lecture16
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1) Stars form from dense fragments within interstellar clouds that collapse under gravity over millions of years. 2) Protostars form within the collapsing fragments and grow in mass through accretion until their cores reach temperatures high enough for nuclear fusion. 3) After 10 million years, the protostar becomes a true star on the main sequence, where nuclear fusion powers the star for its lifetime.
lecture16
- 1. Introduction to modern astronomy16 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
- 2. More details of star formation How do stars evolve to main sequence stars like sun ??
- 3. More details of star formation Stage 1: An interstellar cloud •The core of a dark dust cloud or molecular ( ) divide to fragment into small pieces by its gravity. ∼ 10K •Typically, 10 - 1000 fragments forms. It takes ~million (100万) years. •Multiple stars form. Not one star in one cloud!
- 4. Stage 2: A collapsing cloud fragment •Small pieces of cloud shrink by its gravity. Stage 3: Fragmentation stops •Although the central temperature reaches , outer part is not still increased much. ∼ 10,000K •Contracting cloud fragment begins to form Protostar(原恒星) after 100,000 years when fragment begins to form. The mass of protostar increases because gas is accreting on protostar. More details of star formation
- 5. Stage 2: A collapsing cloud fragment •Small pieces of cloud shrink by its gravity. Stage 3: Fragmentation stops •Although the central temperature reaches , outer part is not still increased much. ∼ 10,000K •Contracting cloud fragment begins to form Protostar(原恒星) after 100,000 years when fragment begins to form. The mass of protostar increases because gas is accreting on protostar. More details of star formation
- 8. More details of star formation Stage 4:A protostar •As the protostar evolves, it shrinks, its density grows, and its temperature at both core and surface rises. The temperature in the core reaches 1,000,000K. But, it has not started nuclear fusion yet.
- 9. More details of star formation Stage 4:A protostar •As the protostar evolves, it shrinks, its density grows, and its temperature at both core and surface rises. The temperature in the core reaches 1,000,000K. But, it has not started nuclear fusion yet. T ↑ L = 4πR2 × σT4
- 10. More details of star formation Stage 4:A protostar •As the protostar evolves, it shrinks, its density grows, and its temperature at both core and surface rises. The temperature in the core reaches 1,000,000K. But, it has not started nuclear fusion yet. T ↑ L = 4πR2 × σT4 We can calculate the luminosity of protostar.
- 11. More details of star formation Stage 4:A protostar •As the protostar evolves, it shrinks, its density grows, and its temperature at both core and surface rises. The temperature in the core reaches 1,000,000K. But, it has not started nuclear fusion yet. T ↑ L = 4πR2 × σT4 We can calculate the luminosity of protostar. •The luminosity of protostars is larger than the sun!
- 12. Stage 4:A protostar •As we studied before, protostellar disk formed around the protostar and it evolves planet systems such as solar system. More details of star formation
- 13. Stage 5 : Protostellar evolution More details of star formation •After stage 4, the protostar on the HR diagram moves down(toward lower luminosity) and slightly to the left (higher temperature). But, the surface temperature remains almost constant. •The path from point 4 to point to 6 is called Hayashi-track. •By stage 5 on the Hayashi track, the protostar approaches the main sequence. The central temperature reaches 5,000,000K. It has not started nuclear fusion yet!
- 14. Stage 6 : A newborn star More details of star formation •After 10 million years after its first appearance, the protostar finally becomes a true star! •At stage 6, the radius of newborn star is about 1,000,000km and the concentration raised the central temperature to 10,000,000K, enough to ignite nuclear burning. Stage 7 : The main sequence at last •The gravity and pressure are finally balanced, and nuclear fusion is efficient. •Finally, newborn stars are listed in main sequence.
- 15. More details of star formation
- 16. More details of star formation
- 17. Stars of other masses •Previously, I explained the evolutionary track for 1 solar-mass star. How about other stars of different mass? •We easily guess low mass fragments within interstellar cloud produce low mass stars and high mass fragments produce high mass stars.
- 18. Zero-Age main sequence •More massive stars become bluer stars and lower massive stars become redder stars. •The “theoretically” predicted main-sequence is called zero-age main sequence (ZAMS). ZAMS depends on mass of stars •ZAMS agrees with main sequence very well (Note) Stars DO NOT evolve along main sequence line! Stars stop at main-sequence and stay there.
- 19. Zero-Age main sequence •More massive stars become bluer stars and lower massive stars become redder stars. •The “theoretically” predicted main-sequence is called zero-age main sequence (ZAMS). ZAMS depends on mass of stars •ZAMS agrees with main sequence very well (Note) Stars DO NOT evolve along main sequence line! Stars stop at main-sequence and stay there. NO!!!
- 20. Observations of cloud fragments and protostars •The time scale of stellar evolution is longer than human being.Thus, we cannot take movie of stellar evolution! •By observing many stars and clouds, we can study each stage of stellar evolution.
- 21. Observations of cloud fragments and protostars Stage 1: An interstellar cloud Stage 2: A collapsing cloud fragment •Region A and region B are dense cloud regions (revealed by radio observation.) Stage 6 : A newborn star •The glowing region of ionized gas results directly from a massive O-type stars. Stage 7 : The main sequence at last •Central star is already fully formed.
- 22. Observations of cloud fragments and protostars Stage 1: An interstellar cloud Stage 2: A collapsing cloud fragment •This image combines infrared, millimeter ,and radio observations of small molecular cloud •The cloud is so dense that it absorbs the infrared background, and so appears dark. •Red blobs are dense pre stellar fragments within the cloud observed by ALMA.
- 23. Observations of cloud fragments and protostars At these regions, Stage 5 : Protostellar evolution Stage 4:A protostar Stage 3: Fragmentation stops
- 24. Observations of cloud fragments and protostars Stage 4:A protostar Stage 5 : Protostellar evolution Stage 6 : A newborn star
- 25. Protostellar winds •Protostars often emit strong winds. These winds may be related to the violent surface activity associated with many protostars Infrared observation. The powerful wind blown from hot young stars evaporates（蒸发） disk.
- 26. Summary • Stars form in the interstellar cloud starting from fragments • After some processes, the protostar is listed in the main sequence star and stars start nuclear fusion.