Chapters 12 and 13 Review: The Life Cycle and Death of Stars. How are stars born, and how do they die? 4/1/2009 Habbal Astro Lecture 27 1

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1 Chapters 12 and 13 Review: The Life Cycle and Death of Stars How are stars born, and how do they die? 4/1/2009 Habbal Astro Lecture 27 1

2 Stars are born in molecular clouds Clouds are very cold: ~10-30 K. (273 K = water freezes) Stars form when gravity overcomes thermal pressure. Then gas clumps begin to collapse. 4/1/2009 Habbal Astro Lecture 27 2

3 From Molecular Cloud to Protostar Gravity causes dense cores in molecular clouds to collapse. Collapsing cores heat up. When core gets hot enough, fusion begins and stops the shrinking. New star achieves long-lasting state of balance. Sometimes protostars can drive an outflow (jets) of material Protostars rotate rapidly, and some may spin so fast that they split to form close binary star systems. 4/1/2009 Habbal Astro Lecture 27 3

4 From Protostar to Main-sequence Star Protostar contracts and heats until core temperature is sufficient for hydrogen fusion. Contraction ends when energy released by hydrogen fusion balances energy radiated from surface. Takes 50 million years for star like Sun from birth to reach the main sequence. (Takes less time for more massive stars). 4/1/2009 Habbal Astro Lecture 27 4

5 What prevents protostars from continually collapsing ever smaller? If M > 0.08 M Sun, then gravitational contraction heats the core until fusion begins. Energy generated by fusion provides thermal pressure to stop the collapse star. If M < 0.08 M Sun, degeneracy pressure stops gravitational contraction before fusion can begin brown dwarf. 4/1/2009 Habbal Astro Lecture 27 5

6 Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in same place 4/1/2009 Habbal Astro Lecture 27 6

7 Thermal Pressure: High T expansion Depends on heat content The main form of pressure in most stars High gravity Degeneracy Pressure: Electrons (or neutrons) cannot be in same state in same place Doesnʼt depend on heat content 4/1/2009 Habbal Astro Lecture 27 7

8 Mass ranges of stars The Hertzprung -Russell Diagram is a compilation of stellar luminosities versus temperature Luminosity It gives the different stages of the evolution of stars It does not show how the actual evolution occurs Temperature Star birth starts along the main sequence, and then branches off depending on initial mass 4/1/2009 Habbal Astro Lecture 27 8

9 Using star clusters in the HR diagram to test stellar evolution Star clusters contain stars of same age but at different life stages 4/1/2009 Habbal Astro Lecture 27 9

10 A subset of the HR diagram: Main sequence stars Different classes of stars High-Mass Stars > 8 M Sun Intermediate-Mass Stars (~2-8 M sun ) Low-Mass Stars < 2 M Sun Brown Dwarfs (no H-burning) < 0.08 M Sun 4/1/2009 Habbal Astro Lecture 27 10

11 Overview of stellar evolution In general, the story of stellar evolution is the ongoing struggle of stars to generate internal energy in order to resist collapsing under their own self-gravity. A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. About 90% of total lifetime is spent on main sequence Stable energy generation during this phase. Once the core H fuel is gone, the star evolves rapidly off the main-sequence, spending much of the remaining time as a cooler & more luminous star. Exact fate will depend on its original mass. In the end, gravity always wins. 4/1/2009 Habbal Astro Lecture 27 11

12 Stages of stellar evolution 1. Main sequence phase (H He in core) 2. Red Giant phase (Inert He core, H-burning shell) 3. Helium core fusion (He C + H-burning shell) 4. Double shell burning (Inert C core, H and He-burning shells) Low M star Heavy mass loss of outer shells planetary nebula White dwarf with inert C core High M star Advanced nuclear burning in multiple shells Change C N, O N, O heavier elements Supernova explosion leaving neutron star or black hole 4/1/2009 Habbal Astro Lecture 27 12

13 The life stages of a low-mass star (<2 M Sun ) 1. Main sequence: core burns H in to He. 2. Red giant: inert He core, H-burning shell. 3. He-core burning star: core burns He into C. Also H-burning shell. 4. Double-shell burning: inert C core, H and He-burning shells. 5. Planetary nebula: heavy mass loss. 6. White dwarf: inert C core, no energy generation 4/1/2009 Habbal Astro Lecture 27 13

