Slide 1 Death of Stars “All hope abandon, ye who enter here” Dante.
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Transcript of Slide 1 Death of Stars “All hope abandon, ye who enter here” Dante.
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Slide 1
Death of StarsDeath of Stars
“All hope abandon, ye who enter here” Dante
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Slide 2
The Dumbbell nebula (M76): a dying sun-like star
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Slide 3
Life of stars:Gravity is everything
• Stars are born due to gravitational collapse of gas clouds• Star’s life is a battle between thermal pressure generated by
nuclear reactions and gravity
• Eventually, a star loses this battle, and gravity overwhelms
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Slide 4
What happens when all hydrogen is converted into helium in the core??
Mass defines the fate of the star
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Slide 5
Evolution on the Main Sequence
Luminosity L ~ M3.5
A star’s life time T ~ energy reservoir / luminosity
T ~ M/L ~ 1/M2.5
Energy reservoir ~ M
Massive stars have short
lives!
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Slide 6
Evolution on the Main Sequence
Zero-Age Main
Sequence (ZAMS)
Main-Sequence stars live by
fusing H to He.
Finite supply of H => finite life time.
MS evolution
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Slide 7
1. Accumulation of helium ash in the core
Helium burning requires higher temperaturesThe star loses the ability to generate nuclear energy
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Slide 8
2. Core collapses and gets hotter, while the envelope expands and cools down
Note the hydrogen fusion in a shell surrounding the core!It is now hot enough there.
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Slide 9
Evolution off the Main Sequence: Expansion into a Red Giant
Hydrogen in the core completely converted into He:
H burning continues in a shell around the core.
He Core + H-burning shell produce more energy than needed for pressure support
Expansion and cooling of the outer layers of the
star Red Giant
“Hydrogen burning” (i.e. fusion of H into He) ceases in the core.
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Slide 10
Red Giant Evolution
4 H → He
He
H-burning shell keeps dumping He
onto the core.
He-core gets denser and hotter until the
next stage of nuclear burning can begin in
the core:
He fusion through the
“Triple-Alpha Process”
4He + 4He 8Be + 8Be + 4He 12C +
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Slide 11
If M > 0.4 Msun, the temperature reaches 100 million K Nuclear fusion of helium and heavier elements starts
Carbon and then oxygen are produced
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Slide 12
Red Dwarfs: no red giant phase
Stars with less than ~ 0.4
solar masses are completely
convective.
Hydrogen and helium remain well mixed throughout the entire star.
No phase of shell “burning” with expansion to giant.
Star not hot enough to ignite He burning.
Mass
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Slide 13
Sunlike Stars
Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.
Expansion to red giant during H burning shell phase
Ignition of He burning in the He core
Formation of a degenerate C,O core
Mass
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Slide 14
In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.
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Slide 15
Outer layers expand due to radiation pressure from a hot core
The star becomes a Red Giant
• Surface temperature drops by a factor of ~ 2• The radius increases by a factor of ~ 100• Luminosity increases ~ R2 T4 ~ 100-1000 times
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Slide 16
Future of the Sun
(SLIDESHOW MODE ONLY)
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Slide 17
Supergiant
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Slide 18
A star leaves the main sequence and becomes a red giant
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Slide 19
Simulated evolution of the main sequence
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Slide 20
While outer layers are expanded, inner helium core contracts, and its temperature rises
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Slide 21
Helium Fusion
When pressure and temperature in the He core
become high enough,
He nuclei can fuse to build heavier elements:
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Slide 22
• For stars with M > 4 Msun, helium fusion proceeds gradually as the core heats up.
• For stars with 0.4 Msun < M < 4 Msun, electrons in helium core becomes degenerate and their pressure does not increase with temperature. Helium fusion results in runaway explosion – helium flash.
• In any case, carbon ash accumulates in the core. Core contracts and becomes degenerate. Then helium burns in the shell. Subsequent evolution depends on the core mass. Eventually all reactions stop and the star dies.
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Slide 23
Inactive He
C, O
Red Giant Evolution (5 solar-mass star)
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Slide 24
Fusion Into Heavier Elements
Fusion into heavier elements than C, O:
requires very high temperatures; occurs only
in very massive stars (more than 8 solar masses).
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Slide 25
The Life “Clock” of a Massive Star (> 8 Msun)
Let’s compress a massive star’s life into one day…
12 12
3
45
67
8
9
1011
12 12
3
45
67
8
9
1011
Life on the Main Sequence
+ Expansion to Red Giant: 22 h, 24 min.
H burning
H He
H He
He C, O
He burning:
(Red Giant Phase) 1 h, 35 min, 53 s
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Slide 26
The Life “Clock” of a Massive Star (2)
H HeHe C, O
C Ne, Na, Mg, O
Ne O, Mg
H He He C, O
C Ne, Na, Mg, O12 1
23
4567
8
910
11
C burning:
6.99 s
Ne burning:
6 ms 23:59:59.996
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Slide 27
The Life “Clock” of a Massive Star (3)
H HeHe C, O
C Ne, Na, Mg, ONe O, Mg
O burning:
3.97 ms 23:59:59.99997
O Si, S, P
H HeHe C, O
C Ne, Na, Mg, ONe O, Mg
Si burning:
0.03 ms
The final 0.03 msec!!
