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An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification scheme. By definition, a galaxy is a congregation of stars held together by gravity. The first intermediate spiral galaxy discovered is the Milky Way, by Galileo, in 1610. 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. 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. 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. Hubble proved that the Universe was full of galaxies, and disproved that the Milky Way was the extent of the Universe. 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.
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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, and as the gas coalesces, the gas cloud will flatten out to form a disk shape. The mass remaining constant, to comply with the physical laws which govern the conservation of angular momentum, its rotational velocity will speed up. 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.
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, with high surface temperatures, causing the disk to glow blue. Surrounding the galaxy is a halo that is made up of old Population II stars, as well as globular clusters, which are groups of Population II stars that are spherical in shape and contain roughly one million 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 postulated 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.
Historical background
In 1920, at the Academy of science, two scientists argued about the distance to nebulae. Harlow Shapley argued that these nebulae were within the Milky Way and located in the halo as globular clusters. Heber D. Curtis disagreed and argued that these nebulae were much more distant, and were located in distant galaxies. Both debated that their idea was right, yet neither of them could provide concrete data to prove they were right, ending the debate in a stalemate. In 1923, Hubble took a picture of the Andromeda Galaxy and noticed something peculiar that no one had notice before, a Cepheid variable located in a globular cluster. Using the equation m-M=5log d-5, Hubble was able to find the distance to these remote stars, and finally prove the existence of other galaxies and the size of the Universe. Another discovery was made in 1930, by Robert J Trumpler, which unlocked more understanding of distant galaxies. While conducting an experiment on the brightness of a galaxy and its distance from Earth, his result of the galaxy's luminosity did not match its experimental value. He concluded that there was dust between Earth and the galaxy that obscures some of the light, making it dimmer. This dust is called interstellar extinction, and is made up or small rock and dust particles that lie within the plan of a galaxy. The dust particles scatter blue light, which has a smaller waves length, and allows most red light, which has longer wavelengths than blue light, to pass through.