1. Chapter 16: A Universe of Galaxies
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2. 16.1 Islands of Stars Our goals for learning:
What are the three major types of galaxies?
How are galaxies grouped together?
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3. Hubble Deep Field
Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away.
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4. Galaxies and Cosmology
A galaxy's age, its distance, and the age of the universe are all closely related.
The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe.
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5. What are the three major types of galaxies?
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6. Hubble Ultra Deep Field
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7. Hubble Ultra Deep Field
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8. Hubble Ultra Deep Field
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9. Hubble Ultra Deep Field
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10. Hubble Ultra Deep Field
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11. Hubble Ultra Deep Field
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12. Hubble Ultra Deep Field
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13. Spiral galaxy
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14. Spheroidal Component: bulge and halo, old stars, few gas clouds
Disk Component:
stars of all ages,
many gas clouds
Spheroidal Component:
bulge and halo, old stars,
few gas clouds
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15. stars of all ages, many gas clouds
Disk Component:
stars of all ages, many gas clouds
Spheroidal Component:
bulge and halo, old stars,
few gas clouds
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16. stars of all ages, many gas clouds
Disk Component:
stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation.
Spheroidal Component:
bulge and halo, old stars,
few gas clouds
Red-yellow color indicates older star population.
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17. Thought Question
Why does ongoing star formation lead to a blue-white appearance?
A. There aren't any red or yellow stars.
B. Short-lived blue stars outshine others.
C. Gas in the disk scatters blue light.
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18. Thought Question
Why does ongoing star formation lead to a blue-white appearance?
A. There aren't any red or yellow stars.
B. Short-lived blue stars outshine others.
C. Gas in the disk scatters blue light.
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19. Barred Spiral Galaxy: Has a bar of stars across the bulge.
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20. Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical).
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21. Elliptical Galaxy: All spheroidal component, virtually no disk component
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22. Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population.
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23. Irregular Galaxy: Neither spiral nor elliptical
Irregular Galaxy: Neither spiral nor elliptical. Blue-white color indicates ongoing star formation.
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24. Hubble's galaxy classes Disk dominates
Spheroid
dominates
Hubble's galaxy classes
Disk dominates
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25. How are galaxies grouped together?
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26. Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group).
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27. Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies).
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28. What have we learned? What are the three major types of galaxies?
Spiral galaxies, elliptical galaxies, and irregular galaxies
Spirals have both disk and spheroidal components; ellipticals have no disk.
How are galaxies grouped together?
Spiral galaxies tend to collect into groups of up to a few dozen galaxies.
Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies.
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29. 16.2 Distances of Galaxies Our goals for learning:
How do we measure the distances to galaxies?
What is Hubble's law?
How do distance measurements tell us the age of the universe?
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30. How do we measure the distances to galaxies?
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31. Brightness alone does not provide enough information to measure distance.
Are Bright Stars Nearby or Luminous?
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32. Determine size of solar system using radar.
Step 1
Determine size of solar system using radar.
Radar Pulses
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33. Step 2
Determine distances of stars out to a few hundred light-years using parallax.
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34. Luminosity passing through each sphere is the same.
Area of sphere:
4 (radius)2
Divide luminosity by area to get brightness.
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35. The relationship between apparent brightness and luminosity depends on distance.
We can determine a star's distance if we know its luminosity and can measure its apparent brightness.
Distance =
4 × Brightness
A standard candle is an object whose luminosity we can determine without measuring its distance.
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36. Step 3
Apparent brightness of star cluster's main sequence tells us its distance.
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37. Knowing a star cluster's distance, we can determine the luminosity of each type of star within it.
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38. Thought Question
Which kind of stars are best for measuring large distances?
A. High-luminosity stars
B. Low-luminosity stars
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39. Thought Question
Which kind of stars are best for measuring large distances?
High-luminosity stars
Low-luminosity stars
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40. Cepheid variable stars are very luminous.
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41. Cepheid Variable Stars
The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period.