O Si, S, PSi Fe, Co, Ni
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Slide 28
Inside Stars
(SLIDESHOW MODE ONLY)
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Slide 29
The Fate of Our Sun and the End of Earth• Sun will expand to a
Red giant in ~ 5 billion years
• Expands to ~ Earth’s radius
• Earth will then be incinerated!
• Sun may form a planetary nebula (but uncertain)
• Sun’s C,O core will become a white dwarf
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Slide 30
The Final Breaths of Sun-Like Stars: Planetary Nebulae
The Helix Nebula
Remnants of stars with ~ 1 – a few Msun
Radii: R ~ 0.2 - 3 light years
Expanding at ~10 – 20 km/s ( Doppler shifts)
Less than 10,000 years old
Have nothing to do with planets!
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Slide 31 p. 192
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Slide 32
The Dumbbell Nebula in Hydrogen and Oxygen Line Emission
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Slide 33
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Slide 34
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Slide 35 p. 193
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Slide 36 p. 193
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Slide 37 p. 193
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Slide 38 p. 193
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Slide 39
What is left??
A stellar remnant: white dwarf, composed mainly of carbon and oxygen
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Slide 40
Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A.
Wobbling motion of Sirius A
1850: a strange star was discovered
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Slide 41
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Slide 42
Sirius B is very hot: surface temperature 25,000 KYet, it is 10,000 times fainter than Sirius A
It should be very small: R ~ 2 Rearth
Its mass M ~ 1 Msun
It should be extremely dense!
M/R3 ~ 106 g/cm3
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Slide 43
White DwarfsDegenerate stellar remnant (C,O core)
Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
Chunk of WD material the size of a beach ball would outweigh an ocean liner!
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Slide 44
All atoms are smashed and the object is supported by pressure of degenerate electrons
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Slide 45
Degenerate gas
-The core is compressed until the inter-particle distance = de Broglie wavelength-One particle occupies finite volume in space and in momentum space- we can decrease the particle size by increasing its velocity (energy)- Pauli exclusion principle permits only one particle per each state
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Slide 46
secJ102
; 34
hh
kmvp
Perhaps one of the key questions when Bohr offered his quantized orbits as an explanation to the UV catastrophe and spectral lines is, why does an electron follow quantized orbits? The response to this question arrived from the Ph.D. thesis of Louis de Broglie in 1923. de Broglie argued that since light can display wave and particle properties, then perhaps matter can also be a particle and a wave too.
Energy and momentum of a particle are related to wavelength:
Wave-particle duality
Wave packetm
k
m
pE
22
222
mv
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Slide 47
Your de Broglie wavelength:
msmkg
sJ
mv31
2
34
10/1010
10
de Broglie wavelength for the electron in an atom:
msmkg
sJ
mv10
529
34
10/1010
10
Note the velocity dependence!
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Slide 48
Degenerate matter
N particles in volume V
Density of particles = N/V
Share of volume per particle: V = V/N = 1/nDistance between particles = d
V ~ d3 ; therefore d ~ (V)1/3 = n-1/3
d
Particle density
3
3deBroglie )(
1~
mv
V
Nn
When particles are so close that d ~ deBroiglie, gas becomes degenerate
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Slide 49
Pauli exclusion principle:Only one electron per quantum state
With increasing density, particles occupy states with higher velocity and kinetic energy
Pressure increases; however the temperature does not increase!
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Slide 50
Equation of State:
Tk
P
Molecular weight =1 for pure hydrogen, 4 for helium, etc.
Classical gas
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Slide 51
Equation of State:
3/5KP
Degenerate gas
Low density,small kinetic energy
High density, large kinetic energy
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Slide 52
Strange properties of degenerate matter
• It strongly resists compression: P ~ 5/3
• Pressure does not depend on temperature
• Gas becomes more ideal with increasing density
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Slide 53
Evolution of sun-like stars on H-R diagram
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Slide 54
White Dwarfs (2)
Low luminosity; high temperature => White dwarfs are found in the lower left corner of the
Hertzsprung-Russell diagram.
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Slide 55
White dwarfs in a globular cluster
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Slide 56 p. 152
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Slide 57
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Slide 58
As it cools, carbon crystallizes into diamond lattice.Imagine single diamond of mass 1030 kg!Don’t rush, you would weigh 15,000 tons there!
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Slide 59
Summary of Post Main-Sequence Evolution of Stars
M > 8 Msun
M < 4 Msun
Evolution of 4 - 8 Msun stars is still uncertain.
Fusion stops at formation of C,O core.
Mass loss in stellar winds may reduce them all to < 4 Msun stars.
Red dwarfs: He burning never ignites
M < 0.4 Msun
Supernova
Fusion proceeds; formation of Fe core.