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J B Lester

AST 201S - Stars and Galaxies University of Toronto Mississauga Chapter 14 Summary Essential Cosmic Evolution, 5 Edition th 1. Discovering the Milky Way • We see a faint band of light circling around the whole sky = Milky Way = Galaxy (from Greek word for milk) • Galileo used his telescope to observe that the faint band is due to the combined light of a huge number of faint stars • Milky Way is a large, thin disk of stars, surrounding and including the Sun 2. Size of the Milky Way • Count the number of stars in different directions • Assume that counting many stars → great extent, counting few stars → short extent • Estimate distances to faintest stars in the disk ≈ 8000 ly • Sun seems near centre → “radius” of disk ≈ 8000 ly, diameter of disk ≈ 16,000 ly • Angular width of band × “radius” = 1000 ly thick 3. True Size of the Milky Way • Problem: interstellar is not empty - it is filled with a very thin distribution of tiny dust particles that limits our view • 8000 ly is only as far as we can see, not the edge of the disk • Need to observe objects above/below the disk to avoid dust, example globular star clusters, which are very bright • Find that the Sun is 28,000 ly from the centre of Milky Way • Outer edge is 20,000 ly beyond the Sun → total radius ≈ 50,000 ly, diameter ≈ 100,000 ly continued on page 2 AST201H5S – Chapter 14 Summary page 2 of 3 4. Structure of the Disk • Learn structure by locating particular types of stars • Concentrate on hot stars because – they are extremely luminous, making them visible to the greatest distances – they must be young, so still where they were formed • Near the Sun, hot, young stars are lined up in lanes or “arms” • To see the whole disk, observe radio emission with wavelength of 21 cm emitted by hydrogen atoms – wavelength is much larger than dust particles, cannot be blocked – hydrogen gas is most concentrated where the young, hot stars form • Observe that the whole disk of the Milky Way has a spiral structure • The spiral arms cannot be due to as a simple “wind up” because the spiral would be wound too tightly in the age of the Milky Way • The spiral pattern might be just a concentration of matter where stars and gas spend more time 5. Milky Way as a recycling system • Supernova expl
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