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Lecture Slides CHAPTER 12: Evolution of Low-Mass Stars Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College Copyright © 2015, W. W. Norton & Company Evolution of Low-Mass Stars Understand the role of stellar mass in the evolution of a star.The goal is to understand this process. Explain the future evolution of the Sun. Utilize the H-R diagram to determine the evolution of stars. Role of Stellar Mass: Star’s Life Main sequence stars generate energy by converting hydrogen to helium in their cores. Eventually the fusion sources change, then halt. A star’s life depends primarily on mass and composition (to a lesser extent). Low-mass stars and high-mass stars evolve differently. Low-mass stars: M < 8 M Role of Stellar Mass: Main-Sequence Lifetimes Higher mass leads to higher temperature and pressure in the core. Higher core temperature means faster nuclear fusion => Stars with higher masses burn their fuel more quickly. Role of Stellar Mass: Main Sequence Star Recall that a protostar becomes a star when nuclear fusion begins. => That is when it becomes a main sequence star! Mass establishes a star’s evolutionary track. Role of Stellar Mass: Changes in Structure The star’s structure will change as it uses fuel. => Must maintain balance between pressure and gravity. Fusion Reactions Main-sequence stars fuse hydrogen to helium in their cores. Eventually, much of the hydrogen in the core is converted to helium. A core of non-fusing helium builds up. Fusion Reactions (Cont.) Fusion Reactions (Cont.) Fusion Reactions (Cont.) Class Question Prediction: When hydrogen fusion in the core stops, its temperature ______ and its thermal pressure _______ so the size of the core ______. A. B. C. D. Decreases, decreases, decreases. Increases, increases, increases. Decreases, increases, decreases. Increases, decreases, increases. Fusion Reactions: Electron-Degenerate At this point, hydrogen fusion only takes place in a shell around the helium core. Because the helium is not fusing, gravity begins to win over the pressure, causing the helium core to shrink. The core becomes more dense, and becomes electron-degenerate. • This means pressure is due to a quantum mechanical effect: there’s a limit to how tightly electrons can be packed together. Red Giant: Hydrogen Shell Burning When the core shrinks, its gravitational pull gets stronger. Stronger gravity => higher pressure => faster nuclear reactions in the hydrogen burning shell => more energy produced! Red Giant: Hydrogen Shell Burning (Cont.) Red Giant: Hydrogen Shell Burning (Cont.) Red Giant: A Larger Star Increase in pressure and energy production results in larger size and lower surface temperature. Star: larger, more luminous, cooler, redder => Red Giant! Red Giant: A Larger Star (Cont.) Red Giant: A Larger Star (Cont.) Class Question What causes a low-mass star, like the Sun, to evolve away from the main sequence? A. When hydrogen is exhausted in the core. B. When all of the hydrogen becomes helium. C. When carbon fusion begins. Red Giant: Branch Red Giant: Branch (Cont.) Red Giant: Branch (Cont.) Helium Fusion As the helium core shrinks, its density and temperature increase. When hot and dense enough, helium fusion begins. Helium fuses to carbon via the triple-alpha process. Helium Fusion: The Helium Flash Within seconds of helium ignition, the thermal pressure to the point that the helium core literally explodes. This explosion is called the helium flash. Helium Fusion: Horizontal Branch Star After the helium flash, the stars are on the horizontal branch of the H-R diagram. Helium fuses to carbon in the core, while hydrogen fuses to helium in a shell around the core. Helium Fusion: Asymptotic Giant Branch After helium is used up in the core, hydrogen and helium fusion continues in shells around non-fusing carbon core. Outer layers expand and cool => asymptotic giant branch of the H-R diagram. Planetary Nebula As the star expands, some of its material leaves as a stellar wind. This mass loss means the star cannot hold onto the outer layers easily. Eventually the outer layers are ejected into space. Planetary Nebula: The Ejected Material The ejected material creates a planetary nebula. • Planetary nebulae having