1. Lecture 30: The Milky Way

2. topics: structure of our Galaxy structure of our Galaxy components of our Galaxy (stars and gas) components of our Galaxy (stars and gas) how we observe different components how we observe different components

3. Our Galaxy

4. Edge-on View

6. The structure of our Galaxy not so easy to determine the structure of our Galaxy from the inside because of the effects of dust in our disk not so easy to determine the structure of our Galaxy from the inside because of the effects of dust in our disk initially thought that the Sun was at the center of the Galaxy initially thought that the Sun was at the center of the Galaxy

7. Herschel’s Map of the Galaxy

9. Period-Luminosity relation

10. Globular cluster with RR-Lyrae

11. The real center of the Galaxy globular clusters can be seen above the “smog” of the disk globular clusters can be seen above the “smog” of the disk period-luminosity relation for RR-Lyrae stars used to find distances to globular clusters period-luminosity relation for RR-Lyrae stars used to find distances to globular clusters GC’s in a huge sphere, but the center is located several kpc away from the Sun – this is the true center of the Galaxy GC’s in a huge sphere, but the center is located several kpc away from the Sun – this is the true center of the Galaxy

12. Verified by Infra-red Observations dust extinction inversely proportional to wavelength – longer wavelengths less obscured dust extinction inversely proportional to wavelength – longer wavelengths less obscured dust emits light at far infra-red wavelengths (30-300 microns) dust emits light at far infra-red wavelengths (30-300 microns) infra-red observations give us a direct view of the structure of our Galaxy infra-red observations give us a direct view of the structure of our Galaxy

13. The Infra-red view

14. A Census of Our Galaxy: stars stars – about 50 billion solar masses stars – about 50 billion solar masses disk stars (about 80 percent) disk stars (about 80 percent) young, metal-rich, blue young, metal-rich, blue bulge stars (about 20 percent) bulge stars (about 20 percent) mixture of young and old, red and blue mixture of young and old, red and blue halo stars (less than 1 percent) halo stars (less than 1 percent) uniformly old and red, metal-poor uniformly old and red, metal-poor

15. A Census of Our Galaxy: gas molecular hydrogen (H 2 ) molecular hydrogen (H 2 ) about 5 billion solar masses about 5 billion solar masses atomic (neutral) hydrogen (H I ) atomic (neutral) hydrogen (H I ) about 5 billion solar masses about 5 billion solar masses ionized hydrogen (H II ) ionized hydrogen (H II ) very small amount of mass, but fills most of the volume of the disk very small amount of mass, but fills most of the volume of the disk density

16. Gas in the interstellar medium molecular cloud cores H2H2H2H2 60 K 10,000 molecular clouds H2H2H2H2 30 K 300 atomic clouds atomic H 100 K 100 diffuse atomic gas atomic H 10,000 K 1 hot bubbles ionized H 10 6 K 0.01 density (atoms/cm 3 ) temperature

17. Chemical Composition 70 percent hydrogen 70 percent hydrogen 28 percent helium 28 percent helium 2 percent “heavy elements” (metals) 2 percent “heavy elements” (metals)

18. Observing neutral Hydrogen neutral hydrogen emits radiation in the radio due to magnetic spin-flip transitions neutral hydrogen emits radiation in the radio due to magnetic spin-flip transitions often called ’21-cm’ transition often called ’21-cm’ transition

19. spin-flip transition

20. Magnetic Resonance Imaging

22. Map of Neutral Hydrogen in our Galaxy

23. observing molecular hydrogen molecular hydrogen is too cold to produce emission lines molecular hydrogen is too cold to produce emission lines we can use Carbon Monoxide (CO) or other molecules as a tracer we can use Carbon Monoxide (CO) or other molecules as a tracer these molecules produce emission lines in the radio these molecules produce emission lines in the radio

24. Molecular clouds in Orion

25. Observing hot ionized gas supernovae and massive stars produce “bubbles” of hot gas supernovae and massive stars produce “bubbles” of hot gas as the bubbles expand outwards, they produce shock waves, which cause X-ray radiation as the bubbles expand outwards, they produce shock waves, which cause X-ray radiation

26. X-ray emission from Tycho’s SN

27. Observing hot ionized gas because of the high temperatures, the gas gets ionized. because of the high temperatures, the gas gets ionized. when electrons fall back from their excited state, they produce emission lines at optical wavelengths when electrons fall back from their excited state, they produce emission lines at optical wavelengths

28. Cygnus Loop

29. Observing hot ionized gas shock waves from SN also act as subatomic particle accelerators shock waves from SN also act as subatomic particle accelerators charged particles (electrons) moving in a magnetic field produce radio- wavelength synchrotron radiation charged particles (electrons) moving in a magnetic field produce radio- wavelength synchrotron radiation

30. Cassiopeia A Radio X-ray

31. Radio CO FIR NIR vis. X-ray gamma ray

32. Summary structure of our Galaxy structure of our Galaxy disk, bulge, halo disk, bulge, halo components of our Galaxy components of our Galaxy stars, atomic gas, molecular gas, ionized gas stars, atomic gas, molecular gas, ionized gas how we can observe these components how we can observe these components different physics producing radiation different physics producing radiation characteristic wavelengths of each process characteristic wavelengths of each process

33. atomic gas molecular clouds star formation heavy elements supernovae and stellar winds hot bubbles

34. Star formation

35. Reflection Nebula

36. Horsehead Nebula