Evolution of Stars (Part I: Solar-type stars) 1 Death of the Sun Parts I and II 2 Learning goals: Be able to …. ! summarize the future of the Sun on a rough timescale; ! apply the basics of the conservation of energy and the battle between gravity and outward pressure to what “drives” a star to evolve at each major stage of evolution; ! explain what is meant by main sequence, subgiant, red giant branch, electron degeneracy, helium flash, horizontal branch, asymptotic giant branch, planetary nebula, white dwarf. 3 Be able to summarize the future of the Sun in a rough timescale. Apply the basics of the conservation of energy and the battle between gravely and pressure to what “drives” a star to evolve at each major stage of evolution. Explain what is meant by red giant branch, electron degeneracy, helium flash, horizontal branch, planetary nebula, white dwarf 4 5 http://chandra.harvard.edu/xray_sources/browndwarf_fg.html Brown Dwarf "In between a star and a planet "Jupiter < BD mass < 0.08 Sun "Radiates in infrared due to low temperature. "Mass too low to start fusion in core. "Slowly cools over trillions of years. 6 7 Core producing energy--Fusing H to He 8 A review of what we know the Sun is doing as a main-sequence star 9 Number of particles in core is decreasing. NET RESULT IN: 6 H OUT: 1 He + 2 H FUSION RATE MUST INCREASE TO OFFSET DECREASE IN PARTICLE PRESSURE! Sun is slowly becoming more and more luminous. 10 Explain how the Sun maintains a constant balance in its interior (solar thermostat) Part of the pressure depends on the number of particles Pressure decreases, core shrinks slightly, pressure evens out, fusion rate must increase to offset the smaller pressure provided by the number of particles present. Luminosity of the Sun must increase! Summary for #1 on flowchart Stars on the main sequence: What is happening in the core? How does the rest of the star support itself? Describe the interior of the star. 11 12 Expanding Subgiant 13 MS very stable long lasting stage but H can fuse only in very center of star Why does core have to contract? Why doesn’t it keep on contracting? GPE-->KE-->thermal energy Discuss shell fusion and luminosity generated 14 Summary for #2 on flowchart Stars on the sub giant branch: Core runs out of hydrogen to fuse. What is happening in the core? How does the rest of the star support itself? 15 16 Core collapse stops Surface of star very far away! H # He fusion shell Helium ash Electron Degenerate degeneracy He Core supports the core. [No two electrons can occupy identical states.] 17 Degeneracy is a strange state of material. The pressure in the core does not depend on temperature. The core cannot expand and cool. Neither can it shrink any more. “Human” (molecular?) “Degeneracy” 18 Explain degeneracy. Illustrate the core gaining mass. Summary: star changes dramatically! Future Sun Today’s Sun 19 “climbing the red giant branch” What’s happening in the star ? Source of luminosity causes star’s radius to increase. Because surface is so far away from source of luminosity and there is such a huge area over which radiation escapes, star’s surface is cool. 20 Summary for #3 on flowchart Stars going up the red giant branch: Core core stops contracting; no fusion is occurring. What supports the core? How does the rest of the star support itself? Are gravity and outward pressure balanced? 21 No support from fusion, the core contracts. Potential energy converted to thermal energy. Thermal energy released initiates shell fusion of H to He. Extreme luminosity expands the rest of star Outward pressure greater than inward pull of gravity Core continues to contract until electron degeneracy sets in. Rest of star supported by thermal pressure. Shell fusion dumps helium ash on core. 22 What initiates helium fusion? 4 Helium flash! Core does not expand and cool so nuclear runaway occurs - the fusion of 3He -> carbon begins. 23 Compare the sizes from main sequence star to red giant to horizontal branch star. 5 6 7 24 What is happening in the core? What supports the star now? Helium - to - carbon fusion (triple-alpha fusion) Only 2 gamma rays produced. Why does helium to carbon fusion have to have high temperatures and a high fusion rate? 25 Where is the star on HRD? 26 After He flash, temperature and luminosity depend on how much mass the star lost as it “climbed” the red giant branch Core gets enough mass that temperature reaches He fusion limit. Luminosity drops as equilibrium starts again. Core supported by He to C fusion. Rest of star supported by radiation and thermal pressure. Star is on horizontal branch; second longest stage of its life. 27 Summary for #5 on flowchart Stars is a horizontal branch star: What is happening in the core? Anything happening around the core? How does the rest of the star support itself? 28 5 6 7 29 Core runs out of helium. "Core: contracting until electron degeneracy sets in again; giving off energy GPE TE " Shells start fusion " H#He " He#C C 30 Fusion of He to c operates at very high Pressures and temps and in a much smaller volume than H to He. Eventually the supply gives out. Explain what happens in the core, shells, atmosphere, etc. Draw a diagram. 31 Summary for #6 on flowchart Star leaves the horizontal branch: Core runs out of helium. What happens in the core? What happens around the core? How does the rest of the star support itself? Are outward pressure and gravity balanced? 32 5 6 7 33 "Rest of star responding to huge luminosity coming from shell fusion " Outer parts of star start to reach escape velocity – star is unstable! 34 35 Summary for #7 on flowchart Star is at the tip of the “asymptotic” branch: Core stops contracting; no fusion is occurring. What supports the core? How does the rest of the star support itself? Are gravity and outward pressure balanced? 36 End of the road for Sun-like stars 8 37 Fusion stops at carbon for solar-mass stars; Atmosphere is not in gravitational equilibrium; ~ 30% of mass of the star gets expelled; Central remnant called a white dwarf -- size of the Earth -- EXTREMELY dense; Gaseous object called a Planetary nebula " White dwarf will 9 slowly cool to a cold, black carbon cinder of about 0.7 solar “WHITE DWARF” masses " “Mass sink” # . material does not go back into ISM. http://news.bbc.co.uk/2/hi/science/nature/3492919.stm 38 End of the road for our Sun and similar stars. Material does not go back into the ISM. Core contracts and heats up; thermal energy ignites shell fusion. Double-shell fusion is very unstable process. Rest of star expands enormously due to extreme luminosity. Thermal pulses give outer regions escape velocity. Carbon core eventually supported by electron degeneracy. Rest of star expands away into interstellar space. Fusion of sun-like stars stops with carbon. Core never gets hot enough to fuse carbon. Core turns becomes a white dwarf. White dwarfs cool over trillions of years. Evolution of the Sun Notice “rapid” increase in luminosity Trouble starts here! 39 Evolutionary tracks of the Sun SUMMARY 40 .