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The Milky Way

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The Milky Way

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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?

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