1. The Most Astonishing Fact About the Universe: The Life Cycle of Stars
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The Most Astonishing Fact About the Universe: The Life Cycle of Stars
The Sun
Andrew Rivers, Northwestern University

2. 2/6/2018

3. 2/6/2018
Range of star properties dramatized: Which is the closest star in this picture?
Alpha Centauri A and B are of star type G and K respectively. Proxima Centauri is M class and thereform much fainter
Alpha Centauri and companions Beta and Proxima Centauri. The stars are at similar distances; Proxima is closest, but is M-class and therefore far fainter.

4. Where does a star’s energy come from?
How much energy?
3.901026 Watts for the sun
Largest earth power plants=5 109 Watts.
Possibilities
Chemical reactions?
Most efficient: 2H + O = H20 +energy.
Sun would last 18,000 years.
Gravitational “settling”.
If Sun still contracting, convert PE to KE to light
30 million years worth of energy!
Contradicts biology, geology, astronomy

5. Solution: Nuclear Fusion
Energy by combining light nuclei like hydrogen to make heavy nuclei.
In the sun 4 Hydrogen nuclei are fused into a helium nucleus: Efficiency 0.08%
Lifetime 10 billion years= O.K.

6. Why is fusion of two nuclei difficult?
Short range, attractive strong nuclear force
long range, repulsive Coulomb force
Charged nuclei must get close enough, then SN force can overcome Coulomb repulsion

7. How do two positive nuclei get close enough for fusion?
Like charges repel
If traveling fast enough, they get close
Attractive SN force overcomes repulsive Coulomb force

8. Foundation Principle 1: Power from Fusion
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Foundation Principle 1: Power from Fusion
P-P chain, Fusion in Main Sequence stars

9. Binding Energy Curve Iron (Fe) most stable element
As more nucleons are in nucleus, they are more tightly bound by SN force
Energy released through Fusion

10. Foundation Principle 2: Hydrostatic Equilibrium Balance
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Foundation Principle 2: Hydrostatic Equilibrium Balance
Fusion in core = outward pressure

11. Image credit: NASA / CXC / M. Weiss.
Hydrostatic Equilibrium: outward photon pressure from fusion reactions in core balances inward gravitational pressure.

12. Principle: Hydrostatic Balance Principle: Fusion in Core
Mass Luminosity Law
Principle: Hydrostatic Balance
Principle: Fusion in Core
Larger mass stars have greater internal pressure from gravity
Fusion rate must be greater for more massive stars in order to balance greater inward gravity pressure.
More massive stars have a greater fusion burn rate (luminosity)

13. Why are more massive stars brighter?
For main-sequence stars, there is a direct correlation between mass and luminosity—the more massive a star, the more luminous it is. A main-sequence star of mass 10 M (that is, 10 times the Sun’s mass) has roughly 3000 times the Sun’s luminosity (3000 L); one with 0.1 M has a luminosity of only about L.
Conclusion: Though more massive stars have more nuclear fuel (M), they have greater burn rates (L) and therefore have shorter lifetimes

14. Tool of the Trade: Hertzsprung-Russell Diagram
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Tool of the Trade: Hertzsprung-Russell Diagram
Purpose: classify stars, explore stellar evolution
Where do we expect to find stars on this plot? Anywhere? In some places and not others?
Regions of the HR diagram, from

15. Recall: Stars are approximate Blackbodies
Figure 17-7: These graphs show the intensity of light emitted by three hypothetical stars plotted against wavelength (compare with Figure 5-11). The rainbow band indicates the range of visible wavelengths. The star’s apparent color depends on whether the intensity curve has larger values at the short-wavelength (blue) or long-wavelength (red) end of the visible spectrum. However, the human brain combines all of the colors in a spectrum, so where the intensity peaks is just one factor shaping the color perceived by the eye. The image insets show stars of about these surface temperatures. UV stands for ultraviolet, which extends from 10 to 400 nm. See Figure 5-12 for more on wavelengths of the spectrum. (inset a: Andrea Dupree [Harvard-Smithsonian CfA], Ronald Gilliland [STScI, NASA, and ESA]; inset b: NSO/AURA/ NSF; inset c: NASA, H. E. Bond, and E. Nelan [STScI]; M. Barstow and M. Burleigh [U. of Leicester, U.K.]; and J. B. Holberg [U. of Arizona])
A Star’s color depends on its surface temperature. Cooler=redder, hotter=bluer

16. HR diagram “scatter-plot of the nearest stars.
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A cool, bright star
25 closest stars, from
The Main Sequence
Some faint, hot stars
Goal: understand this diagram.
HR diagram “scatter-plot of the nearest stars.
From

