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1. The Milky Way Center, Shape
Globular cluster system
Galactic coordinates
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
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?
16. Size of the Milky Way Pulsating stars
Size of the Milky Way
17. But how do we find the distance to the Galactic center? We need the distances to the globular clusters
Use pulsating stars in the clusters
Animation
18. If a star is neither expanding nor contracting, we may assume that throughout the star there is a balance between pressure and temperature
density
luminosity
gravity
19. Pulsating stars
20. Pulsation cycle
21. Pulsating stars
22. Pulsating stars
23. Why is this useful? Flux versus luminosity relation
24. A Cepheid has the same pulsation period, but is 1/16 the brightness of another Cepheid known to be at a distance of 2 kpc. How far away is the dimmer star? 2 kpc
4 kpc
8 kpc
16 kpc
32 kpc
25. Size of Milky Way
26. What causes Cepheid variables to vary in brightness? they pulsate
eclipses
changes in nuclear energy generation rate
obscuration by clouds of dust
27. 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?
28. 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
29. 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.
31. 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
32. 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
34. 800 pc
8 kpc
8,000 kpc
8 Mpc
Variable, between 1 and 10 kpc The distance between the Sun and the Galactic center is closest to:
35. 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)
36. Simplified form of Kepler’s 3rd law using convenient units
37. Mass of the Milky Way within the Sun’s orbit
38. Kepler’s 3rd Law applied to Binary Stars
39. Kepler’s 3rd Law applied to Galaxy
40. Rotation curves
41. Rotation curve of the Milky Way
42. Rotation curve of Milky Way
43. Mass of the Milky Way
44. 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
45. 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
46. The Milky Way in various wavelengths Scattering of light, or why is the sky blue?
Milky Way in infrared, radio
The 21 cm line of Hydrogen
47. Milky Way from Australia
48. Scattering of light Light is completely absorbed by very dense clouds of dust
For less dense clouds, some light is transmitted
Does the transmitted light have the same color as the scattered light?
Do demo 6F40.10
49. Scattering light Blue light is scattered more
Red light is transmitted more
This is why the sky is blue
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?
50. Electromagnetic spectrum
51. Milky Way in optical light
52. Milky Way in infrared light
53. Milky Way in radio waves
54. Hydrogen emits 21 cm radio waves
55. Same effect in other atoms is used to do magnetic resonance imaging (MRI)
56. Spiral arms Tracers of spiral structure
Objects found in spiral arms
How are the spiral arms formed?
57. Spiral arms
58. Tracing spiral arms
59. Spiral arms can be traced from the positions of clouds of atomic hydrogen
60. 21 map of spiral arms
61. 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
62. Spiral arms
63. 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.
64. So what causes spiral arms?
67. Density waves
68. 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
69. Density waves
70. Density waves
71. M74
72. 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
73. 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.
74. Definitely in the form of brown dwarfs
Probably cold, dark hydrogen molecules
Likely super-massive black holes
Definitely cold gas, unknown composition
Not known
The nature of the dark matter in the Milky Way is:
75. 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
From the rotation curve of a hypothetical galaxy as shown in the figure, one could infer
76. The Galactic Center Center of the Milky Way
Stars orbiting the black hole
X-ray flares from the black hole
77. Fish eye MW
78. Radio image, central 3 ly Center is the red ellipse at the center
Called Sgr A*
79. 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.
80. Stellar Orbits in the Galactic Center
81. 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
82. X-ray image, central 3 ly Sgr A* is the bright object in the center of the image.
Makes flares in X-rays.
Movie.
83. 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 The best estimate of the mass of the black hole at the galactic center was made using:
84. 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?