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: ,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

51. Tracing spiral arms

52. Spiral arms can be traced from the positions of clouds of atomic hydrogen
To do this yourself, read

53. 21 map of spiral arms

54. Tracers of spiral arms
Young stars and related objects also trace spiral arms
Emission nebulae = H II regions
Molecular clouds
Clusters of young (O and B) stars

55. Spiral arms

56. Why can't we see visible radiation from the central region of the galaxy?
No visible light is emitted by the central region of the galaxy.
Interstellar dust blocks our view.
Too many stars are in the way.
Gravity curves the light away from the earth and Sun.

57. So what causes spiral arms?

60. Density waves

61. Spiral arms are patterns
According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy
The gravitational field of this spiral pattern causes stars and gas to slow down near the arm
This compresses the interstellar clouds, triggering the formation of stars
The entire arm pattern rotates around the Milky Way once every 500 million years

62. Density waves

63. Density waves

64. M74

65. Which of the following objects are not found primarily in the spiral arms of the Galaxy?
white dwarf stars
HII regions
supernovas
O and B stars

66. Review Questions How can we measure the mass of the Milky Way?
Why do stars behind dust clouds appear red?
Why is the sky blue?
Why are wavelengths of light outside the visible useful in studying the Milky Way?
How is the 21 cm line of Hydrogen produced?
Describe the spiral arms of the Milky Way and what causes them.

67. The nature of the dark matter in the Milky Way is:
Definitely in the form of brown dwarfs
Probably cold, dark hydrogen molecules
Likely super-massive black holes
Definitely cold gas, unknown composition
Not known

68. From the rotation curve of a hypothetical galaxy as shown in the figure, one could infer
A concentration of dark matter inside 2 kpc
A spherical distribution of dark matter
Dark matter only outside 2 kpc
Uniformly distributed dark matter in the disk
No dark matter is needed to explain this plot

69. The Galactic Center Center of the Milky Way
Stars orbiting the black hole
X-ray flares from the black hole

70. Fish eye MW

71. Radio image, central 3 ly Center is the red ellipse at the center
Called Sgr A*

72. Infrared image, central 3 ly
Sgr A* does not appear.
There are about 1,000,000 stars in the area covered by this image.
Stars are only 1000 AU apart.

73. Stellar Orbits in the Galactic Center

74. Mass of Sgr A* can be measured using stellar orbits
Fastest moving star moves at 2% of the speed of light, 5000 km/s
Mass is about 3 million solar masses
Emits radio and X-rays
Almost certainly a black hole

75. X-ray image, central 3 ly
Sgr A* is the bright object in the center of the image.
Makes flares in X-rays.
Movie.

76. The best estimate of the mass of the black hole at the galactic center was made using:
Observations of nearby star orbits in IR
Radio observations of accretion disk
Orbit of stars in spiral arms [Kepler’s 3rd law]
X-ray observations of coronal gas
Dark matter studies using rotation curves

77. Review Questions
What properties of a star’s orbit around the Galaxy enable one to measure the mass inside its orbit?
What is the shape of the rotation curve of the Milky Way and why is was it unexpected?
What lies at the center of the Milky Way?