14 Life of a low-mass star (<2 M Sun ) as viewed on the H-R diagram /1/2009 Habbal Astro Lecture 27 14

15 Phase 1: Main sequence (core hydrogen burning in helium) /1/2009 Habbal Astro Lecture 27 15

16 Phase 2: Red giant (inert helium core, hydrogen burning shell) /1/2009 Habbal Astro Lecture 27 16

17 Phase 3: Core helium burning (core burns helium into carbon, also H-burning shell) /1/2009 Habbal Astro Lecture 27 17

18 Phase 4: Double shell burning (inert C core, H and He-burning shells) /1/2009 Habbal Astro Lecture 27 18

19 Phase 5: Planetary nebula (heavy mass loss of outer layers) /1/2009 Habbal Astro Lecture 27 19

20 Phase 6: White dwarf (inert carbon core, no energy generation) /1/2009 Habbal Astro Lecture 27 20

21 Post-MS evolution High-mass stars become supergiants after core H runs out. Luminosity doesnʼt change much but radius gets much larger. 4/1/2009 Habbal Astro Lecture 27 21

22 How do high mass stars make the elements necessary for life? Helium fusion can make carbon in low-mass stars 4/1/2009 Habbal Astro Lecture 27 22

23 How do high mass stars make the elements necessary for life? High-mass stars can change C into N and O 4/1/2009 Habbal Astro Lecture 27 23

24 Helium-capture reactions add two protons at a time. Build even heavier elements in this fashion. 4/1/2009 Habbal Astro Lecture 27 24

25 Helium capture builds C into O, Ne, Mg, 4/1/2009 Habbal Astro Lecture 27 25

26 Advanced nuclear reactions make heavier elements 4/1/2009 Habbal Astro Lecture 27 26

27 Iron is the dead end for fusion because nuclear reactions involving iron do not release energy. Iron has lowest mass per nuclear particle. starʼs core cannot make any more energy once it has made iron. 4/1/2009 Habbal Astro Lecture 27 27

28 Electron degeneracy pressure in the iron core disappears at very high densities b/c electrons combine with protons, making neutrons and neutrinos. These neutrons collapse to the center, forming a neutron star (supported by neutron degeneracy pressure). The iron core has a starting mass of ~1 M Sun and size of the Earth. Collapses into a ball of neutrons only a few km across!! 4/1/2009 Habbal Astro Lecture 27 28

29 Energy and neutrons released in supernova explosion enable elements heavier than iron to form 4/1/2009 Habbal Astro Lecture 27 29

30 Life of a low (~1 M Sun ) mass star Life of a high (>8 M Sun ) mass star white dwarf supernova, leaving a neutron star or black hole 4/1/2009 Habbal Astro Lecture 27 30

31 Testing our comprehension/ knowledge 4/1/2009 Habbal Astro Lecture 27 31

32 What happens when the starʼs core runs out of helium? A. The star explodes B. Carbon fusion begins C. The core cools off D. Helium fuses in a shell around the core 4/1/2009 Habbal Astro Lecture 27 32

33 What happens when the starʼs core runs out of helium? A. The star explodes B. Carbon fusion begins C. The core cools off D. Helium fuses in a shell around the core The core is not hot enough to burn the produced carbon. So analogous to the red giant phase, the core shrinks and the surrounding layers get denser. Get double-shell burning: (1) H-burning outer shell, and (2) He-burning inner shell. 4/1/2009 Habbal Astro Lecture 27 33

34 What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off. B. Core shrinks and heats up. C. Core expands and heats up. D. Helium fusion immediately begins. 4/1/2009 Habbal Astro Lecture 27 34

35 What happens when a star can no longer fuse hydrogen to helium in its core? A. Core cools off. B. Core shrinks and heats up. C. Core expands and heats up. D. Helium fusion immediately begins. Remember gravitational equilibrium: outward pressure from core energy generation balances the inward push of gravity. W/o the energy generation, the core will be compressed and will heat up. 4/1/2009 Habbal Astro Lecture 27 35

36 What happens as a starʼs inert helium core starts to shrink? A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D. Core pressure sharply drops 4/1/2009 Habbal Astro Lecture 27 36

37 What happens as a starʼs inert helium core starts to shrink? A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D. Core pressure sharply drops The pressure from the outer portion of the star compresses the interior enough that H-burning starts in a narrow shell around the (inert) helium core. H-burning shell generates much more energy than during the main sequence. The star swells up due to this energy generation. 4/1/2009 Habbal Astro Lecture 27 37

38 Question The following question refers to the sketch below of an H-R diagram for a star cluster. Consider the star to which the arrow points. How is it currently generating energy? by hydrogen shell burning around an inert helium core by gravitational contraction by core hydrogen fusion by both hydrogen and helium shell burning around an inert carbon core 4/1/2009 Habbal Astro Lecture 27 38

