Copyright © 2009 Pearson Education, Inc. Chapter 12 Star Formation & DeathCopyright © 2009 Pearson Education, Inc. 12.2 Life as a Low-Mass Star Our goals for learning: • What are the life stages of a low-mass star? • How does a low-mass star die?Copyright © 2009 Pearson Education, Inc. What are the life stages of a low-mass star?Copyright © 2009 Pearson Education, Inc. A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. Main-Sequence Lifetimes and Stellar MassesCopyright © 2009 Pearson Education, Inc. Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Its core cools off. B. Its core shrinks and heats up. C. Its core expands and heats up. D. Helium fusion immediately begins. E. It does a hissy fit.Copyright © 2009 Pearson Education, Inc. Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Its core cools off. B. Its core shrinks and heats up. C. Its core expands and heats up. D. Helium fusion immediately begins.Copyright © 2009 Pearson Education, Inc. Life Track After Main Sequence • Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.Copyright © 2009 Pearson Education, Inc. Broken Thermostat • As the core contracts, H begins fusing to He in a shell around the core. • Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting.Copyright © 2009 Pearson Education, Inc. Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion. The fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon.Copyright © 2009 Pearson Education, Inc. Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up. B. Hydrogen fusion stops. C. Helium fusion rises very sharply. (Hint: Degeneracy pressure is the main form of pressure in the inert helium core.)Copyright © 2009 Pearson Education, Inc. Thought Question What happens in a low-mass star when core temperature rises enough for helium fusion to begin? A. Helium fusion slowly starts up. B. Hydrogen fusion stops. C. Helium fusion rises very sharply. (Hint: Degeneracy pressure is the main form of pressure in the inert helium core.)Copyright © 2009 Pearson Education, Inc. Helium Flash • The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core. • The core temperature rises rapidly when helium fusion begins. • The helium fusion rate skyrockets until thermal pressure takes over and expands the core again.Copyright © 2009 Pearson Education, Inc. Helium-burning stars neither shrink nor grow because the core thermostat is temporarily fixed.Copyright © 2009 Pearson Education, Inc. Life Track After Helium Flash • Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.Copyright © 2009 Pearson Education, Inc. Life Track After Helium Flash • Observations of star clusters agree with those models. • Helium-burning stars are found in a horizontal branch on the H-R diagram.Copyright © 2009 Pearson Education, Inc. Combining models of stars of similar age but different mass helps us to age-date star clusters. Using the H-R Diagram to Determine the Age of a Star ClusterCopyright © 2009 Pearson Education, Inc. How does a low-mass star die?Copyright © 2009 Pearson Education, Inc. Thought Question What happens when a star’s 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.Copyright © 2009 Pearson Education, Inc. Thought Question What happens when a star’s 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.Copyright © 2009 Pearson Education, Inc. Double Shell Burning • After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer. • This double shell–burning stage never reaches equilibrium—the fusion rate periodically spikes upward in a series of thermal pulses. • With each spike, convection dredges carbon up from the core and transports it to the surface.Copyright © 2009 Pearson Education, Inc. Planetary Nebulae • Double shell–burning ends with a pulse that ejects the H and He into space as a planetary nebula. • The core left behind becomes a white dwarf.Copyright © 2009 Pearson Education, Inc. Planetary Nebulae • Double shell–burning ends with a pulse that ejects the H and He into space as a planetary nebula. • The core left behind becomes a white dwarf.Copyright © 2009 Pearson Education, Inc. Planetary Nebulae • Double shell–burning ends with a pulse that ejects the H and He into space as a planetary nebula. • The core left behind becomes a white dwarf.Copyright © 2009 Pearson Education, Inc. Planetary Nebulae • Double shell–burning ends with a pulse that ejects the H and He into space as a planetary nebula. • The core left behind becomes a white dwarf.Copyright © 2009 Pearson Education, Inc. End of Fusion • Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some He fuses to C to make oxygen). • Degeneracy pressure supports the white dwarf against gravity.Copyright © 2009 Pearson Education, Inc. Life stages of a low-mass star like the Sun The Death Sequence of the SunCopyright © 2009 Pearson Education, Inc. Life Track of a Sun-Like StarCopyright © 2009 Pearson Education, Inc. What have we learned? • What are the life stages of a low-mass star? — H fusion in core (main sequence) — H fusion in shell around contracting core (red giant) — He fusion in core (horizontal branch) — Double shell-burning (red giant) • How does a low-mass star die? — Ejection of H and He in a planetary nebula leaves behind an inert white dwarf.Copyright © 2009 Pearson Education, Inc. 12.3 Life as a High-Mass Star Our goals for learning: • What
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