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42. Cepheid variable stars with longer periods have greater luminosities.
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43. Step 4
Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles.
Using Cepheid Variables as Standard Candles
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44. White dwarf supernovae can also be used as standard candles.
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45. Step 5
Apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years).
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46. What is Hubble's law?
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47. The Puzzle of "Spiral Nebulae"
Before Hubble, some scientists argued that "spiral nebulae" were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way.
The debate remained unsettled until someone finally measured the distances of spiral nebulae.
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48. Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles.
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49. Hubble also knew that the spectral features of virtually all galaxies are redshifted ⇒ they're all moving away from us.
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50. By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way.
Discovering Hubble's Law
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51. Hubble's law: velocity = H0 × distance
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52. Redshift of a galaxy tells us its distance through Hubble's law:
velocity H0
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53. Distances of the farthest galaxies are measured from redshifts.
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54. We measure galaxy distances using a chain of interdependent techniques.
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55. How do distance measurements tell us the age of the universe?
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56. Thought Question
Your friend leaves your house. She later calls you on her cell phone, saying that she's been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?
1 minute
30 minutes
60 minutes
120 minutes
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57. Thought Question
Your friend leaves your house. She later calls you on her cell phone, saying that she's been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?
1 minute
30 minutes
60 minutes
120 minutes
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58. Thought Question
You observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?
1 million years
14 million years
10 billion years
14 billion years
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59. Thought Question
You observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?
1 million years
14 million years
10 billion years
14 billion years
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60. Hubble's constant tells us the age of the universe because it relates velocities and distances of all galaxies.
Distance
Velocity
Age =
~ 1 / H0
Estimating the Age of the Universe
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61. The expansion rate appears to be the same everywhere in space.
The universe has no center and no edge (as far as we can tell).
Two Possible Explanations of the Cause of Hubble's Law
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62. One example of something that expands but has no center or edge is the surface of a balloon.
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63. Cosmological Principle
The universe looks about the same no matter where you are within it.
Matter is evenly distributed on very large scales in the universe.
No center and no edges
Not proved but consistent with all observations to date
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64. Distances between faraway galaxies change while light travels.
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65. Distances between faraway galaxies change while light travels.
Astronomers think in terms of lookback time rather than distance.
distance?
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66. Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time.
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67. What have we learned? How do we measure the distances to galaxies?
The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system.
Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles.
We can measure distances greater than 10 billion light-years using white dwarf supernovae as standard candles.
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68. What have we learned? What is Hubble's law?
The faster a galaxy is moving away from us, the greater its distance:
velocity = H0 × distance
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69. What have we learned?
How do distance measurements tell us the age of the universe?
Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach its current distance.
Measuring Hubble's constant tells us that amount of time: about 14 billion years.
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70. 16.3 Galaxy Evolution Our goals for learning:
How do we study galaxy evolution?
Why do galaxies differ?
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71. How do we study galaxy evolution?
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72. Deep observations show us very distant galaxies as they were much earlier in time (old light from young galaxies).
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73. © 2015 Pearson Education, Inc.

74. Modeling Galaxy Formation:
Matter originally filled all of space almost uniformly.
Gravity of denser regions pulled in surrounding matter.
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75. Denser regions contracted, forming protogalactic clouds.
H and He gases in these clouds formed the first stars.
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76. Leftover gas settled into a spinning disk.
Supernova explosions from the first stars kept much of the gas from forming stars.
Leftover gas settled into a spinning disk.
Conservation of angular momentum
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77. But why do some galaxies end up looking so different?
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78. Why do galaxies differ?
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79. Why don't all galaxies have similar disks?
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80. Conditions in Protogalactic Cloud?
Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk.
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81. Conditions in Protogalactic Cloud?
Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk.
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82. Distant Red Ellipticals
Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe.
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83. We must also consider the effects of collisions.
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84. Collisions were much more likely early in time, because galaxies were closer together.
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85. Many of the galaxies we see at great distances (and early times) do indeed look violently disturbed.
