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Milky Way (Optical)

Milky Way (Optical). Lund Observatory, Sweden. Milky Way (IR). IRAS view: Our Galaxy without the dust!. Milky Way (21 cm). NRAO 21-cm map: Hydrogen is concentrated near the plane. Our Galaxy from the outside. NGC 1232; courtesy ESO. NGC 891; courtesy WIYN. A Schematic View.

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Milky Way (Optical)

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  1. Milky Way (Optical) Lund Observatory, Sweden

  2. Milky Way (IR) IRAS view: Our Galaxy without the dust!

  3. Milky Way (21 cm) NRAO 21-cm map: Hydrogen is concentrated near the plane

  4. Our Galaxy from the outside NGC 1232; courtesy ESO NGC 891; courtesy WIYN

  5. A Schematic View

  6. Vital Statistics Diameter: 30 kpc (+/- 10%) • Thickness: typically ~300 pc • low-mass stars have highest scale height • high-mass stars and GMCs have lowest • Number of Stars: 200 billion • Mass: ~1012 solar masses • estimated from rotation curve • Rotation Period (at Sun's orbit): 250 million years • estimated from solar space velocity

  7. How did we get here?...a little history • 1610 • Galileo Galilei (Italy) makes first observations of the Milky Way with a telescope: "For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters." • 1750 • Thomas Wright (England) first suggests that the Sun was one of innumerable stars distributed in a flat slab of stars

  8. Wright's Model

  9. 18th Century • 1755 • Immanuel Kant (Germany) suggested that the Milky Way was a rotating disk of stars, one of many such "Island Universes" • 1785 • William Herschel (England) counted the density of stars in many directions with his 48-inch (1.2 meter) telescope, and constructed a model for the shape of the Milky Way

  10. Herschel's Model

  11. Kapteyn: The First Modern Picture • 1922 • Jacobus Kapteyn (Holland) accurately measured the density of stars in the Milky Way from photographic plates. Using distances of nearby stars measured by parallax, estimated the Milky Way had a diameter of about 15 kpc and a thickness of 3 kpc -- fairly close to modern values! • Assumed that the Sun was near the center of the Galaxy

  12. Problems of Scale • Problem: How do you measure distances accurately at the scale of the Galaxy? • Parallax? Distances are too great! • Spectroscopic Parallax • Obtain spectrum of far-away star, match it to the HR diagram, and obtain intrinsic stellar parameters • Hard to get spectra for faint stars... • Variable Stars! • Henrietta Leavitt & Harlow Shapley (1910s, 1920s) derive period-luminosity relationship for Cepheid variables

  13. Period-Luminosity Relation Measuring the period gives the intrinsic luminosity, which can be combined with the apparent luminosity to get distance!

  14. Enter My Hero: Harlow Shapley • 1920 • Harlow Shapley (Harvard) observed that the globular clusters were centered about a point that was displaced from the Sun. • Used Cepheids to measure distances to globular clusters • Assumed (correctly!) that globulars form a spherical distribution around the galactic center • Estimated distance to center as 15 kpc (we now estimate 8.5 kpc); direction to center toward Sagittarius • Direction was correct, distance overestimated due to presence of dust

  15. Shapley's Milky Way

  16. A more modern picture... COBE IR image with globular cluster positions overplotted

  17. The Finishing Touches... • 1927 • Jan Oort (Holland) measures rotation rate of Milky Way and infers its mass • 1940-1960s • Jan Oort & Hendrik van de Hulst (Holland) map the spiral arms of the Milky Way through observations of 21-cm radio emission

  18. The Current Picture

  19. Two images of M74, in red light and UV light, trace the red giants (red) and hot young stars (UV).

  20. Nearby Structure: MW is a Spiral Galaxy

  21. Radio Observations: Spiral Structure

  22. The "Monster in the Middle"

  23. What's Happening at the Center? • Infrared light shows a dense star cluster which peaks at the center, near Sag A*. • ~ 2 x 106 Msun within 1 pc • Stars are only 1000 AU apart • A collision every 106 years! • Radio observations in molecular lines show a massive "molecular ring" of gas and dust that rotates around this star cluster.

  24. Structures outside the molecular ring • 20 pc long linear structures tracing Galactic magnetic field lines • Isolated star forming regions and supernova remnants.

  25. X-ray and gamma-ray observations • The Galactic Center region is a strong source of x-rays and gamma-rays from the annihilation of antimatter. • X-rays from black-hole binary star systems and supernovae near the galactic center. • 0.5 MeV gamma rays from a "fountain" of antimatter positrons from the Galactic Center region, perhaps the result of many many • Numerous supernovae in the central regions of the Milky Way. • Is there a massive black hole at the center??

  26. A Big Remaining Mystery: "Dark Matter" The Milky Way rotation curve, like that of most other galaxies, is "non-Keplerian." This implies that lots of the galaxies mass lies at large distances from the center. But we don't see much out there in the halo to account for it: not stars, not dust, not gas. We only know the stuff is there because of the gravity it exerts. What is it?

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