovered is the Milky Way, by Galileo, in 1610.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕 He was the first person with a telescope powerful enough to make such a discovery. Before Galileo, it was thought that all bright objects in the sky were either the planets in the Solar System, moons, comets, or stars. Until the beginning on the twentieth century, astronomers did not know the size of the Universe, but speculated it to be about as big as the Milky Way.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕 In 1920, at the National Academy of Science, there was a big debate between Harlow Shapley and Heber D. Curtis on whether nebulae are small globular clusters surrounding the Milky Way, or separate galaxies located farther away. Nothing was resolved at the debate; neither side was able to provide conclusive evidence to prove their side correct over their opponent.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕 In 1923, Edwin Hubble resolved the matter with a photograph that he took of the Andromeda Galaxy. What he found in his photograph was a very bright light source pulsing at a certain rate, a Cepheid variable, located outside the Milky Way. This can be used to determine the distance to it.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕 Hubble proved that the Universe was full of galaxies, and disproved that the Milky Way was the extent of the Universe.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕 There are many types of galaxies in the Universe, elliptical, barred spiral galaxies; they vary in shape and size, but on average spiral galaxies are the most abundant.〔Redd, Nola Taylor "Spiral Galaxy Facts & Definition | Space.com." Space.com. N.p., 15 Aug. 2013. Web. 11 Feb. 2015.〕
== Structure ==
A galaxy starts out as a giant cloud of cold gas. The cloud of gas must be close to absolute zero, if the gas cloud is too hot, the atoms will have too much kinetic energy, and gravity will not be able to condense the cloud. When a cloud reaches a mass of about 109−1011 times the mass of the Sun, the gas cloud will collapse under gravity. For an intermediate spiral galaxy to form, the gas cloud must be rotating, as the gas coalesces, the gas cloud will flatten out to form a disk shape, and its rotational velocity will speed up to conserve angular momentum. At the very center of the galaxy, the gas will condense so much under gravitational pressure that a supermassive black hole will form. All spiral galaxies have a black hole at the center of their galaxy, called the galactic nucleus, which range from 106 to 109 times the mass of the Sun.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕
The Central Bulge contains Population II stars, which are old stars, devoid of metals, and have low surface temperatures. For this reason the central bulge glows an orange/red color. The disk of the galaxy is full of star formation, and Population I stars, which are metal-rich, and have high surface temperatures, causing the disk to glow blue. Surrounding the galaxy are is a halo that is made up of old Population II stars, as well as globular clusters, which are groups of Population II stars of about that are circular in shape and contain roughly 106 stars. Star formation takes place in the spiral arms of the galaxy. The rotation velocity of the galaxy is nearly uniform throughout the entire disk. The reason for this is due to an even bigger halo of dark matter that extends beyond the size of the galaxy. This dark matter is to date made up of an unknown substance which radiates no light, and accounts for up to 90% of the mass in a total galaxy.〔Kaufmann III, William. Universe. W.H. Freeman and Company, 2014.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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