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The Milky Way

The Milky Way. Types of Stars. Population I – Second generation stars containing elements heavier than Helium ( the Sun is population 1 ) Population II – first generation ( primordial ) stars consisting of only Hydrogen and Helium

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The Milky Way

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  1. The Milky Way

  2. Types of Stars Population I – Second generation stars containing elements heavier than Helium ( the Sun is population 1 ) Population II – first generation ( primordial ) stars consisting of only Hydrogen and Helium Variable Stars – Horizontal branch stars whose luminosity ( and hence their distance ) can be determined by measuring the period of their cycle of brightening/dimming

  3. Variable Stars

  4. Types of Clusters Globular – tightly bound elliptically-shaped systems of 10,000 to 1,000,000 old population II stars, typically found orbiting outside the disk of a spiral galaxy Open – loosely bound irregularly-shaped systems of population I stars typically found in the galactic disk

  5. Open Cluster ( M6 - Butterfly )

  6. Colour-Magnitude for M6

  7. M45 Pleiades

  8. Globular Cluster M2

  9. M13 – Great Hercules Cluster

  10. Types of Gas • H I regions • very cold hydrogen gas giving off light at 21 cm due to transitions involving flipping electron spin • Typically found in the disk of a spiral galaxy far removed from stars

  11. HI regions

  12. Types of Gas • H II regions • Heated ionized Hydrogen gas surrounding young stars or protostars • Emits light characteristic of atomic transitions. Protons capture electrons in an excited state , a photon is emittted when the electron drops into the ground state • Typically found in the spiral arms of a spiral galaxy

  13. HII regions

  14. Milky Way at various frequencies

  15. Classification Scheme

  16. Elliptical Galaxies • Elliptical galaxies are not flattened into a disk • The three dimensional configuration is very efficient in producing stars • S0 …. • Ellipticals contain very little dust • Almost all the stars are old population II stars • Stellar orbits are approximately random ( no rotation curves )

  17. Elliptical Galaxy

  18. Regions of Spiral Galaxies

  19. Bulge: • Many RR Lyrae stars • A little gas & dust • Halo: • Old metal-poor stars • Globular clusters • RR Lyrae Stars • Dark Matter ! • Thick disk of Stars (~1000 pc thick) • Open Clusters & loose Associations of stars • Mix of young & old stars • Cepheid Stars in young clusters • Thin disk of Gas & Dust (~100 pc thick) • Mostly cold atomic Hydrogen gas • Dusty Giant Molecular Hydrogen Clouds

  20. Spiral Galaxy

  21. Barred Spiral

  22. Rotation curves

  23. Rotation Curve

  24. Rotation Curves

  25. Interpretation of Rotation Curves • The velocity of rotating stars is a probe of the distribution of mass within the galaxy • The wiggles indicate the presence of spiral arms, where the matter density is higher than average • Rotation curves for spiral galaxies are typically flat out to distances beyond where any stars can be seen. This indicate the presence of DARK MATTER • The data is consistent with spherical Halos of dark matter extending far beyond the visible part of the galaxy

  26. Galactic Halos

  27. Spiral Arm Wrapping

  28. Spiral Arms as traffic jam

  29. Galaxy Collisions

  30. Collision Remnants

  31. Cosmic Distance Ladder

  32. Parallax

  33. Cepheid Variables

  34. Tully Fischer Relation

  35. Tully-Fischer Relation

  36. Hubble’s Law

  37. Get velocity from Doppler Shift

  38. Cosmic Expansion

  39. Raisin Bread Model

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