Transcript Stellar Evolution

Stellar Evolution
Astronomy 315
Professor Lee Carkner
Lecture 13
Changes
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This is an illusion due to the fact that
stellar lifetimes are very long
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We can’t watch as any one star
changes, so we have to examine
different stars at different stages
Keys to Evolution
Stars change properties as they go through
their lives
The evolution of a star is based on two basic
things:
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The star will change so that it can get back into
hydrostatic equilibrium
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The mass of the star determines the evolutionary
path it will follow
Fusion and Evolution
The outward force for a star is the
thermal pressure
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If the rate of energy generation in the
core changes it will change the point at
which hydrostatic equilibrium occurs
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Each star follows an evolutionary path
on the HR diagram (as T and L change)
Pre-Main Sequence Evolution
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H.E. is not reached again
until fusion begins
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Starts above the main
sequence and moves down
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The Main Sequence
As hydrogen is converted to
helium the core gets a little
denser and reactions speed
up raising the luminosity
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What happens when all the
hydrogen is gone?
Post Main Sequence
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The star then begins a series of other
types of fusion reactions
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Star leaves the main sequence and
becomes a giant
Mass and Evolution
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Few million years
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Billions of years
Main sequence lifetime (T) is inversely
proportional to mass:
T = 1/M2.5
Cluster Evolution
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All members of the cluster were born at
the same time but have different masses
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High mass stars first
Cluster Evolution
Cluster Ages
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The higher mass the stars the lower the
age
The point at which the cluster diverges
from the main sequence is called the
turn-off point
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The Pleiades
NGC 3293
Evolution
of a Cluster
Extrapolation
If A0 stars live for 440 million years and F0 stars
live for 3 billion year, how long do A3 stars live?
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3X109 – 4.40X108 = 2.56X109
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2.56X109/10 = 2.56X108
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(2.56X108)(3) = 7.68X108
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(4.40X108)(7.68X108) = 1.21X109 = 1.21 billion
years
Post Main Sequence Evolution
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Core becomes denser and contracts
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Called the shell hydrogen burning
Star burns from the inside out
Above the Main Sequence
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This energy expands the outer layers of
the star
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The expansion cools the outer layers as
well
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The star moves up and right above the
main sequence becoming a giant
Becoming a Giant
Structure of a Giant
Helium Burning
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In some stars this happens very rapidly
in a helium flash
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Star becomes hotter and less luminous
as the core readjusts
Burning Other Elements
Helium burning happens very rapidly
and soon the core is full of carbon and
oxygen
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If the star is massive enough it will
burn C and O into other elements
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This is where everything heavier than
He comes from
The End
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Number of elements a star processes depends
on mass
Elements end up in layers around the core
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A star spends only about 10% of its life as a
giant (for solar mass star about 1 billion
years)
Evolution of a Solar Mass Star
7) Main Sequence
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8) Red Giant
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9) Helium Flash
Star rapidly
burns He in core
Evolution of a Solar Mass Star
10)Horizontal
Branch
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11)Asymptotic
Giant Branch
C and O core
contracts, He and H
burns in shell, star
expands and cools
Which Way Does the Star Go?
Up and to the
right (8-9, 10-11)
Contracting core
and shell burning
cause move to
higher L, lower T
Down and to the
left (9-10)
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Next Time
Read Chapter 21.1-21.5