39 Question The following question refers to the sketch below of an H-R diagram for a star cluster. Consider the star to which the arrow points. How is it currently generating energy? by hydrogen shell burning around an inert helium core by gravitational contraction by core hydrogen fusion by both hydrogen and helium shell burning around an inert carbon core 4/1/2009 Habbal Astro Lecture 27 39

40 Question Consider the star to which the arrow points. Which one of the following statements in NOT true It is brighter than the Sun Its core temperature is higher than the Sunʼs It is significantly less massive than the Sun. Its surface temperature is lower than the Sun's It is larger in radius than the Sun. 4/1/2009 Habbal Astro Lecture 27 40

41 Question 2 Consider the star to which the arrow points. Which one of the following statements in NOT true It is brighter than the Sun Its core temperature is higher than the Sunʼs It is significantly less massive than the Sun. Its surface temperature is lower than the Sun's It is larger in radius than the Sun. 4/1/2009 Habbal Astro Lecture 27 41

42 4/1/2009 Habbal Astro Lecture 27 42

43 Question What is the CNO cycle the period of a massive star's life when carbon, nitrogen, and oxygen are fusing in different shells outside the core a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts the process by which helium is fused into carbon, nitrogen, and oxygen the period of a low-mass star's life when it can no longer fuse carbon, nitrogen, and oxygen in its core the process by which carbon is fused into nitrogen and oxygen 4/1/2009 Habbal Astro Lecture 27 43

44 Question What is the CNO cycle the period of a massive star's life when carbon, nitrogen, and oxygen are fusing in different shells outside the core a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts the process by which helium is fused into carbon, nitrogen, and oxygen the period of a low-mass star's life when it can no longer fuse carbon, nitrogen, and oxygen in its core the process by which carbon is fused into nitrogen and oxygen 4/1/2009 Habbal Astro Lecture 27 44

45 Question Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission? hydrogen silicon oxygen iron 4/1/2009 Habbal Astro Lecture 27 45

46 Question Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission? hydrogen silicon oxygen iron 4/1/2009 Habbal Astro Lecture 27 46

47 Question What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure? The star explodes violently, leaving nothing behind. The core contracts and becomes a white dwarf. Gravity is not able to overcome neutron degeneracy pressure The core contracts and becomes a black hole. The core contracts and becomes a ball of neutrons. 4/1/2009 Habbal Astro Lecture 27 47

48 Question What happens when the gravity of a massive star is able to overcome neutron degeneracy pressure? The star explodes violently, leaving nothing behind. The core contracts and becomes a white dwarf. Gravity is not able to overcome neutron degeneracy pressure The core contracts and becomes a black hole. The core contracts and becomes a ball of neutrons. 4/1/2009 Habbal Astro Lecture 27 48

49 Question 12.8 When does a star become a main-sequence star? when the protostar assembles from its parent molecular cloud the instant when hydrogen fusion first begins in the star's core when a star becomes luminous enough to emit thermal radiation when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium when hydrogen fusion is occurring throughout the star's interior 4/1/2009 Habbal Astro Lecture 27 49

50 Question 12.8 When does a star become a main-sequence star? when the protostar assembles from its parent molecular cloud the instant when hydrogen fusion first begins in the star's core when a star becomes luminous enough to emit thermal radiation when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium when hydrogen fusion is occurring throughout the star's interior 4/1/2009 Habbal Astro Lecture 27 50

51 Final question You observe an object that periodically dims and brightens at a rapid rate, such as once per second. A reasonable explanation for this dimming and brightening is that the object is: generating energy in its core by nuclear fusion, and rapid changes in the core temperature cause rapid changes in the fusion rate rapidly rotating so that we see a bright spot with each rotation periodically passing in front of and behind an ordinary star, so that we see changes in brightness with these transits and eclipses 4/1/2009 Habbal Astro Lecture 27 51

52 Final question You observe an object that periodically dims and brightens at a rapid rate, such as once per second. A reasonable explanation for this dimming and brightening is that the object is: generating energy in its core by nuclear fusion, and rapid changes in the core temperature cause rapid changes in the fusion rate rapidly rotating so that we see a bright spot with each rotation periodically passing in front of and behind an ordinary star, so that we see changes in brightness with these transits and eclipses Recall that a typical neutron star is only a few kilometers in radius while a typical white dwarf is about the size of Earth. An Earthsize white dwarf could not hold itself together if it were rotating once a second, so such a rapid rotation rate means the object must be a neutron star. 4/1/2009 Habbal Astro Lecture 27 52

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