17. “O” Stars
“M” Stars

18. Physical Basis for Stellar Evolution
Hydrostatic equilibrium
Inward gravitational pressure and outward pressure (usually radiation pressure) must balance in any stable star
Gravity, though the weakest force is always attractive and omnipresent.
If there is no outward pressure, the star must collapse.
“The War Against Gravity”

19. Why should stars have a “life cycle”?
Only set amount of Hydrogen gas to use in nuclear fusion.
Must find some other way to counteract gravitational pressure
Initial mass determines how quickly fuel will burn (The luminosity) to maintain equilibrium
L~M3.5 More massive stars, even with more fuel to start should burn quicker

20. Life-story of a Sun-like Star, Stage 1: Formation from a molecular cloud/solar nebula
Protostar
Main-sequence
Planetary nebula
White dwarf
Red giant
Raw material for star formation strewn throughout the galaxy
Giant Molecular Clouds. Cool clouds of H2 (molecular hydrogen) and some CO.
When do they collapse?
Pressure wave from Supernova or or collision between clouds or other compressing event..
Gas must be slow moving (little KE) to collapse

21. Observation: Dust in Molecular Clouds Blocks Visible Radiation
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Observation: Dust in Molecular Clouds Blocks Visible Radiation
Reddening: red wavelengths pass through dust more readily than blue.
Star forming regions often cold (easier to collapse) therefore they do not radiate visible light (dark clouds).
Molecular Cloud Barnard 68

22. Longer wavelength IR not blocked by dust
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Observation: Molecular Clouds from the Spitzer Infrared Space Telescope
Optical
Longer wavelength IR not blocked by dust

23. How Can We See Molecular Clouds?
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How Can We See Molecular Clouds?
The Observation: Carbon Monoxide emission line in the Milky Way
Emission Line Spectrum
The Physics: Molecules have rotational transition, releasing radio waves

24. Life Story Stage 2: Protostar
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Life Story Stage 2: Protostar
Stage 1
Stage 2
Molecular cloud
Protostar
Fusion has not yet begun, but clump of gas has condensed.
Radiates in red and infrared (Temperature is 2000 to 3000K)
Optical light is blocked by dust
Infrared gets through (wavelength large compared to size of dust grains.
Protostars only around for short time (few million years)

25. (a) This visible-light view shows a dark nebula (or Bok globule) called L1014 in the constellation Cygnus (the Swan). No stars are visible within the nebula. (b) The Spitzer Space Telescope was used to make this false-color infrared image of the outlined area in (a). The bright red-yellow spot is a protostar within the dark nebula. (NASA/JPL-
Caltech/N. Evans [University of Texas at Austin])

26. Protostar stage: forming planets
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Protostar stage: forming planets
Proto-planet accretion
To final solar system
Proto-planets
Condensing into
dust particles which
build up
Collapsing gas &
dust cloud
“pancaking”

27. Artist’s rendition of a forming proto-planetary disk with newborn planets

28. Life Story, Stage 3: The Main Sequence
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Life Story, Stage 3: The Main Sequence
Stage 1
Stage 2
Stage 3
Molecular cloud
Protostar
Main-sequence
As cloud collapses, temp rises in the core until fusion is possible
When fusion “turns on” the protostar becomes a star.
The stars stay on the main sequence for 90% of their lifetimes
Stars form in groups (open clusters)

29. Q: What do all stars on the main sequence have in common?
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Q: What do all stars on the main sequence have in common?
A: PP-Chain. All are burning Hydrogen into Helium in their cores.

30. NGC 3603 Star Birth Sequence
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NGC 3603 Star Birth Sequence
Q: Why are the stars we see in this newborn cluster all blue?
Consider the Blackbody spectrum. Which stars are we biased to see?

31. Sun Formation Life-story told by HR diagram
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Sun Formation Life-story told by HR diagram
Evolution of the sun onto the Main Sequence

32. Q: Which cluster is older?
Pleiades
Hyades
Image Credit & Copyright: Rogelio Bernal Andreo
Q: Which cluster is older?

33. HR Diagram of Pleiades Cluster
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Bright, blue O stars
Faint, red M stars

34. Sun Life Story Stage 4: Red Giant Expansion
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Stage 1
Stage 4
Stage 2
Stage 3
Molecular cloud
Protostar
Main-sequence
Red giant
Star runs out of hydrogen fuel in core
Gravity doesn’t go away
Star collapses and heats up
Core inert, Shell burning begins
Fusion is rapid because the shell layer is still compressing
Luminosity of star increases
Radius of star increases, becoming giant
During this stage the sun would expand to take up most of the sky. Perhaps the Earth would even be inside the sun; models of stellar evolution are not precise.
Artist’s rendition of Earth’s future
with red giant Sun

35. Shell Hydrogen burning after core of Hydrogen has been exhausted.
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Shell Hydrogen burning after core of Hydrogen has been exhausted.
New source of fusion means the core of the star becomes hotter causing the star size to grow.