nothing to do with planets! The remaining star shrinks and becomes hotter, moving rapidly from right to left across the H-R diagram. Planetary Nebula: Lifespan The star ionizes the gas in the expanding outer layers, causing the planetary nebula that we can observe. Planetary nebulae do not last forever – eventually the gas disperses. Planetary Nebula: Facts The first observed planetary nebulae had circular appearances (hence the name), but now we observe examples with much more complex structure. White Dwarf Leftover core of star remains as white dwarf. Masses 0.6–1.4 M, size like Earth. They are hot, but not very luminous due to small size. White dwarfs cool off because no nuclear fusion is occurring. •Our Sun’s evolution Star Clusters Star clusters are bound groups of stars, all made at the same time. Star Clusters (Cont.) Star Clusters (Cont.) Star Clusters (Cont.) Star Clusters (Cont.) Star Clusters (Cont.) Star Clusters (Cont.) Star Clusters: Young and Old Young clusters still have high-mass stars on main-sequence. In older clusters, high-mass stars have already died. Location of main-sequence turnoff gives cluster age. Star Clusters: Models of Stellar Evolution We compare our models to the observed H-R diagrams of star clusters. Agreement shows that our models of stellar evolution are on the right track! Evolution in Close Binary Systems Recall that most stars are in binary systems. In each pair of low-mass stars, the more massive star evolves first. It can only expand so much before it begins to lose material. Evolution in Close Binary Systems (Cont.) Evolution in Close Binary Systems (Cont.) Evolution in Close Binary Systems: Mass Transfer Material can flow from the giant star to the companion. This is called mass transfer. The giant becomes a white dwarf. When the second star is a giant, it can dump material onto the white dwarf. Evolution in Close Binary Systems: Mass Transfer (Cont.) Evolution in Close Binary Systems: Mass Transfer (Cont.) Evolution in Close Binary Systems: Evolution of Nova As hydrogen collects on the white dwarf, nuclear reactions can start on the surface => gets much brighter temporarily => nova. For a few hours, it can be a halfmillion times more luminous than the Sun. Evolution in Close Binary Systems: Evolution of Nova (Cont.) Evolution in Close Binary Systems: Evolution of Nova (Cont.) Evolution in Close Binary Systems: Evolution of Nova (Cont.) Evolution in Close Binary Systems: The White Dwarf Limit The maximum mass for a white dwarf is 1.4 M, known as the Chandrasekhar limit. If material dumped on the white dwarf pushes it over this limit, the white dwarf will explode. Evolution in Close Binary Systems: The White Dwarf Limit (Cont.) Evolution in Close Binary Systems: The White Dwarf Limit (Cont.) Evolution in Close Binary Systems: The White Dwarf Limit (Cont.) Evolution in Close Binary Systems: Type Ia Supernova This explosion is called a Type Ia supernova. The explosion is briefly as luminous as 10 billion Suns. Nothing of the white dwarf is left behind; the other star continues to evolve on its own. Evolution in Close Binary Systems: Type Ia Supernova (Cont.) Evolution in Close Binary Systems: Type Ia Supernova (Cont.) Evolution in Close Binary Systems: Type Ia Supernova (Cont.) Class Question Which of the following is the correct order for the stages of evolution of a low-mass star, like the Sun? A. Main sequence, white dwarf, planetary nebula, red giant. B. Main sequence, red giant, white dwarf, planetary nebula. C. Main sequence, red giant, planetary nebula, white dwarf. D. Main sequence, planetary nebula, red giant, white dwarf. Chapter Summary Low-mass stars burn through their nuclear fuel more slowly and have longer lifetimes than high-mass stars. A low-mass star will evolve as hydrogen fusion stops in its core. The evolutionary result of an isolated low-mass star: planetary nebula + white dwarf H-R diagrams help us understand stellar evolution. Nebraska Applet HR-Diagram Explorer Click the image to launch the Nebraska Applet (Requires an active Internet connection) Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College This concludes the Lecture slides for CHAPTER 12: Evolution of Low-Mass Stars wwnpag.es/uou2 Copyright © 2015, W. W. Norton & Company