1. The Milky Way Our Galaxy Please press “1” to test your transmitter.

2. The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see the Milky Way as a faint band of light across the sky.

3. Based on this (our only) view of the Milky Way, what would you say the Milky Way would look like from a position far outside the Milky Way? 1.A flat, round disk. 2.A flat, elliptical disk 3.A sphere. 4.A spiral with one arm wound around multiple times 5.A spiral with multiple arms.

4. The Structure of the Milky Way Galactic Plane Galactic Center The actual structure of our Milky Way is very hard to determine because: 1) We are inside. 2) Distance measurements are difficult. 3) Our view towards the center is obscured by gas and dust.

5. In order to get a good outside view of the Milky Way, you are trying to send a spacecraft high above the plane of the Milky Way, to a height equal to our distance from the Galactic center. If you had a spacecraft that could travel at almost the speed of light, how long would it take it to get there? 1) 2 months 2) 5 years 3) 250 years 4) 30,000 years 5) 5 million years Sun Galactic Center

6. The Sun is about 8.5 kpc = 8,500 pc ≈ 30,000 light years from the Galactic center. Sun Galactic Center => No spacecraft will ever travel a significant distance through or even out of the Milky Way.

7. The Voyager Spacecraft (launched in the 1970s) have just reached the “edge” of the solar system!

8. Approx. 1,700 light years The Extended Solar Neighborhood

9. The Milky Way Galaxy Diameter of the Milky Way: Approx. 75,000 light years

10. 1. Select bright objects that you can see throughout the Milky Way; trace their directions and distances. 2. Observe objects at wavelengths other than visible (not absorbed by gas and dust); note their directions and distances. 3. Trace the orbital velocities of objects in different directions. Viable ways to explore our Milky Way:

11. To trace out the structure of our Milky Way, you want to select the brightest stars, which are … 1.A stars. 2.G stars. 3.O stars. 4.M stars. 5.White dwarfs.

12. Remember: O and B stars are the most massive, most luminous stars (unfortunately, also the shortest-lived ones). => Look for very young clusters or associations containing O and B stars: O/B Associations!

13. Which method might astronomers use to measure the distances to star clusters across the Milky Way? 1.Light travel time measurements 2.Cepheid Variables 3.Trigonometric Parallax 4.Cosmological Redshift 5.Gravitational Redshift

14. Exploring the structure of the Milky Way with bright objects O/B Associations Distances to O/B Associations determined using Cepheid Variables O/B Associations trace out 3 spiral arms near the Sun. Sagittarius arm Orion-Cygnus arm Perseus arm Sun

15. Star Formation in Spiral Arms Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation Spiral arms are stationary shock waves, called Spiral Density Waves, initiating star formation

16. Star Formation in Spiral Arms Spiral arms are basically stationary shock waves Stars and gas clouds orbit around the Galactic center and cross spiral arms Shocks initiate star formation Star formation self-sustaining through O and B ionization fronts and supernova shock waves

17. The Nature of Spiral Arms Spiral arms appear bright in visible light, from newly formed, massive stars, Spiral arms are also bright in infrared, from dust.

18. Self-Sustained Star Formation in Spiral Arms Star forming regions get elongated due to differential rotation Star formation is self-sustaining due to ionization fronts and supernova shocks

19. The Whirlpool Galaxy M 51 (Whirlpool Galaxy) Self-sustaining star forming regions along spiral arm patterns are clearly visible

20. Why do the star-forming regions in this image of the Whirlpool Galaxy appear red? 1.This is the red color of Hydrogen Balmer (H  ) emission from Hydrogen that has been ionized by young, massive, hot stars. 2.Their light is highly red-shifted because especially the star-forming regions are moving away from us at high speed. 3.This is the red color of interstellar dust that is present in the molecular clouds out of which stars are formed. 4.Star forming regions are red from the dominant red light of cool, low-mass stars. 5.They are red from the intensive Hydrogen burning that goes on in the newly formed stars.

21. Exploring the structure of the Milky Way (II) Globular Clusters Dense clusters of 50,000 – a million stars Approx. 200 globular clusters in our Milky Way Old (~ 11 billion years), lower-main-sequence stars Globular Cluster M80

22. Globular Cluster M53

23. How do we know that globular clusters are old? 1.They contain many O and B stars. 2.Their light is highly red-shifted. 3.They are at very large distances, so they must have formed a long time ago. 4.The turn-off point in their Hertzsprung-Russell diagram is very low on the main sequence. 5.Stellar surfaces appear wrinkled.

