Stellar Evolution Astronomy 315 Professor Lee Carkner Lecture 13.
Post on 20-Dec-2015
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Changes
This is an illusion due to the fact that stellar lifetimes are very long
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:
The star will change so that it can get back into hydrostatic equilibrium
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
If the rate of energy generation in the
core changes it will change the point at which hydrostatic equilibrium occurs
Each star follows an evolutionary path on the HR diagram (as T and L change)
Pre-Main Sequence Evolution
H.E. is not reached again until fusion begins
Starts above the main sequence and moves down
The Main Sequence
As hydrogen is converted to helium the core gets a little denser and reactions speed up raising the luminosity
What happens when all the hydrogen is gone?
Post Main Sequence
The star then begins a series of other types of fusion reactions
Star leaves the main sequence and becomes a giant
Mass and Evolution
Few million years
Billions of years Main sequence lifetime (T) is inversely
proportional to mass:T = 1/M2.5
Cluster Evolution
All members of the cluster were born at
the same time but have different masses
High mass stars first
Cluster Ages
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
Extrapolation If A0 stars live for 440 million years and F0 stars live for
3 billion year, how long do A3 stars live?
3X109 – 4.40X108 = 2.56X109
2.56X109/10 = 2.56X108
(2.56X108)(3) = 7.68X108
(4.40X108)(7.68X108) = 1.21X109 = 1.21 billion years
Post Main Sequence Evolution
Core becomes denser and contracts
Called the shell hydrogen burning Star burns from the inside out
Above the Main Sequence
This energy expands the outer layers of the star
The expansion cools the outer layers as well
The star moves up and right above the main sequence becoming a giant
Helium Burning
In some stars this happens very rapidly in a helium flash
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
If the star is massive enough it will burn C and O into other elements
This is where everything heavier than He comes from
The End
Number of elements a star processes depends on mass
Elements end up in layers around the core
A star spends only about 10% of its life as a giant (for solar mass star about 1 billion years)
7) Main Sequence
8) Red Giant
9) Helium Flash Star rapidly
burns He in core
Evolution of a Solar Mass Star
10)Horizontal Branch
11)Asymptotic Giant Branch C and O core
contracts, He and H burns in shell, star expands and cools
Evolution of a Solar Mass Star
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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