1. Lecture 22
RIP
The Death of Stars

2. Announcements
Tonight is the last regular Lab. A signup sheet will be posted next to the door for the make-up lab next week. Please indicate which labs you are missing so that I can decide how to do the make-up.

3. The Main Sequence -5 -3 -1 On the HR diagram, the sun starts here. 1 37 9
On the HR diagram, the sun starts here.
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4. Early Red Giant -5 -3 -1 1 3 5 7 9
By the time the sun first becomes a red giant, it is now here on the diagram (in the region for giants).
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5. Just Before The Helium Flash
By the time the sun reaches the helium flash, it is here on the diagram. -5 -3 -1 1 3 5 7 9
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6. The Red Giant Branch -5 -3 -1 1 3 5 7 9
This path stars follow as they become red giants is often called the giant branch of the HR diagram.
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7. A Helium-Burning Star -5 -3-1 1 3 5 7 9
After the helium flash, the sun becomes, smaller, warmer, and dimmer than before.
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8. The Horizontal Branch
Once a solar-type star begins helium burning, it ends up somewhere along this horizontal line on the HR diagram. -5 -3 -1 1 3 5 7 9
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9. The Horizontal Branch
For this reason, helium-burning solar-type stars are called horizontal branch stars. -5 -3 -1 1 3 5 7 9
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10. The Horizontal Branch
The core helium burning phase is sometimes called “the second main sequence” because of similarities to the hydrogen burning phase:
Energy is again produced in the core (but using a different fuel).
Pressure-Temperature thermostat is very effective again: star’s size/temperature stays very stable.

11. The End Of The Reprieve Important Differences:
Helium burning doesn’t last as long.
Helium fusion is not as efficient as hydrogen fusion: produces less energy per kg of nuclear fuel.
Sun is still 40 times brighter than today.
Starts to run out of helium in only about 250 million years.

12. The Second Ascension As helium fuel runs out:
Carbon core starts shrinking.
Helium burning begins in shell around carbon core.
Hydrogen burning begins in shell around helium shell.
The star is swelling into a red giant again! Called the second ascension.

13. About How Big Will Our Sun Get?
This phase is the largest and brightest our sun will ever get.
Here’s original size for comparison.
L = 4,800
T = 3,000 K
R = 260

14. The Death Of Earth? During this phase, the sun will swallow the Earth.
Probably won’t make it out to Mars.

15. The Second Giant Branch-5 -3 -1 1 3 5 7 9
There are two giant branches on the HR diagram, side by side.
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16. The Second Giant Branch
The second one is called the asymptotic giant branch. -5 -3 -1 1 3 5 7 9
So stars in their second ascension are often called AGB stars.
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17. AGB Giants Very large, luminous, and red.
T ~ 3,000 K
Energy source is helium and hydrogen shell fusion.
Star has inert C, N, O core.

18. AGB Giants AGB Giants experience significant mass loss.
Gravity too low to hold onto distended outer layers.
Dust forms in cool outer layers; “reflects” core light, helping to push outer layers out into space.
Lose up to 1 solar mass every 100,000 years.

19. Mass Loss In Giant Stars
Giant stars have strong stellar winds and weak surface gravity.
During the giant phase, these winds carry off a large percentage of the star’s mass.

20. By The End Of The Giant Phase…
Up to half or more of the star’s gasses can end up as a nebula around the giant star.

21. The Planetary Nebula Forms in two stages:
Early in AGB stage, mass loss occurs in the form of a slow cool wind. Forms an expanding shell of gas around the star.
After expulsion of outer layers, core is exposed to space.

22. The Planetary Nebula
Hot, fast stellar wind from core slams into cool expanding shell.
Gas glows by emission. Result is a planetary nebula.

23. Planetary Nebulae Mass loss not necessarily symmetric:
Cold shell may be less dense at poles.
Easier for hot wind to get through the poles.
Results in an asymmetric nebula (like an hourglass).

