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The Nature of Galaxies

The Nature of Galaxies. Chapter 17. Elliptical. Other Galaxies. External to Milky Way established by Edwin Hubble used Cepheid variables to measure distance M31 (Andromeda Galaxy) far outside Milky Way Three basic types: elliptical spiral irregular. Spiral. Irregular.

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The Nature of Galaxies

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  1. The Nature of Galaxies Chapter 17

  2. Elliptical Other Galaxies • External to Milky Way • established by Edwin Hubble • used Cepheid variables to measure distance • M31 (Andromeda Galaxy) far outside Milky Way • Three basic types: • elliptical • spiral • irregular Spiral Irregular

  3. Spiral Galaxies • Similar to Milky Way: • thin disk + nuclear bulge + halo • Disk contains: • dust and gas • H II regions, H I regions, molecular clouds • spiral arms • active star formation • open clusters • mixture of young & old stars • Halo contains: • old stars • Globular Clusters • Bulge contains: • old stars

  4. Spiral Galaxies

  5. Barred Spiral Galaxies • Some spirals have bar in center • “barred spirals”

  6. Elliptical Galaxies • Shape ranges from: • spherical to ellipsoidal • Characteristics: • no disk or spiral arms • old reddish stars • similar to halo or bulge in spirals • little gas or dust • little star formation

  7. Irregular Galaxies • No specific shape • often appear chaotic • Often have intense star formation • gravitational interaction with other galaxies? • Mixture of old and new stars

  8. Interacting Galaxies

  9. Cartwheel Galaxy NASA/HST Simulation by C. Mihos et al., CWRU

  10. Galaxy Masses • For spirals: • use Doppler shift; measure galaxy rotation • make rotation curve • calculate mass using Kepler’s Law • For ellipticals: • use Doppler shift; measure stellar orbital velocities • calculate mass using Kepler’s Law • Results • Giant ellipticals and spirals are most massive; • irregulars & dwarf ellipticals least massive

  11. Mass-to-Light Ratios • ratio of mass to luminosity • for Sun, • M/L = 1 • average star • M/L = 2 to 3 • for entire Galaxy • M/L ~ 100 • 90% of galaxy mass is unseen

  12. Extragalactic Distances • Compare apparent and absolute brightness • Variable Stars: • Cepheids, RR Lyrae • Standard Candles: • brightest stars, supernovae, planetary nebulae • Galaxy techniques: • For spirals: • rotation rate gives mass • mass depends on number of stars, hence luminosity • For ellipticals: • range of stellar velocities depends on mass (hence luminosity)

  13. Fifteen years ago, a quasar was observed that was found to be located 8 billion light years away. If our universe is approximately • 15 billion years old, when did the quasar emit the light that we observe? • 15 years ago • 7 billion years ago • 8 billion years ago • 15 billion years ago

  14. Galaxy Motion • Galaxy spectra: • absorption lines redshifted • more distant galaxies have larger redshift • ALL galaxies moving away

  15. The Hubble Law • Hubble Law: • The more distant the galaxy, the faster it is moving away.v = H d (H is the Hubble constant, d is distance)

  16. Implications of Hubble Law • Every galaxy moving away • Farther away = faster • Conclusion: • Universe is expanding • Predicted by Einstein’s Theory of Relativity • Are we at center? NO • universe same in all directions • there is no center!

  17. The Expanding Universe • A uniformly expanding universe • explains Hubble law • example: expanding loaf of raisin bread • Galaxies (like raisins) not moving, not expanding • Space is expanding

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