19, 2018 by Contributor, Elizabeth Howell. Spiral galaxies are a class of galaxies originally described by astronomer Edwin Hubble in his 1936 work The Realm of the Nebulae. Spiral galaxies are galaxies with spiral arms, a pancake-like disk of stars and a central bulge or concentration of stars. The central bulge is devoid of gas and dust. It is dominated by young, blue Population I stars. The disk is a region of star formation and has a great deal of gas and dust. The disk of the galaxy contains the spiral arms. Its discovery will help scientists understand how galaxies transition from "highly chaotic, turbulent discs" to more organized and thinner discs, like that of the Milky Way. Spiral galaxies like NGC 3310 (right) have two distinct regions. Spiral galaxys are divided into three main types depending on how tightly wound their spiral arms are: Sa, Sb and Sc. He classified spiral and barred spiral galaxies further according to the size of their central. Edwin Hubble invented a classification of galaxies and grouped them into four classes: spirals, barred spirals, ellipticals and irregulars. In 2017, astronomers discovered an 11-billion-year-old ancient spiral galaxy called A1689B11. Andromeda Galaxy, (catalog numbers NGC 224 and M31), great spiral galaxy in the constellation Andromeda, the nearest large galaxy.The Andromeda Galaxy is one of the few visible to the unaided eye, appearing as a milky blur. The arms and bulge are surrounded by a faint halo of stars. Galaxies range from 1,000 to 100,000 parsecs in diameter and are usually separated by millions of parsecs. They don’t have nice spiral arms, but they do have dark patches of gas and dust. They don’t seem to fit into either the spiral or elliptical galaxy categories. One of the largest known spiral galaxies is NGC 6872, which is 522,000 light-years across from the tips of its outstretched spiral arms - that's about 5 times the size of the Milky Way. Irregular galaxies are the most unusual of galaxies. But it's unclear how common elliptical galaxies are as they're made up of older, dimmer stars, and are more challenging to spot. Spiral galaxies are thought to evolve into elliptical galaxies as the spirals get older. Encounters between galaxies could cause such waves as the mass of the smaller galaxy could affect the structure of the larger galaxy as the two combine. One theory suggests the galaxy arms could be the result of density waves traveling through the outer disk. How the spiral arms form continues to puzzle scientists.
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