1) Stars spend most of their lives stably fusing hydrogen into helium on the main sequence. 2) After the main sequence, low mass stars evolve through red giant, asymptotic giant, and planetary nebula stages before becoming white dwarfs. 3) More massive stars evolve faster and burn carbon to produce oxygen in later stages.

1. Introduction to modern astronomy17
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

2. •Last week, we studied how stars form. Stars are born in the interstellar medium and
evolve to the HR diagram's main sequence.

3. •Last week, we studied how stars form. Stars are born in the interstellar medium and
evolve to the HR diagram's main sequence.

4. 10.Stellar evolution
（恒星演化）

5. Evolution of stars
•The meaning of “evolution” for stars is different from evolution in biology.
Biology
The change in the characteristics of a population of plants or
animals over many generations
Stars The change of a single stellar star during its lifetime

6. Evolution of main-sequence stars
Let’s look at the evolution of stars after
main sequence.
•As we saw, once stars start nuclear fusion in the
core, they become main sequence star (stage 7).
•They burn Hydrogen and generates Helium.
H ⟶ 4
He
Stage 8: The sub-giant branch
•As hydrogen burning continues, hydrogen atoms are
depleted（消耗）and hydrogen burning finally stops.

7. Evolution of main-sequence stars
Stage 8: The sub-giant branch
•The number of Hydrogen atom is decreased
and the number of Helium is increased.
•As hydrogen burning continues, hydrogen atoms are
depleted（消耗）and hydrogen burning finally stops.

8. Evolution of main-sequence stars
Stage 8: The sub-giant branch
4
He
•Since hydrogen burning stops, the core
pressure cannot support its gravity and the
core shrinks.

9. Evolution of main-sequence stars
Stage 8: The sub-giant branch
4
He 4
He
•The surface temperature of star is decreased
although the luminosity is slightly changed.
•Since hydrogen burning stops, the core
pressure cannot support its gravity and the
core shrinks.

10. Evolution of main-sequence stars
Stage 9: The Red-Giant Brach
4
He
4
He
•When helium core shrinks, the density and
temperature in the core become higher again. As a
result, energy is generated.
•The energy is carried to surface by convection
（对流）. Then, the star becomes larger and more
luminous. This star is called red-giant.

11. Evolution of main-sequence stars
•Red-Giant stars are largeand luminous
•We see the evolution of stars after red-giant stars.

12. Evolution of main-sequence stars
Stage 10: Helium fusion
•When the temperature of the helium core reaches ,
helium begins to burn in the core.
108
K
4
He + 4
He → 8
Be + energy
8
Be + 4
He → 12
C + energy.
•The reaction transforms helium into carbon in two
steps.
•This process sis called triple-alpha process.
•Because of quantum physical（量⼦⼒学） effect,
helium fusion becomes violent and the temperature
rises up rapidly (helium flash). But, helium flash does
not increase the luminosity.

13. Evolution of main-sequence stars
Stage 11: Back to the giant branch
4
He + 4
He → 8
Be + energy
8
Be + 4
He → 12
C + energy.
•As helium fuses to carbon, a new carbon-
rich core begins to form.
•Star contains carbon core surrounded by a helium-
burning shell, which is in turn surrounded by a
hydrogen-burning shell.
•The outer envelope of the star expands, much as it did earlier during the first red-giant
stage again.

14. Evolution of main-sequence stars
Stage 11: Back to the giant branch
•This second process to become red-giant star is
called asymptotic giant branch（红巨星⽀）.
•The carbon core grows in mass as more and
more carbon is produced in the helium-
burning shell.
•But, it continues to shrink in radius, driving
hydrogen- and helium-burning shells to higher
and higher temperatures and luminosities.

15. The death of low mass stars
•The fate of stars depend on their mass.
Or
Solar-mass stars like the sun
Massive stars
Carbon core stops
nuclear fusion
?
Carbon core
starts new
nuclear fusion
Carbon core

16. The death of low mass stars
•The fate of stars depend on their mass.
Or
Solar-mass stars like the sun
Massive stars
Carbon core stops
nuclear fusion
?
Carbon core
starts new
nuclear fusion
Carbon core

17. The death of low mass stars
Stage 12: A planetary nebula（⾏星状星云）
•Carbon core no longer generates energy. But, the outer core shells continue to burn hydrogen
and helium, and they generates carbon more and more.
C
H,He
•The gas stripped from
stars becomes planetary
nebulae.
•As the core exhausts its last remaining fuel, it contracts and
heats up, moving to the left in the HR diagram.
•The planetary nebulae emits ultraviolet emission.