24. Planetary Nebulae Planetary Nebulae are very short lived:
Expansion of nebula rapidly cools gases.
Emission fades, nebula becomes too dim to observe after a few 10,000’s of years.

25. The Final Collapse Core finishes consuming all nuclear fuel.
Gravity wins!
Core collapses until electron degeneracy prevents further contraction.
What’s left of star is now about the size of Earth, but very, very hot: a white dwarf star.

26. White Dwarf Stars No nuclear fusion. Star is “dead.”
Electron degeneracy (From Quantum Mechanics) provides the pressure that prevents gravity from collapsing the star.
Pauli Exclusion Principle:
No two electrons can be in the same place at the same time, doing the same thing.
Electrons can exert a powerful outward pressure to keep from getting too close together!

27. White Dwarf Stars
Heat is left over from energy released during gravitational collapse.
Star starts out very hot: 100,000 K!
No way to replace heat radiated into space. Star slowly cools down over billions of years.
End stage is black dwarf – but none have formed yet!

28. The Structure Of A White Dwarf
Mostly a sphere of C, N, and O that is completely electron degenerate.
Atmosphere of hydrogen and helium.
Carbon center may crystallize to form a giant diamond!

29. Daily Quiz 22 – Question 1
What prevents gravity from shrinking a white dwarf to a smaller size?
Helium core fusion.
Helium shell fusion.
Hydrogen core fusion
Degenerate electrons (electromagnetic force).

30. White Dwarf Sizes Higher mass results in smaller, denser white dwarf.
Upper mass limit of 1.44 solar masses.
Called the Chandrasekar limit.
Above this mass, gravity overcomes electron degeneracy.
The white dwarf collapses!

31. White Dwarf Sizes

32. Novae! Occur in binary systems.
One star is “normal” (often a giant or supergiant).
Other star is a white dwarf.

33. Novae! Companion star loses mass to the white dwarf.
Forms an accretion disk that deposits hydrogen onto the dwarf’s surface.
Hydrogen crushed to degeneracy.
Pressure and temperature increase as more hydrogen is added.
“Kindling point” is reached, and …

34. Novae! Surface of dwarf is consumed in a thermonuclear explosion!
Light output jumps to 10,000’s to 100,000’s of times normal!
Hydrogen layer is ejected from white dwarf.
White dwarf is not “damaged”
Process begins again.
Most nova recur!

35. A much bigger class of stellar explosion is called a Supernova
And Now: Supernovae!
A much bigger class of stellar explosion is called a Supernova

36. Supernovae have two types:
Supernovae classed by spectrum:
Type I
Spectrum shows no hydrogen lines.
Some Type I SN’s just as bright as Type II: called Type Ib.
Remaining Type I SN’s soar to 4 billion times solar luminosity, then fade quickly: called Type Ia.
Type II
Spectrum shows hydrogen lines.
Caused by core collapse in massive star. Hydrogen lines from exploding outer layers of star.

37. Type Ia Supernova Some supernova are exploding white dwarfs.
How do you blow up a white dwarf?
Start with a star system similar to setup for a nova:
White dwarf drawing material from companion star.

38. Blowing Up White Dwarfs
BUT white dwarf is very close to Chandrasekar Limit (1.44 solar masses).
Matter “stolen” from companion star drives mass above Chandrasekar Limit before a nova can occur.

39. Blowing Up White Dwarfs
White dwarf collapses. Internal temperature reaches kindling point for Carbon before dwarf reaches neutron degeneracy.
Gas still electron degenerate – no pressure/temperature thermostat:
Runaway fusion – all carbon fused all at once!
Resulting thermonuclear explosion totally blasts the white dwarf apart! Result is a Type Ia Supernova!

40. Daily Quiz 22 – Question 2
What can happen to the white dwarf in a close binary system when it accretes matter from the companion giant star?
The white dwarf can become a main sequence star once again.
The white dwarf can ignite the new matter and flare up as a nova.
The white dwarf can accrete too much matter and detonate as a supernova type Ia.
Either the white dwarf can ignite the new matter and flare up as a nova, or the white dwarf can accrete too much matter and detonate as a supernova type Ia.

