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Announcements: -Public Viewing THIS Friday Evergreen Valley College http://www.evc.edu 7PM-10:30 check website for weather information maps available online Pick up copy of handout: required for credit!! Homework #9 due today Exam #3: May 3 (Chp 12, 13). Chapter 13. - PowerPoint PPT Presentation
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Announcements:
-Public Viewing THIS FridayEvergreen Valley Collegehttp://www.evc.edu7PM-10:30check website for weather informationmaps available online
Pick up copy of handout: required for credit!!
Homework #9 due todayExam #3: May 3 (Chp 12, 13)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display2
Chapter 13
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Introduction
• Where do stars come from?Giant Molecular CloudsBok GlobulesInterstellar Medium (ISM)ProtostarsPre-Main Sequence Stars
• How do they age (evolve)
• What is their fate?
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Bi-polar jetsHerbig-Haro objects (HH objects)Brown DwarfsContraction timescales depend on massHydrostatic Equilibrium
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A Star’s Mass Determines its Core TemperatureA 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.
t
M
L
How much fuel is available
How quickly fuel is used
Mass
Luminosity
= 1010 In solar units
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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) 30,000b) 6 millionc) 1,700d) 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.Outer layers expand
Shell burning begins.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 12C
Alpha particlesTriple 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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AB C
Youngest to oldest:a) B, C, Ab) A, C, Bc) C, A, Bd) C, B, A
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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 lyTypical 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:
Massive stars do not stop with helium fusion – a variety of Massive stars do not stop with helium fusion – a variety of nuclear reactions creates heavier elements.nuclear reactions creates heavier elements.
Formation of heavy elements by nuclear burning processes is Formation of heavy elements by nuclear burning processes is called called nucleosynthesis.nucleosynthesis.
Proton-proton chainProton-proton chainTriple-alpha process (helium to carbon)Triple-alpha process (helium to carbon)44He + He + 1212C = C = 1616O + O + where where is a gamma ray photon is a gamma ray photon1616O + O + 1616O = O = 2828Si + Si + 44HeHe
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Old Age of Massive Stars:
As the temperature of the core increases, heavier elements are As the temperature of the core increases, heavier elements are fused forming concentric layers of elements.fused forming concentric layers of elements.
Iron is the heaviest element fused (at about 1 billion K) - larger Iron is the heaviest element fused (at about 1 billion K) - larger elements will not release energy upon being fused.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
