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Homework #9 due today
Exam #3: May 3 (Chp 12, 13)
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Chapter 13
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Introduction
• Where do stars come from?
Giant Molecular Clouds
Bok Globules
Interstellar Medium (ISM)
Protostars
Pre-Main Sequence Stars
• How do they age (evolve)
• What is their fate?
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Bi-polar jets
Herbig-Haro objects (HH objects)
Brown Dwarfs
Contraction timescales depend on mass
Hydrostatic Equilibrium
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A Star’s Mass Determines its Core Temperature
Hydrostatic Equilibrium:
gas pressure balances
gravity
higher gravity, higher
internal pressure, higher
internal temperature!
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Main Sequence Lifetimes
High-mass stars have more fuel available
(larger gas tanks)
However, they burn their fuel more quickly
(always speeding)
In the end, they run out of gas sooner.
How much fuel is available
Mass
t

How quickly fuel is used
Luminosity
10 M
= 10
 In solar units
L
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A B0 Main Sequence star is 17.5 times more massive
than the Sun and 30,000 times more luminous. Such a
star will spend approximately _____ years on the
Main Sequence.
a) 30,000
b) 6 million
c) 1,700
d) 1.7x1013
e) 17.5x1013
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High mass stars are the first to reach the Main
Sequence and the first to leave!
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What happens to the star when it runs out of hydrogen?
No hydrostatic equilibrium!
Core begins to collapse.
Core temperatures rise.
Hydrogen shell burning.
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What happens to the star when it runs out of hydrogen?
Core contracts and heats up. Shell burning begins.
Outer layers expand Outer layers expand a lot!
Red Giant
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What happens to the star when it runs out of hydrogen?
Radius increases
Surface temperature decreases
Star moves toward upper right corner of HR Diagram
Red Giant!!
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What happens to the star when it runs out of hydrogen?
• Eventually, core temperatures are high enough to
begin fusion of Helium nuclei into Carbon. (T=100
million K)
4He
+ 4He + 4He 
Alpha particles
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12C
Triple Alpha Process
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Sun becomes a Red Giant
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High-mass stars burn their fuel more quickly!
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A
Youngest to oldest:
a) B, C, A
b) A, C, B
c) C, A, B
d) C, B, A
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B
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C
The Demise of a Sun-like Star:
No hydrostatic equilibrium!
Core begins to collapse.
Core temperatures rise.
Hydrogen and Helium shell burning.
Second “red giant” ascent.
And then……
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At the end of its life, a star like the Sun will shed its
outer layers.
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Planetary Nebulae:
The Ring Nebula
Typical size: 0.25 ly
Typical velocity of
expanding material:
20 km/s
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Typical Shape: conical along rotation axis
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Catseye Nebula
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Which of the following sequences correctly describes
the evolution of the Sun from young to old?
a) white dwarf, red giant, main sequence,
protostar
b) red giant, main-sequence, white dwarf,
protostar
c) protostar, red giant, main sequence, white
dwarf
d) protostar, main sequence, white dwarf, red
giant
e) protostar, main sequence, red giant, white
dwarf
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Old Age of Massive Stars:
stars do not stop with helium fusion – a variety of
nuclear reactions creates heavier elements.
Massive
Formation
of heavy elements by nuclear burning processes is
called nucleosynthesis.
Proton-proton chain
Triple-alpha process (helium to carbon)
4He + 12C = 16O + g where g is a gamma ray photon
16O + 16O = 28Si + 4He
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Old Age of Massive Stars:
As
the temperature of the core increases, heavier elements are
fused forming concentric layers of elements.
Iron
is the heaviest element fused (at about 1 billion K) - larger
elements will not release energy upon being fused.
 CORE COLLAPSE!
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NGC 3603: 2 million years old
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Stars like the Sun probably do not form iron cores
during their evolution because
a) all of the iron is ejected when they become
planetary nebulae
b) their cores never get hot enough for them to
make iron by nucleosynthesis
c) the iron they make by nucleosynthesis is all
fused into carbon
d) their strong magnetic fields keep their iron
in the atmosphere
e) none of the above
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