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The Milky Way and Beyond

The Milky Way and Beyond. Galaxies and the Larger Scale Structure of the Cosmos. The Milky Way. Our Home Galaxy We live on the “fringes” 75% of the distance out from center Our solar systems makes one orbit of Galactic center every 250 million years!

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The Milky Way and Beyond

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  1. The Milky Way and Beyond Galaxies and the Larger Scale Structure of the Cosmos

  2. The Milky Way • Our Home Galaxy • We live on the “fringes” • 75% of the distance out from center • Our solar systems makes one orbit of Galactic center every 250 million years! • Makes Galaxy difficult to describe due to our perception

  3. The Milky Way • Home to some 100 billion stars • Believed to have a Spiral Structure • This is inferred from various observations • Interstellar dust thwarted early observers • Advent of Radio and IR telescopes improved model

  4. Shapley’s work • His work disputed earlier work by Kapteyn • Shapley said that the Milky Way was larger than initially believed • Kapteyn didn’t take into account the dust present causing dimming • Shapley took the dust into account but didn’t think about dimming

  5. Shapley’s work • Also, he didn’t realize there were TWO classes of variable stars • RR Lyraes • Cepheids • The two classes have different brightness and different periods for variablity • Using the Period-Luminosity relationship (Leavitt) he estimated distance

  6. What Shape is the Milky Way? • Dust initially confused observers • Thought we were in the center of the Galaxy • Stars seemed to be equally distributed • Shapley demonstrated that the 100+ Globular Clusters didn’t orbit us • They orbit a point 30,000 ly distant

  7. The Milky Way Galaxy Side View Nuclear Bulge Disk Sun Halo Globular Clusters

  8. Side View Structure • Disk- 100,000 ly across, 2000 ly thick • Contains Spiral Arms • Nuclear Bulge- 20,000 ly across • Contains Nucleus of Galaxy • Halo- 300,000 ly across • Contains Orbiting Globular Clusters and Dark Matter • Each part has a different population of Stars

  9. Stellar Populations • Mass function- # of stars of each mass • Observations tell us that a variety of masses are made • Observations tell us that star formation is ongoing • Baade grouped the stars according to location and color

  10. Stellar Populations • Blue disk stars = Population I • Red bulge and halo stars = Population II • Further study yields • Bulge- Aging Population I Stars and Pop II • Disk- Young Population I Stars • Halo- Old Population II stars

  11. Stellar Populations • Population I • lots of metals • Young and blue • Circular orbits • Population II • metal poor • Old and red • Elliptical and tilted orbits

  12. Stellar Populations • Not all stars (i.e. Sun) fit easily into either category • Subdivisions include extreme and intermediate Populations, and the “old disk” category • Open clusters contain Pop I • Globular clusters contain Pop II

  13. Galactic Motions • Globular Clusters orbit around the nucleus randomly • Bulge stars are “semi-random”

  14. Galactic Motions • Sun and other disk material orbits nucleus of Galaxy in an orderly way • Experiences Differential rotation • Observed in other Spiral Galaxies • “Rotation” occurs due to a Density Wave • It is not a rigid motion of an “arm” • Wind-up problem

  15. Density Waves • Material in the wave is not fixed • Material can move through the wave • Not a material wave but a disruption wave • Like a traffic jam behind a slow moving vehicle • Wave passes through ISM and triggers star formation

  16. Spiral Arm Structure • Number of Arms isn’t well know. • All numbers between 2-10 have been suggested • Use Spiral Arms Tracers to map the arms • Molecular Clouds (Radio) • H II regions (Optical) • Cepheid Variables (Optical) • OB Stars (Optical)

  17. Variable Stars • Cepheid and RR Lyrae Variables • Variable Stars • Luminosity Varies in predictable ways • RR Lyrae vary over 0.5-1 day • Cepheids vary 1-100 days • Both on Instability Strip of HR diagram

  18. Period-Luminosity Relation • Relationship of Period of Pulse and Luminosity of Star • Linear for Cepheids • Constant for RR Lyraes • Cepheid distances can then be determined • Used for large distances because they are brighter

  19. Period Luminosity

  20. RR Lyrae • Found in Globular Clusters • Shapley used observations to establish distances to GC

  21. Other Tracers • Molecular Clouds emit in Radio • Use Doppler shift to map arm structure • H II regions and OB stars • Luminosity is known • Distance obtained from Inverse Square Law • Group objects by distance, spiral structure seen

  22. Nucleus • Very Obscured • Very crowded • Sagittarius A- powerful radio source, x-ray jets • Million M Black Hole? • Radio reveals two H arms shooting out

  23. Nucleus • Jansky first looked into the heart of the Galaxy with Radio waves • Evidence of star formation ongoing with giant molecular clouds and HII regions • Cool hydrogen and a ring of molecule rich gas exist even closer to the center • As we approach the center, we use many “eyes” to see

  24. The Heart of the Galaxy • Swarm of stars circle the center of the Galaxy • Millions packed into a cubic light year • At the very center is a ring of dust and gas • This surrounds a very small (10 AU) but very powerful source • This is the suspected black hole

  25. In the Halo • If mass were condensed in the center of the Galaxy, rotation would obey Kepler’s 3rd law • More distant objects would orbit more slowly and we can calculate speeds • This relationship doesn’t hold true

  26. Rotation Curve • Plotting speeds of objects based on distance from Galactic center • Appears that most of the mass is contained in the halo

  27. Rotation Curve

  28. Rotation Curve

  29. Formation of the Galaxy • Similar to Star Formation • Everything is on a much larger scale • Halo objects form first • Globular Clusters • Halo Stars • Disk and Nucleus collapse next • Collapse generates star formation

  30. A Universe of Galaxies • Normal Galaxies come in 3 types • Spirals • Ellipticals • Irregulars • Each galaxy has a different morphology • Also different stellar populations • Classified on Hubble Tuning Fork Diagram

  31. Hubble Tuning Fork Diagram

  32. Spiral Galaxies S0 (Sa?)

  33. Spiral Galaxies (Sb)

  34. Spiral Galaxies (Sc)

  35. Spiral Galaxies (Sc)

  36. Elliptical Galaxies (E1)

  37. Elliptical Galaxies (E2)

  38. Elliptical Galaxies

  39. Barred Spirals

  40. Barred Spiral (w/ Star Formation)

  41. Barred Spiral

  42. Irregular Galaxies (LMC)

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