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Spiral Galaxies

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  1. Spiral Galaxies Ron Buta University of Alabama

  2. What are spiral galaxies? • Flattened systems of stars, gas, and dust • stars seem to concentrate in spiral “arms” • all bound by gravity into a single unit • typical size:50,000-100,000 light years • typical mass: 10billion-500billion suns • often there is also a prominent bulge

  3. Face-on spiral galaxy

  4. Highly inclined spiral galaxy

  5. Edge-on spiral galaxy

  6. Edge-on Spiral Galaxy

  7. Why are spirals interesting? • An intriguing morphology • difficult to explain • common and widespread thoughout the Universe • new stars tend to form in spirals but not in other types of galaxies • part of process of galaxy evolution and interaction • our Galaxy is a spiral

  8. Discovery of Spirals • 1845 William Parsons, Third Earl of Rosse, Ireland • 72-inch reflector • visually detected spiral arms in the Whirlpool Nebula M51 • many other “nebulae” found to be spirals

  9. William Parsons, “Lord Rosse” 1800-1867

  10. Discovery of spiral structure in M51

  11. What causes spiral structure? • Natural phenomenon in flat, rotating stellar disks • possibly triggered by an interaction with another galaxy • or generated by bars • “density waves”

  12. Spiral galaxy research at UA • morphology • sense of winding of spiral arms • gravitational torques due to bars and spiral arms • star formation in galactic rings

  13. The de Vaucouleurs Atlas of GalaxiesGerard de Vaucouleurs 1918-1995

  14. The de Vaucouleurs Atlas: what is it? • Authors: R. J. Buta (U. Alabama) • H. G. Corwin, Jr. (Caltech) • S. C. Odewahn (U. Arizona) • publisher: Cambridge Univ. Press • Purpose: to illustrate the de Vaucouleurs revised Hubble classification system with modern digital images

  15. NGC 7457 SA(rs)0-

  16. NGC 1553 SA( r )0

  17. NGC 1291 (R)SB(s)0/a

  18. NGC 2713 (R’)SB(rs)ab

  19. NGC 4736 (R)SA( r )ab

  20. NGC 5850 SB( r )b

  21. NGC 4501 (M88) SA(rs)b

  22. NGC 4321 (M100) SAB(s)bc

  23. NGC 1566 SAB(s)bc

  24. NGC 6643 SA(rs)c

  25. NGC 7479 SB(s)c

  26. NGC 6946 SAB(rs)cd

  27. NGC 5585 SAB(s)d

  28. NGC 4618 SB(rs)m

  29. Sense of winding of spiral arms • Trailing arms: follow direction of rotation • Leading arms: oppose direction of rotation • most spiral arms trail • but one galaxy has leading arms!

  30. Trailing arm spiral M81

  31. Leading arm spiral galaxy NGC 4622

  32. Barred spiral galaxies • A bar-like pattern of old stars crosses the center • arms break from the ends of this bar • 70% of spirals have a bar, including the Milky Way

  33. Barred spiral galaxy NGC 1365

  34. Barred spiral NGC 1300

  35. How stars move in presence of a bar • In solar system, planets follow elliptical paths with Sun at one focus • no torque on planet as it orbits • in presence of a bar, star may follow a centered elliptical orbit • there is a torque on star as it orbits • amount of torque is a measure of “bar strength”

  36. Rings of Star Formation • Real bar orbit • “lit up” by new stars • often not circular

  37. Gallery of spirals from the Hubble Space Telescope

  38. Typical spiral galaxy: NGC 4414

  39. Starburst spiral galaxy NGC 3310

  40. Ringed spiral galaxy NGC 7742

  41. Superposed spiral galaxies NGC 3314

  42. Edge-on spiral galaxy ESO 510-13

  43. Edge-on spiral galaxy NGC 4013