lecture17
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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.
lecture17
- 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.
- 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.