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Stellar Evolution
Astronomy 315
Professor Lee Carkner
Lecture 13
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 Evolution
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

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?

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
Becoming a Giant
Structure of 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)
Evolution of a Solar Mass Star
7) Main Sequence

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

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)

Next Time
Read Chapter 21.1-21.5