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Stellar Evolution
Lecture 14 NGC 7635: The Bubble Nebula (APOD)
Prelim Results
0
1
2
3
4
5
6
7
8
9
1 6
11
16
21
26
31
36
41
46
51
56
61
66
71
76
81
86
91
96
Mean = 75.7
Stdev = 14.7
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Energy Transport in Stars
• How does the energy produced get out?
• Energy can be transported by
– Conduction
– Convection
– Radiation
• Stars mainly use the latter two methods
– Trade between convection and radiation depends on the star and region within a star
– White dwarfs and neutron stars are exceptions
A Model of the Sun
CORE
Tsurface ~ 6000 K
Radiative Zone
Temp. Density Energy
(106 K) g/cm3 Transport
Core ~ 15 100 Convective
Rad. ~ 3 1 Radiative
Conv. ~ 1 0.1 Convective
Convective Zone
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The Interiors of Stars
Radiative Zone
3.5 solar masses
1 solar mass
0.5 solar masses
Convection Zone
Radiative Zone
Energy Transport Summary
• Massive stars (> 2 Msun) have small convective cores and large radiative envelopes.
• Low mass stars (< 1 Msun) have small radiative cores and large convective envelopes.
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The Balance of Stellar Life
• Hydrostatic Equilibrium is the balance of gravity and pressure in each layer of a star.
• This keeps a star from collapsing (or expanding).
• This balance is maintained as stars age, so that the size might shrink or grow to maintain it.
The Life Cycle of Stars
• Birth: Grav. Collapse of Interstellar Clouds
“Hayashi Contraction” of Protostar
• Life: Stability on Main-Sequence
Long life - energy from nuclear reactions in the core (E = mc2)
• Death: Lack of fuel, instability, variability,
expansion (giants, supergiants),
explosions!!
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Main-Sequence Evolution
• Fusion is occurring in the cores of stars.
• H is being converted to He.
• Since 4 particles are converted to 1, the pressure drops.
• The core collapses and heats up.
• This heats the outer layers which expand outward.
Stars evolve, even on the Main-Sequence
Lum
ino
sity
(Lsu
n)
O B A F G K M
Zero Age Main-Sequence
Temperature Ref: Seeds
30 M
3 M
Initial Sun
Present Sun
1010 yrs
6x108 yrs
5x106 yrs
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Sun: On the Main-Sequence
• 5 billion years ago:
– Beginning of its life on main-sequence
– Sun had 1/3 luminosity it has now.
• 5 billion years from now:
– End of its life on main-sequence
– Sun will have twice the luminosity it has now.
Stellar Evolution
• When H is exhausted, the core shrinks.
• It heats up but can not yet burn He, which needs 100,000,000 K!
• The high temperatures ignite a shell of H around the core.
• The increased pressure drives the envelope of the star outward.
• Creating a giant or supergiant.
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Giant and Supergiant Stars
• Luminosity steadily climbs as shell fusion of H accelerates
• Expanded star: very large radius => large luminosity ( L = 4pR2 sT 4 ) • Uneasy stellar evolutionary stage • Variability • Mass loss since gravity on the surface is weak • Very high, and increasing temperature in the core
L 103 L
!
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Stellar Evolutionary Tracks
Late stages of Stars
• The Helium Flash:
- When Tcore ~ 108 K, He begins to fuse:
He4 + He4 (Be8)*
• Be8 is highly unstable – another He4 must come along within 10-8 seconds!
He4 + (Be8)* C12
Triple process: 3 He4 C12
• First realized by Salpeter (Cornell)
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Late stages of Stars • Onset of He4 fusion is explosive in solar mass stars
– Core was largely supported by electron degeneracy pressure – not dependant on T
– As T 108 K, reactions take off T, but P stays the same!
– Reactions run away since T40 – bomb!
– L ~ 1011 L
– but we don’t see this!
– L lifts the core and enables stable He4 fusion at core: Luminosity stabilizes at L ~ 30 to 100 L
– H fusion in shell continues, but at a much slower rate L goes down
Late stages of Stars
• Eventually He in core is exhausted – Core then must begin contracting again, raising its
temperature
– Ignites He shell burning around core
– We now have twin layers of He and H shell burning – at ever increasing rates
– Eventually, for solar mass stars, core stabilizes under electron degeneracy pressure
– Envelope is ejected as a “planetary nebula”
– Core remains as a “white dwarf”
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Evolution of a star in the H-
R diagram
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Evolution of the Sun Lu
min
osi
ty (L
sun)
O B A F G K M
Temperature
Hayashi Contraction
Explo- sion
Red Giant
Protostar
Instability
White Dwarf
Cloud Collapse
Lum
ino
sity
(Lsu
n)
O B A F G K M
Temperature
Hayashi Contraction
Protostar
Cloud
SuperGiant
Evolution of a 20 Solar Mass Star Supernova
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