The Milky Way

1. The Milky Way • Center, Shape • Globular cluster system • Galactic coordinates • Size of the Milky Way

2. Milky Way from Iowa

3. Milky Way from Australia

4. Milky Way from Australia

5. How do we locate the center of the Milky Way? • Can’t see center directly with visible light because of obscuring clouds in the plane of the Galaxy • Look above the plane of the galaxy

8. M15

9. M13

10. Globular clusters • Compact, spherical group of stars • Up to several 100,000 stars • All stars formed together, same age • Form a halo around the Milky Way

11. Globular cluster system

12. Globular cluster system • Centered on the center of the Milky Way • Extends far above and below the plane • By observing globular clusters, we can determine the direction to the center of the Milky Way (and, later, our distance from the center).

13. Globular clusters in Sagittarius

14. Galactic coordinates

15. Globular clusters are found primarily in what part of the Milky Way? • Halo • Disk • Central 1 kpc • Bulge • Disk and Bulge

16. Size of the Milky Way • How do we find the size of the Galactic halo or the distance to the Galactic center? • Star counts, as described in the text book, are a bad way.

17. Pulsating stars

18. Use known luminosity to determine distance Flux versus luminosity relation We can figure out the luminosity of a pulsating star by timing the pulsations. Since, we can measure its flux, we can then find the distance to the star.

19. Size of Milky Way

20. Luminosities of components • With distances, can do star counts, correct for dust absorption and estimate luminosities • Disk: 19109 L • Bulge: 2109 L • Halo: 2109 L • Total: 23109 L • Average star is dimmer than the Sun, total number stars is ~200 billion.

21. Review Questions • What are globular clusters? How are they distributed in the Galaxy? • What are Galactic coordinates? • Why do some stars pulsate? Why are pulsating stars useful in measuring distances? • What is the size and shape of the Milky Way?

22. Orbits of stars in the Milky Way • Stellar orbits in disk and halo • Finding the mass from the orbit • Mass of the Milky Way • Rotation curves • Dark matter

23. What keeps the planets in orbit around the Sun? • The force of gravity from the Sun • To orbit, a planet at a particular distance from the Sun must have a particular orbital speed.

25. Orbits of stars in the Milky Way • The orbit of a star is determined by the total mass lying inside the orbit • By measuring the speed of the star’s orbit and its distance from the center, we can figure out the total mass lying inside the orbit of the star

26. Stellar Orbits in the Galaxy • Stars in the disk all orbit the Galactic center: • in the same direction • in the same plane (like planets do) • they “bobble” up and down • this is due to gravitational pull from the disk • this gives the disk its thickness • Stars in the bulge and halo all orbit the Galactic center: • in different directions • at various inclinations to the disk • they have higher velocities • they are not slowed by disk as they plunge through it • nearby example: Barnard’s Star

28. The distance between the Sun and the Galactic center is closest to: • 800 pc • 8 kpc • 8,000 kpc • 8 Mpc • Variable, between 1 and 10 kpc

29. Mass of the Galaxy We can use Kepler’s Third Law to estimate the mass of the Milky Way inside the Sun’s orbit • Sun’s distance from center of Milky Way: 8,500 pc = 1.8 x 109 AU • Period of Sun’s orbit around the center of the Milky Way: 230 million years (2.3 x 108 yr)

30. Simplified form of Kepler’s 3rd law using convenient units Where M in solar masses a in AU P in Earth years

31. Mass of the Milky Way within the Sun’s orbit • Where M in solar masses, a in AU, P in Earth years • Mass within Sun’s orbit is 1011 M • Total mass of MW Galaxy is ~ 1012 M • Total number of stars in MW Galaxy  2 x 1011

32. Kepler’s 3rd Law applied to Binary Stars • Where: • G is gravitational constant • G = 6.67·10-11 m3/kg-s2 in SI units • m1, m2 are masses (kg) • P is binary period (sec) • A is semi-major axis (m)

33. Kepler’s 3rd Law applied to Galaxy Where M(r) is mass inside r (kg) Change from P to velocity = v

34. Rotation curves

35. Rotation curve of the Milky Way

36. Rotation curve of Milky Way

37. Mass of the Milky Way density  R-2

38. Dark Matter • Dark – it doesn’t produce light (any kind) • Does have mass, produces gravity • Nature is unknown • Might be normal matter in a form that doesn’t emit much light – very small and dim star, little black holes • More likely it is elementary particles other than normal matter

39. What properties of the sun could be used to measure the total mass enclosed within the sun's orbit? • mass and orbital speed • mass and distance from the center • mass and age • orbital speed and distance from the center

40. The Milky Way in various wavelengths • Milky Way in infrared, radio • The 21 cm line of Hydrogen

41. Milky Way from Australia

42. Scattering of light • Blue light is scattered more • Red light is transmitted more • Stars seen through dust appear redder than they really are • If we want to try to see through dust, what kind of light should we use?

43. Electromagnetic spectrum ---------- radio ------------

44. Milky Way in optical light

45. Milky Way in infrared light

46. Milky Way in radio waves

47. Hydrogen emits 21 cm radio waves

48. Same effect in other atoms is used to do magnetic resonance imaging (MRI)

49. Spiral arms • Tracers of spiral structure • Objects found in spiral arms • How are the spiral arms formed?

50. Spiral arms