36. Sudden onset  Helium flash
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As core temp rises, Helium atoms eventually reach speeds required for Helium fusion
Sudden onset  Helium flash
Extra outward pressure from the core star expands
Hydrostatic equilibrium means that the star will then contract
Variable stars: the pulsations are not damped, but periodic.

37. Hydrogen shell burning Temp rises, Helium ignites in core
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Hydrogen shell burning
Temp rises, Helium ignites in core
Triple Helium fusion process
Q: Why is He fusion harder, than H fusion, requiring greater temperatures?

38. Life Story, Stage 5: Red Giant rundown
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Life Story, Stage 5: Red Giant rundown
Stage 1
Stage 4
Stage 2
Stage 5
Stage 3
Molecular cloud
Protostar
Main-sequence
Planetary nebula
Red giant
What happens when no more Helium?
Star center compresses, Burn Carbon + Helium= Oxygen
Planetary Nebula
Low mass stars  the increased number of photons from core push on outer carbon/silicon in layers  puff off the star
Much of mass returned to Interstellar Medium and is heated by the star

39. Ring Nebula as seen by the Hubble Space Telescope.
Image Credit: NASA, ESA, and C.R. O'Dell (Vanderbilt University)

40. Cats Eye Nebula Image Credit: NASA,ESA, HEIC, Hubble Heritage Team
Named after its shape, this nebula was first discovered in The exposed core of the central star—clearly visible in the image—is about 10,000 times more luminous than the Sun, and its intense ultraviolet radiation powers the nebula by ionizing its gas. While the nebula is hotter than the Sun, the gas is too diffuse to shine by thermal radiation as the Sun does. Instead, like all planetary nebula, the visible light comes from emission lines: After electrons are excited to higher energies, photons are emitted as the electrons cascade down to lower energies in the gas’s atoms. As with many planetary nebulae, the mechanisms shaping the intricate visible structures are not well understood. (NASA, ESA, HEIC, The Hubble Heritage Team STScI/AURA)
Image Credit: NASA,ESA, HEIC, Hubble Heritage Team
Cats Eye Nebula

41. Pictorial story of the evolution of Sun from proto-star birth to white dwarf cool-down.
Story of the sun as told on an HR diagram. Note that the longest time in the normal evolution is on the Main Sequence

42. 2/6/2018
An HR diagram story of the evolution of the sun after the main sequence.

43. Theoretical evolution of a star cluster: Birth to Main Sequence
Figure part 1: This series of H-R diagrams shows the evolution of 100 stars of different masses in a hypothetical cluster. Each dot represents a star, and “Age” indicates the time that has elapsed since all the stars formed. At each age, the luminosity and surface temperature of each star were calculated using the equations of stellar structure. After a star passes through the red-giant stage, it is deleted from the diagram (Adapted from R. Kippenhahn)
Theoretical evolution of a star cluster: Birth to Main Sequence

44. Theoretical evolution of a star cluster: Main Sequence to Red Giants
Figure part 2: This series of H-R diagrams shows the evolution of 100 stars of different masses in a hypothetical cluster. Each dot represents a star, and “Age” indicates the time that has elapsed since all the stars formed. At each age, the luminosity and surface temperature of each star were calculated using the equations of stellar structure. After a star passes through the red-giant stage, it is deleted from the diagram (Adapted from R. Kippenhahn)
Theoretical evolution of a star cluster: Main Sequence to Red Giants

45. Part 3: This series of H-R diagrams shows the evolution of 100 stars of different masses in a hypothetical cluster. Each dot represents a star, and “Age” indicates the time that has elapsed since all the stars formed. At each age, the luminosity and surface temperature of each star were calculated using the equations of stellar structure. After a star passes through the red-giant stage, it is deleted from the diagram (Adapted from R. Kippenhahn)
Theoretical evolution of a star cluster: Red Giants and the Turn-off Point

46. Stage 4B: The Alternate Ending Red Giant rundown, high mass
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Stage 4B: The Alternate Ending Red Giant rundown, high mass
Stage 2
Stage 3
Molecular cloud
Protostar
MS Blue Giant
Red giant
Running out of fuel
Star center compresses
Burn Carbon + Helium to get Oxygen
Shell burning with other reactions
Reactions become less efficient.
Hydrogen burning most efficient
Other reactions less so
Iron=most stable element, no energy available
But Gravity remains!