18. The death of low mass stars

19. C
C
The death of low mass stars
H,He
Stage 13: A white dwarf（⽩矮星）
•The carbon core continues to evolve and the core
becomes visible as the envelope recedes.
•The size of carbon core is around ~5000km (similar
to the earth), but the mass is around solar
mass( )
∼ 2 × 1030
kg
The density is ∼ 109
− 1010
kg/m3
So huge!!
•This core has new name, white dwarf.
•If the star is low-mass but close to , the following
nuclear reactions ( ) would
occur.
8M⊙
16
O + 4
He → 20
Ne + energy

20. C
C
The death of low mass stars
Stage 13: A white dwarf（⽩矮星）
•The carbon core continues to evolve and the core
becomes visible as the envelope recedes.
•The size of carbon core is around ~5000km (similar
to the earth), but the mass is around solar
mass( )
∼ 2 × 1030
kg
The density is ∼ 109
− 1010
kg/m3
So huge!!
•This core has new name, white dwarf.
•If the star is low-mass but close to , the following
nuclear reactions ( ) would
occur.
8M⊙
16
O + 4
He → 20
Ne + energy

21. C
C
The death of low mass stars
Stage 13: A white dwarf（⽩矮星）
•The carbon core continues to evolve and the core
becomes visible as the envelope recedes.
•The size of carbon core is around ~5000km (similar
to the earth), but the mass is around solar
mass( )
∼ 2 × 1030
kg
The density is ∼ 109
− 1010
kg/m3
So huge!!
∼ 103
− 104
kg/cm3
•This core has new name, white dwarf.
•If the star is low-mass but close to , the following
nuclear reactions ( ) would
occur.
8M⊙
16
O + 4
He → 20
Ne + energy

22. The death of low mass stars
•Sirius B is a white dwarf star, companion to
Sirius A
•Once an isolated star becomes a white dwarf, its
evolution is over (As we will see, while dwarfs in
binary systems may undergo further activity).

23. The death of low mass stars
•Sirius B is a white dwarf star, companion to
Sirius A
•Once an isolated star becomes a white dwarf, its
evolution is over (As we will see, while dwarfs in
binary systems may undergo further activity).

24. Evolution of stars more massive than the sun
•We next consider the fate of more massive stars( ).
≳ 8M⊙
•High mass stars evolve much faster than low mass
stars.
(e.g)
The sun It spends 10 billion years on
main sequence and leave it.
star
10M⊙
It spends 20 million years on
main sequence and leave it.
They leave the main sequence when they run out of
hydrogen in their cores
**Remember that the lifetime of massive stars is
shorter than less massive stars

25. Evolution of stars more massive than the sun
•We next consider the fate of more massive stars( ).
≳ 8M⊙
•This figure compares the post-main-sequence
evolution of three different stars, 1M⊙,4M⊙,10M⊙

26. Evolution of stars more massive than the sun
•We next consider the fate of more massive stars( ).
≳ 8M⊙
•This figure compares the post-main-sequence
evolution of three different stars, 1M⊙,4M⊙,10M⊙
•For star, the path to red-giant is almost
vertically.
1M⊙

27. Evolution of stars more massive than the sun
•We next consider the fate of more massive stars( ).
≳ 8M⊙
•This figure compares the post-main-sequence
evolution of three different stars, 1M⊙,4M⊙,10M⊙
•For star, the path to red-giant is almost
vertically.
1M⊙
•For stars, the path to red-giant
becomes more horizontally
4M⊙,8M⊙

28. Evolution of stars more massive than the sun
•We next consider the fate of more massive stars( ).
≳ 8M⊙
•This figure compares the post-main-sequence
evolution of three different stars, 1M⊙,4M⊙,10M⊙
•For star, the path to red-giant is almost
vertically.
1M⊙
•For stars, the path to red-giant
becomes more horizontally
4M⊙,8M⊙
•More important divergence occurs at .
At higher mass of stars, oxygen is generated
after carbon by nuclear fusion, while it is not
generated in low mass stars.
≳ 8M⊙

29. Evolution of stars more massive than the sun
Betelgeuse Rigel
•Betelgeuse and Rigel are famous post-main sequence stars
•Astronomers think they are currently fusing helium into carbon and oxygen.

30. Evolution of stars more massive than the sun
Betelgeuse Rigel
•Betelgeuse and Rigel are famous post-main sequence stars
•Astronomers think they are currently fusing helium into carbon and oxygen.
•Recently, the brightness Betelgeuse has been decreased.

31. Evolution of stars more massive than the sun
Betelgeuse Rigel
•Betelgeuse and Rigel are famous post-main sequence stars
•Astronomers think they are currently fusing helium into carbon and oxygen.
•Recently, the brightness Betelgeuse has been decreased.
Supernova happens soon!?
Unfortunately, No…it may be due to expansion and shrink by itself.

32. Stellar evolution in binary systems
•Many stars in galaxy are not alone. They are members of binary star system.
•Each star in a binary system is surrounded by its own zone of influence, inside of which
its gravitational pull dominates the effects of both the other star and the overall rotation of
the binary. This region is called Roche lobes.

33. Stellar evolution in binary systems
(a)Algol is a binary-system which comprised of
massive blue-giant star and a smaller companion
similar to the sun.
(b)The blue-giant eventually becomes red-giant
and gas is transferred to its companion star.
(c) The initially smaller star has grown to
become a more massive blue-giant.
In binary system, mass is transferred!

34. Stellar evolution in binary systems

35. Stellar evolution in binary systems

36. Summary
• Stars spend most of their lives on the main sequence, in
the core-hydrogen-burning phase of stellar evolution
stably fusing hydrogen into the helium
• Low mass stars usually evolves red-giant star, asymptotic
giant branch, planetary nebula and white dwarf after they
leave main sequence.
• Massive stars evolve more rapidly than low mass stars
and they burn carbon to produce oxygen.
• Most of stars are binary.