Ch 13 The Deaths of Stars Study Guide.pdf - Chapter 13...

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Chapter 13 Study Guide 13-1 C H A P T E R 1 3 THE DEATHS OF STARS Perhaps you were surprised to learn in the previous chapters that stars are born and that they grow old. In fact, astronomers can tell the life stories of stars right to their ends. In this chapter you will learn how stars die; as you follow those stories, you see more examples of how scientists test hypotheses against evidence. Here you will find answers to five important questions: How will the Sun and other low-mass stars die? What happens if an evolving star is a member of a close binary system? How do massive stars die? What do we learn about stellar evolution from observations of supernovae and supernova remnants? What will be the ultimate fate of Earth as the Sun evolves and dies? 13-1 LOW-MASS STARS Red dwarfs are stars on the main sequence of the HR diagram that have less than about 0.5 Mand are completely mixed internally by convection. They cannot ignite a hydrogen fusion shell, so they cannot become giant stars. Because they have little weight to support and can fuse nearly all of their hydrogen fuel, they will remain on the main sequence for many times the present age of the Universe. When red dwarfs use up the hydrogen fuel in their cores, models indicate they should contract, increase their surface temperatures, and become bluer. Because no new energy is being generated in the core, they are expected to then cool and fade to white dwarfs. The Universe is not old enough for any white dwarfs to have formed in this way. Medium-mass stars with initial masses between about 0.5 and 8 M, including the Sun, evolve to become red giants that eventually fuse helium into carbon and oxygen. Stars with initial masses less than 8 M(which, after mass loss, become red giants with less than 4 M) can never become hot enough to fuse carbon. Red giant stars lose mass rapidly into space in superwinds, initially by a slow stellar wind and later a fast stellar wind. As a giant cools, it becomes more opaque and its internal pressure increases, which causes the giant to expel the gaseous layers above the core outward in one or more surges, producing a planetary nebula. After the planetary nebula stage, the dying star collapses to become a white dwarf. White dwarfs are the remains of low- and medium-mass stars that have collapsed until they become degenerate matter. Because white dwarfs are small, dense, and generate no fusion energy, they are sometimes called compact objects, along with neutron stars and black holes. White dwarfs slowly cool and should eventually become black dwarfs. A white dwarf cannot support its own weight by its electron degeneracy pressure if its mass is greater than the Chandrasekhar limit, which is 1.4
Chapter 13 Study Guide 13-2 M. Main-sequence medium-mass stars up to about 8 or 10 Mcan lose enough mass during their lives to end as white dwarfs with masses below the Chandrasekhar limit.

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