Introducing the Milky Way Galaxy

1. Introducing the Milky Way Galaxy

2. Next two weeks • Today: The Milky Way Galaxy (material will be covered in Exam #3) • Next Tues: Exam #2 Review • Next Thurs: Exam #2 (60 questions) • And Keep in Mind • Homework #7:  Due Friday at 9am (this week only) • Telescopes Project F–L: Last night is tonight • Telescopes Project M–R: Begins next week

3. The Milky Way: Our Home Galaxy

4. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I

5. Emission from H I Gas The lowest orbital of the hydrogen atom is not one level – it is actually 2 levels, separated by a miniscule amount of energy. Occasionally, (weak) collisions will push electrons into this state.

6. Emission from H I Gas When the electron decays, a photon with a wavelength of 21 cm is produced, corresponding to radio wavelengths. Radio waves are not blocked by dust, so we can see the emission from H I across the entire galaxy.

7. Milky Way at 21 cm All-sky picture of the Galaxy at 21 cm

8. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I

9. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2

10. Molecules in the Interstellar Medium In the coldest, densest regions of the ISM, atoms can stick together to form molecules, such as H2O, NH3, CO2, HCN, CH2O, CH3OH, CH2O, CH3CH2CN, HC5N, H2C2O, CH3CH2CN, NH2CH2COOH, and many others. Many of these are organic molecules!

11. The Milky Way in CO The most common molecule is H2, but it has no easily observable emission lines. Fortunately, the molecule CO is relatively common, and has strong transitions at some radio wavelengths.

12. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO

13. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II

14. H II Regions When H I is located near an extremely hot (O or B) star, it can be ionized. You will then see emission lines from recombination.

15. The Milky Way in H II All sky picture of the Galaxy at 6563 Å

16. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II

17. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II • Dust

18. The Milky Way in Optical Light Dust blocks background light (blue more than red)

19. The Milky Way in the Near-Infrared At near-infrared wavelengths, the dust in more transparent, so we can see more stars in the Milky Way.

20. The Milky Way in the Near-Infrared At near-infrared wavelengths, the dust in more transparent, so we can see more stars in the Milky Way.

21. The Milky Way in the Far Infrared At longer infrared wavelengths, interstellar dust glows (via blackbody radiation).

22. Different Ways of Seeing the Milky Way

23. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II • Dust

24. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II • Dust • Stars

25. Star Clusters • Open Clusters • Hundreds or thousands of stars • Loosely bound; self gravity not strong enough to hold the stars together over time • No more than a few billion years old

26. Open Clusters

27. Open Clusters

28. Open Clusters

29. Open Clusters

30. The Random Motions of Clusters

31. Measuring the Age of a Star Cluster The older the cluster, the fainter the turn-off.

32. Ages of Open Clusters For open clusters, the main sequence extends to bright magnitudes. These clusters are young!

33. Star Clusters • Open Clusters • Hundreds to thousands of stars (i.e., low mass) • Loosely bound; self gravity not strong enough to hold the stars together over time • No more than a few billion years old • Globular Clusters • Tens of thousands to a million stars (i.e., high mass) • Self-gravity strong enough to keep stars from wandering off • As old as about 13 billion years

34. Globular Clusters

35. Globular Clusters

36. Globular Clusters

37. Globular Clusters

38. Ages of Globular Clusters In globular clusters, the main sequence turnoff is at spectral type G or K. These clusters are very old.

39. Different Ways of Seeing the Milky Way • Interstellar Medium • Atomic Hydrogen – H I • Molecular Hydrogen – H2 • Traced by the molecule CO • Ionized Hydrogen –H II • Dust • Stars • Open clusters • Globular clusters

40. Structure of the Milky Way Galaxy

41. Structure of the Milky Way • We would like to determine the structure of the Milky Way: How large is it? What is its shape? Where are we in the Milky Way? • To do this, we need to construct a 3-D map of the Milky Way, and for this we need to measure distances to many, many (very distant) stars • It will help if we can distinguish old stars from young stars, so we would like to measure ages of stars as well

42. Stars with different Ages: Stellar Populations • Astronomers divide the stars in our the Galaxy into two populations according to age: • Population I: young stars • Population II: old stars

43. Measuring Ages For star clusters, measuring the age is easy.

44. Measuring Ages But for individual stars that aren’t in clusters (like the Sun), we can’t use the cluster turnoff method to measure an age. For instance, a lone G star might be young, or it might be 10 billion years old. How do we tell the age?

45. The Astronomer’s Periodic Table

46. The Astronomer’s Periodic Table

47. The Astronomer’s Periodic Table

48. The Astronomer’s Periodic Table

49. Measuring Ages But for individual stars that aren’t in clusters (like the Sun), we can’t use the cluster turnoff method to measure an age. For instance, a lone G star might be young, or it might be 10 billion years old. How do we tell the age? In these cases, one can examine the metallicity of the star. Because supernovae have created metals over time, stars being formed today have more metals than old stars. high metallicity = young low metallicity = old

50. Determining a Star’s Metallicity The metallicity of a star can be determined through its spectrum. The hydrogen absorption lines indicate that the temperatures of the two stars are similar. But in the second star, all the “metal” lines are much, much weaker!