41. And Now: Type Ib and II Supernovae!
The Times Listed Are For An M=25 Star

42. The Supergiants Core runs low on H fuel. Collapses and ignites He.
He burning creates C, N, and O.
Ignites H to He burning shell around core.
Star’s luminosity increases. Swells in size.

43. Countdown to Disaster After 7 million years:
H to He fusion in core ends.
He to C, N, O fusion in core begins.
H to He burning shell forms.
Star becomes supergiant.

44. Countdown to Disaster 500,000 years later: 600 years later:
He in core exhausted.
Core collapses, heats up to 800 million K.
C, N, O burning begins, producing Ne and Mg.
600 years later:
Core C, N, O supply used up.
Core collapses, heats up to 1.5 billion K.
Ne and Mg burning begins, producing Si.

45. Countdown to Disaster Six months later:
Core supply of Ne and Mg used up.
Core collapses, heats to 3 billion K.
Si fusion begins, producing Fe.
Now there’s a problem! Remember, we can’t fuse iron into heavier elements and make energy!

47. Countdown to Disaster One day after Silicon fusion begins:
Si is running low in the core.
Heat/Pressure from Si fusion cannot support Fe core.
Fe core begins to collapse. Core heats up.
Fe cannot be fused into heavy elements (and still release energy)!

48. Countdown to Disaster Only milliseconds to go:
Temperature in Fe core soars above 100 billion K!
Two nuclear reactions can occur at this temperature:
Neutronization – protons and electrons react to form neutrons.
Photodisintegration – photons hit Fe nuclei and shatter them into He nuclei!

49. Countdown to Disaster
Both reactions require energy! Core rapidly cools down!
Loss of heat/pressure speeds up collapse!
Result is a catastrophic, runaway collapse of the Fe core!

50. The Fuse is Lit! 500 km Fe core collapses to 10 km across.
Reaches same density as nuclear matter.
Core collapse stops abruptly as core becomes unimaginably rigid.
Outer layers of star slam into now rigid core at extreme speeds.
Shockwave forms, rocketing outward through the star!

51. KABOOM! One hour later: Shockwave erupts through surface of star.
Everything but collapsed core blasted into space: star dies in a spectacular explosion!

52. The Supernova Star-destroying explosion called a supernova.
Light output exceeds 600 million solar.
Extreme heat/energy in shockwave results in nuclear fusion in outer layers of star.
Fusion reactions in supernova create elements heavier than Fe.

53. Type Ib Supernovae
Similar to how a type II supernova happens, but without hydrogen lines in the spectra.
How to “get rid” of hydrogen lines?
Eject hydrogen-rich outer layers before core collapse.
Example: Wolf-Rayet stars.
>40 solar masses.
Extremely unstable: violent stellar wind eventually ejects outer layers of star.
After core collapse and supernova, very little hydrogen is left in star to create spectral lines.

54. Type Ib Supernovae
Similar to how a type II supernova happens, but without hydrogen lines in the spectra.
How to “get rid” of hydrogen lines?
Could also strip outer layers by being part of a binary system.

55. Daily Quiz 22 – Question 3
Why can't massive stars generate energy from iron fusion?
The temperature at their centers never gets high enough.
The density at their centers is too low.
Iron fusion consumes energy.
Not enough iron is present.

56. Observations of Supernovae
Supernovae can easily be seen in distant galaxies.

57. Local Supernovae and Life on Earth
Nearby supernovae (< 50 light years) could kill many life forms on Earth through gamma radiation and high-energy particles.
At this time, no star capable of producing a supernova is < 50 ly away.
Most massive star known (~ 100 solar masses) is ~ 25,000 ly from Earth.