47. Shell burning in a star at the end of its days ( Freedman Geller & Kaufmann, Universe)

48. Recall: Binding Energy Curve
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Iron (Fe) most stable element
Less tightly bound as nuclei get bigger, short range SN not as effective
As more nucleons are in nucleus, they are more tightly bound by SN force
Energy released through Fusion

49. Image credit: Wikipedia user Sakurambo.

50. 2/6/2018
Supernova
Stage 4
Stage 2
Stage 3
Molecular cloud
Protostar
MS Blue Giant
Red giant
Supernova
Iron core No more reactions can produce energy to hold out core
Star begins to collapse due to gravity
What stops the collapse?
Electron degeneracy pressure
Electrons resist when we try to place them in the same place (not the same thing as electrostatic repulsion)
As soon as the collapsing core reaches the density where electrons “see” each other, the star becomes stable and stops collapsing

51. 2/6/2018
Inner layers are still moving inward and hit the “solid wall” of the new white dwarf and….
Bounce!
The supernova emits as much energy per second as all stars in the galaxy combined (for awhile).

52. 2/6/2018
Supernova 1987A: Closest supernova during age of telescopes (in Large Magellanic Cloud)
Before…..
After!

53. Supernova 1994D outshines its entire host galaxy
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Supernova 1994D outshines its entire host galaxy

54. Life Story end-game: White Dwarf stars
Stage 4
Stage 1
Stage 2
Stage 5
Stage 3
Stage 6
Molecular cloud
Protostar
Main-sequence
Planetary nebula
White dwarf
Red giant
A white dwarf star remains after the supernova and as a product of evolution of low mass stars
Theoretical calculations  the maximum white dwarf = 1.4Msun
Chandrasekar mass
What then?

55. The Old open cluster NGC 6971
The Old open cluster NGC The current HR diagram reveals age and stages of evolution.

56. Fighting the War Against Gravity
Normal Stars
Fusion Hydrogen  Helium +energy. Outward pressure from escaping photons balances gravity
Massive stars, after hydrogen
Fusion of heavier elements, pressure is balanced same as above
Iron is the most stable, no energy available to maintain equilibrium through fusion
White Dwarf stars
Gravitational force balanced by “electron degeneracy pressure”

57. Degeneracy pressure
Fermions (electrons, protons, neutrons) cannot occupy the same energy level
If all available energy states are filled, an electron must go to a higher energy level
this takes work! The gas resists compression

59. A White Dwarf Star

61. Summary: Life-story of Stars
Stage 1
Stage 4
Stage 2
Stage 5
Stage 3
Stage 6
Molecular cloud
Protostar
Main-sequence
Planetary nebula
White dwarf
Red giant
Image source:
Life story of sun told as a track on the H-R diagram
High mass stars have different HR tracks and can make heavier elements before going supernova.

62. What is this picture?

63. Pulsars can rotate at a rate of approx 1000 times per second
Jocelyn Bell Burnell, first to notice a radio source, identified as a pulsar
The fastest pulsars can rotate at a rate of a thousand times a second.
Fast rotating neutron star generates strong magnetic field and a “flashlight beamed” periodic source of radio waves, first known as LGM’s
Pulsars can rotate at a rate of approx 1000 times per second

64. Alternative Ending: Neutron Stars

65. A rapidly rotating neutron star lies at the heart of the crab nebula

66. The evolution of an isolated star (one that is not part of a close multiple-star system) depends on the star’s mass. The more massive the star, the more rapid its evolution. If the star’s initial mass is less than about 0.4 M, it evolves slowly over the eons into an inert ball of helium. If the initial mass is in the range from about 0.4 M to about 8 M, it ejects enough mass over its lifetime so that what remains is a white dwarf with a mass less than the Chandrasekhar limit of 1.4 M. If the star’s initial mass is more than about 8 M, it ends as a core-collapse supernova, leaving behind a neutron star or black hole.

67. The Stellar Evolution Cycle
These images summarize the key stages in the cycle of stellar evolution. (top: Infrared Space Observatory, NASA; right: Australian Astronomical Observatory; bottom: NASA; left: NASA; middle: Australian Astronomical Observatory/David Malin Images)
The Stellar Evolution Cycle

68. Summary of star evolution stages
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Summary of star evolution stages

69. Grand Unification We are stardust!
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Grand Unification
We are stardust!
All heavy elements are cooked in stars
Spread through galaxy by supernovae
New stars form from the rich ashes