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Ch 13 (Galaxies) - Chapter 13 Galaxies In this chapter we...

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Galaxies Chapter 13:
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In this chapter we will answer these 5 essential questions: 1. What do galaxies look like? 2. How do astronomers find the distance to galaxies? 3. How do galaxies differ in size, luminosity, and mass? 4. Do other galaxies contain supermassive black holes and dark matter as does our galaxy? 5. Why are there different kinds of galaxies
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Galaxies Contain a few thousand to tens of billions of stars, Large variety of shapes and sizes Star systems like our Milky Way as well as varying amounts of gas and dust
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We will classify the different kinds of galaxies and discover their basic properties Diameter Luminosity Mass We will try to understand how galaxies form and evolve. Amount of gas & dust a critical clue Also interactions between galaxies
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The Family of Galaxies Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals Ellipticals Irregular (some interacting)
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13-1 The Family of Galaxies Galaxies classified according to shape (developed in 1920s by Edwin Hubble) 3 kinds of galaxies …
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Elliptical galaxies No disk, no spiral arms, almost no gas & dust, from small dwarfs to huge giants Spiral galaxies Disk shaped, spiral arms, gas & dust many have a “barred” structure small percentage contain only a little gas & dust Irregular galaxies Highly irregular in shape Rich in gas & dust
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Classification of Elliptical galaxies Round or elliptical Contain no visible gas & dust Lack hot bright stars Given a classification, based on shape, from E0 to E7 (E0’s are round and E7’s are highly elliptical)
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Classification of Spiral Galaxies Contain a disk & spiral arms, halo stars not visible but we assume all spirals have halos. Contain gas & dust & hot, bright O & B stars Classified according to the size of their nucleus, the amount of gas & dust, and the amount of hot, bright stars
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Sa galaxies: large nuclei, less gas & dust, fewer hot bright stars Sc galaxies: small nuclei, more gas & dust, many hot, bright stars Sb galaxies: intermediate Classification of Spiral Galaxies (continued) Galaxies with an obvious disk & nuclear bulge but no visible gas & dust & no hot, bright stars are classified as S0.
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Galaxy Classification Sa Sb Sc Elliptical Galaxies Spiral Galaxies E0 = Spherical Small nucleus; loosely wound arms E1 E6 E0, …, E7 Large nucleus; tightly wound arms E7 = Highly elliptical
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Gas and Dust in Galaxies Spirals are rich in gas and dust Ellipticals are almost devoid of gas and dust Galaxies with disk and bulge, but no dust are termed S0
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Barred Spiral Galaxies Roughly 2/3 of all spiral galaxies are barred spiral galaxies. Barred spirals have an elongated nucleus with spiral arms springing from the ends of the bar (our own galaxy is a barred spiral) Barred spiral galaxies are classified SBa, SBb, or SBc just like non-barred spirals
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Barred Spirals Some spirals show a pronounced bar structure in the center.
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