24. Example: HR diagram of the star cluster M 55 High-mass stars evolved onto the giant branch Low-mass stars still on the main sequence Turn-off point

25. The Structure of the Milky Way Revealed 75,000 light years Disk Nuclear Bulge Halo Sun Globular Clusters Open Clusters, O/B Associations

26. Infrared View of the Milky Way Interstellar dust (absorbing optical light) emits mostly infrared light. Small and Large Magellanic Clouds: Small satellite galaxies of our Milky Way, 160,000 LY and 180,000 from the center of the Milky Way.

27. Infrared View of the Milky Way

28. Radio View of the Milky Way Radio map at a wavelength of 21 cm, tracing neutral hydrogen Interstellar dust does not absorb radio waves => We can observe any direction throughout the Milky Way at radio waves.

29. The Milky Way Across the Electromagnetic Spectrum Radio Waves Infrared Visible light X-rays  -rays

30. The Structure of the Milky Way Revealed Distribution of dust Sun Ring Bar Distribution of stars and neutral hydrogen

31. When the Milky Way was formed, the gas contained almost exclusively H and He; the gas is enriched by heavier elements (“metals”) only through supernovae. For this reason, … 1.Older stars should be more metal-rich than younger ones. 2.There should be no difference in the metal content of old and young stars. 3.Younger stars should be more metal-rich than older ones. 4.The metal content of a star should depend primarily on its mass. 5.The metal content of a star should depend primarily on its temperature.

32. Stellar Populations Population I: Young stars (less than 5 billion years): metal-rich; located in spiral arms and disk Population II: Old stars (more than 5 billion years): metal-poor; located in the halo (globular clusters) and nuclear bulge Younger stars are more metal-rich than older ones.

33. Orbits of Stars in the Milky Way Population I (disk stars) Population II (halo stars)

34. Orbital Motions in the Milky Way (II) Differential Rotation Sun orbits around Galactic center with 220 km/s 1 orbit takes approx. 240 million years.

35. What can we infer from the sun’s orbital period around the Galactic center (knowing its distance from the GC)? 1.The mass of the sun. 2.The diameter of the Milky Way. 3.The angular momentum of the Milky Way. 4.The shape of the Milky Way’s spiral arm structure. 5.The mass of the Milky Way contained inside the sun’s orbit.

36. Mass determination from orbital velocity: The more mass there is inside the orbit, the faster the sun has to orbit around the Galactic center. Combined mass: M = 4 billion M sun M = 11 billion M sun M = 25 billion M sun M = 100 billion M sun M = 400 billion M sun Method similar to mass determination in binary systems (variation of Kepler’s 3 rd law):

37. The Mass of the Milky Way Total mass in the disk of the Milky Way: Approx. 200 billion solar masses Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most of the mass is not emitting any radiation: Dark Matter!

38. Why do we not have a direct view (in visible light) to the Center of the Milky Way? 1.The Galactic Center does not emit visible light. 2.There are too many stars in the way that block our view to the Galactic Center. 3.The Galactic Center is always hidden behind the sun. 4.The Milky Way does not have a center. 5.There is too much dust and gas in the way that blocks our view to the Galactic Center.

39. The Galactic Center Wide-angle optical view of the GC region Galactic center Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Extinction by 30 magnitudes  Only 1 out of 10 12 optical photons makes its way from the GC towards Earth!

40. Radio View of the Galactic Center Many supernova remnants; shells and filaments Sgr A Arc Sgr A*: The Center of our Galaxy The Galactic Center contains a supermassive black hole of approx. 4 million solar masses. Sgr A

42. The Black Hole in the Center of the Milky Way By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 4 million solar masses.

43. The Black Hole in the Galactic Center Black Hole with 4 million M sun

44. What keeps the stars in the disk of the Milky Way on their orbits around the Galactic center? 1.The angular momentum of the Milky Way. 2.The gravitational attraction of the stars near the Galactic center. 3.The gravitational attraction of the supermassive black hole in the Galactic center. 4.The gravitational attraction of the stars in the halo of the Milky Way. 5.The gravitational attraction of all mass (stars, gas, dark matter) in and around the Galactic center.

45. The gravitational attraction of all mass (stars, gas, dark matter) inside the respective orbit keeps the stars in their orbits. Compare 4 million solar masses (supermassive BH in the Galactic center) to the 1 trillion masses of the entire Galaxy! The mass of the supermassive BH in the Galactic center makes up only a tiny fraction of the total mass of the Milky Way!

46. X-Ray View of the Galactic Center Chandra X-ray image of Sgr A* Supermassive black hole in the Galactic center is unusually faint in X-rays, compared to those in other galaxies. Galactic center region contains many black-hole and neutron-star X-ray binaries.

48. History of the Milky Way The traditional theory: Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II); Rotating cloud collapses into a disk- like structure Later populations of stars (pop. I) are restricted to the disk of the Galaxy