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The Milky Way Galaxy

The Milky Way Galaxy Seeds chapter 12 Objective Investigate our home galaxy and its stellar community Introduction Our galaxy ~75,000 ly diameter ~100 billion stars One of the largest star systems in the cosmos via lactea “milky way” How did it form and what is its structure?

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The Milky Way Galaxy

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  1. The Milky Way Galaxy Seeds chapter 12

  2. Objective • Investigate our home galaxy and its stellar community

  3. Introduction • Our galaxy • ~75,000 ly diameter • ~100 billion stars • One of the largest star systems in the cosmos • via lactea “milky way” • How did it form and what is its structure?

  4. The Discovery of our Galaxy • Until this century astronomers assumed stars are scattered uniformly through space

  5. Variable Stars • Variable stars were not understood until the 20th century • Instability strip • RR Lyrae variables • Cepheids • Period-luminosity relation

  6. The Size of the Galaxy • Open clusters distributed along Milky Way • Globular star clusters centred on Sagittarius - the centre of our galaxy

  7. An Analysis of the Galaxy • Disc component • Sun 8.5kpc from centre • spiral arms • Spherical component • halo • nuclear bulge

  8. The Mass of the Galaxy • Differential rotation • Rotation curve • Dark matter • Dark halo ~7  visible radius~10-100 visible mass

  9. The Origin of the Milky Way • Spherical component provides fossil evidence

  10. The Age of the Milky Way • Dating prone to uncertainties • Best estimates are: • Disc ~ 7 Gyr age • Halo ~ 11 Gyr age

  11. Stellar Populations • Population I • Young metal-rich disc population stars • Population II • Old metal poor halo population stars

  12. The Element-Building Cycle • Lighter metals commonly produced in evolved stars • Heavier metals rarer, produced in SNe • Very metal poor stars are old galactic fossils

  13. The History of the Milky Way Galaxy • Traditional model • single cloud forms sphere & then collapsed to a disc • Revised model • include mergers?

  14. Spiral Arms and Star Formation • Spiral arms contain hot blue stars, dust, gas, young clusters • Why are they there despite differential rotation?

  15. Tracing the Spiral Arms • Young objects are spiral tracers • Arms are associated with star formation

  16. Radio Maps of Spiral Arms • 21cm radiation from cool hydrogen and Doppler effect enable arms to be mapped • Arms irregular but dense

  17. The Density Wave Theory • Arms are waves of compression • Trigger star formation • How does wave start? • What causes flocculent (woolly) arms?

  18. Star Formation in Spiral Arms • Self-sustaining star formation affects arms • Grows clumps and spurs of new stars

  19. The Nucleus • Most mysterious part of galaxy • 30 magnitudes visual extinction • Radio/IR reveal crowded fast-moving stars

  20. Observations • Sagittarius A* is powerful radio source only 4AU diameter marking galaxy’s core • Stars only 1000AU apart, gas/dust ring, cavity & stars orbiting compact 3 million solar mass object

  21. The Massive Black Hole Hypothesis • Compact - 13 times larger than Sun • Accretion via disc can provide radio energy • Swirl of matter 3pc across around it • Eats few tenths solar mass/year

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