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86. The collisions we observe nearby trigger bursts of star formation.
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87. Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.
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88. Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together.
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89. Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies.
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90. Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years.
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91. The intensity of supernova explosions in starburst galaxies can drive galactic winds.
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92. X-ray
image
The intensity of supernova explosions in starburst galaxies can drive galactic winds.
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93. What have we learned? How do we study galaxy evolution?
Deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages.
We use computer modeling to match theory with observation.
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94. What have we learned? Why do galaxies differ?
Some of the differences between galaxies may arise from the conditions in their protogalactic clouds.
Collisions can also play a major role because they can transform two spiral galaxies into an elliptical galaxy.
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95. 16.4 Active Galactic Nuclei
Our goals for learning:
What is the evidence for supermassive black holes at the centers of galaxies?
Why do we think the growth of central black holes is related to galaxy evolution?
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96. What is the evidence for supermassive black holes at the centers of galaxies?
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97. The most luminous examples are called quasars.
If the center of a galaxy is unusually bright, we call it an active galactic nucleus.
The most luminous examples are called quasars.
Active Nucleus in M87
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98. The highly redshifted spectra of quasars indicate large distances.
From brightness and distance, we find that luminosities of some quasars are >1012LSun!
Variability shows that all this energy comes from a region smaller than the solar system.
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99. Thought Question
What can you conclude from the fact that quasars usually have very large redshifts?
They are generally very distant.
They were more common early in time.
Galaxy collisions might turn them on.
Nearby galaxies might hold dead quasars.
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100. Thought Question
What can you conclude from the fact that quasars usually have very large redshifts?
They are generally very distant.
They were more common early in time.
Galaxy collisions might turn them on.
Nearby galaxies might hold dead quasars.
All of the above!
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101. Galaxies around quasars sometimes appear disturbed by collisions.
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102. Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures.
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103. Radio galaxies contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at near light speed.
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104. The lobes of radio galaxies can extend over hundreds of millions of light-years.
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105. The speed of ejection suggests that a black hole is present.
An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light.
The speed of ejection suggests that a black hole is present.
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106. Radio galaxies don't appear as quasars because dusty gas clouds block our view of their accretion disks.
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107. Characteristics of Active Galaxies
Luminosity can be enormous (>1012LSun).
Luminosity can vary rapidly (comes from a space smaller than solar system).
They emit energy over a wide range of wavelengths (contain matter with wide temperature range).
Some drive jets of plasma at near light speed.
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108. The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars.
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109. Energy from a Black Hole
The gravitational potential energy of matter falling into a black hole turns into kinetic energy.
Friction in the accretion disk turns kinetic energy into thermal energy (heat).
Heat produces thermal radiation (photons).
This process can convert 10–40% of E = mc2 into radiation.
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110. Jets are thought to come from the twisting of a magnetic field in the inner part of the accretion disk.
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111. Why do we think the growth of central black holes is related to galaxy evolution?
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112. Orbits of stars at center of Milky Way indicate a black hole with mass of 4 million Msun.
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113. Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of at least 3 billion Msun.
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114. Black Holes in Galaxies
Many nearby galaxies—perhaps all of them—have supermassive black holes at their centers.
These black holes seem to be dormant active galactic nuclei.
All galaxies may have passed through a quasar-like stage earlier in time.
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115. Galaxies and Black Holes
The mass of a galaxy's central black hole is closely related to the mass of its bulge.
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116. Galaxies and Black Holes
The development of a central black hole must somehow be related to galaxy evolution.
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117. What have we learned?
What is the evidence for supermassive black holes at the centers of galaxies?
Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type.
The only model that adequately explains our observations holds that supermassive black holes are the power source.
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118. What have we learned?
Why do we think the growth of central black holes is related to galaxy evolution?
Observations of stars and gas clouds orbiting the centers of galaxies indicate that many galaxies, perhaps all of them, have supermassive black holes.
The masses of the black holes are closely related to the properties of their home galaxies, suggesting a connection between the black hole and galaxy evolution.
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