58. Remnant of a supernova observed in 1054
Supernova Remnants
X-rays
The Crab Nebula:
Remnant of a supernova observed in 1054
Cassiopeia A
Optical
The Veil Nebula
The Cygnus Loop

59. The Remnant of SN 1987A Most recent nearby SN was in February 1987.
The Remnant of SN 1987A
Most recent nearby SN was in February 1987.
Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays.

61. Daily Quiz 22 – Question 4
Which type of star eventually develops several concentric zones of active shell fusion?
Low mass stars.
Medium mass stars.
High mass stars.
White dwarfs.

62. The Neutron Star
Formed from the collapsing iron core of a massive star.
Core collapses until neutron degenerate.
Often 1-2 solar masses squeezed into a ball 20 km across!

63. The Neutron Star
A neutron star has an outer crust (2 km thick) made from super-dense iron.
Inside is an ocean of superfluid neutrons that form whirlpools under the surface of the star.

64. The Neutron Star What are they like? Extremely hot: 1 million K
Rotate very fast: conservation of angular momentum.
Extremely powerful magnetic fields.
Extreme surface gravity.

65. The Neutron Star Very powerful magnetic field.
Believed to create beams of electromagnetic radiation.

66. Pulsars
As a neutron star rotates, the beams sweep through space.

67. Pulsars
When a beam sweeps over the Earth, we see a flash of light.

68. Pulsars
Since the neutron star rotates so quickly, the flashes (“pulses”) of light happen many times a second.
When observed with telescopes, these rapidly flashing (“pulsing”) objects were originally called pulsars.
Pulsars are just neutron stars that are easy to observe because the pulsing makes them stand out.

69. The Neutron Star Mass Limit
Like white dwarfs, neutron stars have a mass limit.
Believed to be solar masses (not known for sure)
If a neutron star is over this limit, nothing can stop its collapse. But what does it become?

70. Forming a Black Hole
ANY object that shrinks enough will develop surface gravity high enough to prevent everything from escaping.
One example is a collapsing neutron star: if it collapses enough its surface gravity will get intense enough to form a black hole.

71. Anatomy of a Black Hole
Event Horizon
Singularity
Ergosphere

72. Anatomy of a Black Hole The Event Horizon The Ergosphere
The point of no return – once you enter, you can never leave.
Inside all paths lead to the singularity.
The Ergosphere
Space itself getting dragged around the black hole.
Nothing can stay stationary within.
Once you enter, half your mass must go into the event horizon so the other half can escape.

73. Anatomy of a Black Hole
Why don’t black holes suck in everything in the Universe?
Only dangerous if you are very close

74. Black Holes Are Simple Objects
No Hair Theorem – Black holes have no hair.
“Hair” represents “details” – a black hole is described by only three quantities: mass, electric charge, and rotation.
The Law of Cosmic Censorship – There can be no naked singularities.
Weird, universe-destroying things happen there! They must be “shielded” by an event horizon.

75. The Schwarzschild Radius
The size of a black hole’s event horizon is related to its mass:
R = (3 km) M (in solar masses)
So a 20 solar mass black hole has a Schwarzschild Radius of 60 km.
As a black hole eats, it gets bigger!

76. The Accretion Disk
Although we can’t directly observe the black hole, we can see the X-rays created by superheated gas flowing into the hole in an accretion disk.

77. Observing Black Holes Mass > 3 Msun => Black hole!
Observing Black Holes
No light can escape a black hole
=> Black holes can not be observed directly.
If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity.
Mass > 3 Msun
=> Black hole!

78. Cygnus X-1
The first X-ray source discovered in Cygnus was found to be a very compact object (more than 3 solar masses) in orbit around the blue supergiant star HDE
First example of an X-ray source believed to be a black hole.

79. And Others… But it was certainly not the last.
Many others have been found.
AND the best evidence for real black holes lurks at the centers of galaxies!

80. The Galactic Nucleus The most mysterious part of the galaxy.
The very center is a powerful radio source called Sagittarius A*.

82. Next Time
Read Units 70 and 74