the lives of stars chapter 12. life on main-sequence zero-age main sequence (zams) –main sequence...
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The Lives of Stars
Chapter 12
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Life on Main-Sequence
• Zero-Age Main Sequence (ZAMS) – main sequence location where stars are born
• Bottom/left edge of main sequence• H fusion begins
• As star ages– Energy source is H fusion
• composition changes• H -> He
– Location in H-R diagram slowly changes• begins to move away (right/up) from ZAMS• broadens (smears out) main sequence
Stellar Lifetimes
• 90% of star’s life spent in main sequence
• Lifetime depends on mass
Main Sequence to Red Giant
• H in core used up– He “ash” in core
– no more fuel for energy
• Gravity begins to win – core contracts, gets hotter
– start H fusion in shell surrounding core
– outer layers expand
• Star becomes Red Giant
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Further Evolution
• As core contracts– temperature increases– becomes hot enough– Begins to fuse
He into C
• Energy production– stops core collapse– star is stable again
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Beginning of the End
• When He exhausted, star out of fuel again– core collapse resumes– He shell burning begins
• outer layers expand
• Star becomes Red Supergiant– strong mass loss occurs via stellar wind
Stellar Evolution
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Evolution of Massive Stars
• Up to C-O core, evolution same for all stars• From then on, different paths• Low-mass stars:
– no C burning– core energy generation complete– star dies
• High-mass stars:– C burning begins in core– Eventually fuse heavier
and heavier elements
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Making Heavy Elements• High-mass stars fuse heavier elements in cores
C -> Ne O -> Si -> Fe– at each step, core collapses further
• This nucleosynthesis produces most elements up to iron (Fe)
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Evolutionary Tracks
Star Clusters
• Star clusters – Stars born same location, same time– contains stars with different masses – permits study of stellar evolution
• Age of cluster – determined by which stars have departed main
sequence
Globular Clusters
• spherical “ball” of stars– concentrated toward center
– 10,000 - 100,000 stars
• about 150 around our Galaxy– very distant from Sun
(>10,000 LY)
– sizes 50-300 LY diameter
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Open Clusters
• 100’s of stars (up to 1000)
– smaller than Globular clusters – no central concentration
• Found within the Galaxy– 1000’s known– diameters < 30 LY
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H-R Diagrams of Clusters
• Cluster ages are different– globular clusters oldest
– open clusters relatively young
• H-R diagrams indicate age– interpret using stellar evolution
theory
Cluster Evolution
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Estimating Cluster Ages
• Make H-R diagram for cluster• Have all stars arrived at ZAMS?
– if not, cluster extremely young
• Have some stars departed Main Sequence?
– cluster is older
– main sequence turn-off point • determines cluster age
• the farther down the turn-off, the older the cluster
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Theoretical NGC 2264
Theoretical 47 Tuc
Ages of Clusters
• Globular clusters– only lowest part of main sequence is present– typical age: 15 billion yrs
• Open clusters– much younger than globulars– all ages: 1 million yrs up to a few billion yrs
Stellar Death
Death of Stars
• Two possibilities– Low mass stars < 5 Msun
• end is planetary nebula WHITE DWARF
– High mass stars• end is type II supernova either:
NEUTRON STAR or BLACK HOLE
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Fate of Low Mass Stars
• During end of red supergiant phase– large mass loss
• star loses entire envelope, revealing core
– core becomes white dwarf• white dwarf slowly cools
• eventually becomes “black dwarf”
Planetary Nebulae
• During transition to white dwarf– outer layers expanding– exposes hot core; – shell material heated; begins to
glow
• Result is a planetary nebula– tens of thousands known in our
Galaxy
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Planetary Nebulae
Evolution to White Dwarf• Low mass stars
– cannot fuse carbon– lose energy source
• gravity wins• core contracts
• Core contraction– produces very high density– electron degeneracy pressure
• stops core collapse
– remnant core becomes white dwarf
White Dwarf Stars
• Properties – diameter ~ same as Earth– very dense
(1 tsp = several tons!)– very hot on surface
• Chandrasekhar limit– Maximum mass = 1.4 Msun– larger stars collapse
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“Diamond stars”??
Fate of Massive Stars• High-mass stars fuse heavier elements in cores
C -> Ne O -> Si -> Fe– at each step, core shrinks further
– fusion stops when iron (Fe) produced
• This nucleosynthesis produces most elements up to iron
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Collapse and Explosion
• When core mass exceeds 1.4 Msun– collapse continues unabated– all protons converted into neutrons– collapse abruptly halted by neutron
degeneracy pressure – results in shock wave & explosion– Produces type II supernova
• Some material falls onto core– M < 2.5 Msun neutron star remains– M > 2.5 Msun black hole produced
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Supernovae
• Supernovae as bright as entire galaxy • Ejection velocities
– millions of miles/hr (~10,000 km/s)
• Supernova explosion – heavy elements (C, N, O, Fe) returned to
interstellar medium for recycling– also produces elements heavier than iron
• elements such as gold, silver, uranium
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Pulsars
• Pulsating radio sources – Periods .001-10 seconds– Very regular– also observed in optical (crab nebula)
• Pulsars = spinning neutron stars– fast period requires very small objects– neutron stars only possibility
• Radiation and particles beamed out from magnetic poles– spinning lighthouse effect results in
observed “pulses”
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Novae
• Novae NOT same as Supernova– less energetic; not as bright
• Binary system with mass transfer onto WD– material accumulates on WD surface
– eventually nuclear detonation occurs
– result is a nova
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White Dwarf Supernovae• As mass accumulates, WD exceeds Chandrasekhar limit
– rapid core collapse occurs
– Resulting explosion = Type I supernova
• Properties somewhat different than Type II SN (caused by massive star explosions)
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