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Chapter 20
Stellar Evolution
(Transparencies)
Evolution of Low-Mass Stars
1. The Sun began its life like all stars
as an intersteller cloud.
2. This cloud collapses due to
gravity into a dense core.
3. In about a million years a small,
hot, dense core called a protostar
forms.
ANNOUNCMENT
This Friday (April 12, 2002)
we will meet in Kennedy
Auditorium again.
Please remind anyone who is
not here today.
4. When the temperature reaches 10
million Kelvin in the core, fusion begins
and transforms the protostar into a
main-sequence star.
5. Low mass stars like the Sun remain on
the main-sequence for about 10 billion
years. Massive stars stay on the mainsequence for about 1 billion years.
 6. Hydrogen fusion begins in a shell
around the core and the star expands
into a Red Giant.
 7. After most of the hydrogen is fused
into helium, helium fusion begins in an
event called the Helium Flash.
 8. Stars can then become unstable and
turn into pulsating stars like RR Lyrae
Variables or Cephied Variables.
 9. As a star burns helium into carbon
the radiation pressure pushes the star's
outer atmosphere away from the core
creating a Planetary Nebula.
 10. This leaves an exposed core called a
White Dwarf. These have about the
same diameter as the Earth.
Evolution of High-Mass Stars
1 to 5. Same as before…
 intersteller cloud  dense core 
  protostar  main-sequence star
6. When a high-mass star exhausts
the hydrogen fuel in its core the
star leaves the main sequence and
begins to burn helium.
7. The star becomes a Red
Supergiant after millions of years of
helium fusion.
8. When helium is depleted, fusion
of heavier elements begins. This
process is called nucleosynthesis.

H  He  C  O  Si  Fe
 9. Fusion stops with iron (Fe) and a star
with an iron core is out of fuel.
 Reason: Iron atoms cannot fuse and
release energy.
 10. The core collapses due to reduced
pressure converting the iron core into
mostly neutrons.
 11. The core pressure then surges and
lifts the outer layers from the star in a
titanic explosion - a supernova!
11. The core pressure then surges and
lifts the outer layers from the star in a
titanic explosion - a supernova!
Nucleosynthesis
Evolutionary Time Scales for a 15 M Star
Fused
Products
H
4
He
12
C
He
12
C
16 20
O, Ne,
24
4
Mg, He
16 24
O, Mg
28
32
Si, S
56
Fe
Neutrons
20
Ne +
16
O
28
Si +
56
Fe
4
Time
7
Temperature
6
10 yrs.
6
Few X 10 yrs
1000 yrs.
4 X 10 K
8
1 X 10 K
8
6 X 10 K
Few yrs.
One year
Days
< 1 second
1 X 10 K
9
2 X 10 K
9
3 X 10 K
9
> 3 X 10 K
9
Changing H-R diagram
of a hypothetical star
cluster.
The Double
Cluster “h and
 Persei
Only 10
million years
old
Glodular cluster 47 Tucanae.
~ 11 billion years old
Evolution of a Binary Star
Each starSystem
can be pictured as being surrounded by a “zone of
influence” or Roche lobe.
What can happen?
What can happen?
H-R Diagram Questions
1. What property is measured along
the horizontal axis?
2.
… along the vertical axis?
3. Where are the red giants?
4.
… the white dwarfs?
5.
… the hottest stars?
6.
… the coolest stars?
7.
… the largest stars?
8.
… the smallest stars?
H-R Diagram Questions
9. Where are O class stars?
10.
… M class stars?
11.
… G class stars?
12. Where is the Sun?
13. Where are the high-mass mainsequence stars?
14. Where are the low-mass mainsequence stars?
15. Where are the oldest stars?
16. Which stars along the main-sequence
live the longest?
End of Chapter 20