ASTR 1102 002 2008 Fall Semester Joel E Tohline Alumni Professor Office 247 Nicholson Hall Slides from Lecture13 Chapter 20 Stellar Evolution The Deaths of Stars and Chapter 21 Neutron Stars Low Moderately Low HighMass Stars along the MS Terminology used throughout Chapter 20 Summary of Evolution Moderately Low Mass Stars like the Sun 0 4 Msun M 4 Msun Helium may ignite via a helium flash In red giant phase core helium fusion converts helium into carbon oxygen hydrogen fusion continues in a surrounding shell After core no longer contains helium star may enter asymptotic giant branch AGB phase helium continues to burn in a shell that surrounds an inert C O core As AGB star star s radius is 1 AU or larger Outer envelope ejected nonviolently to reveal the hot inner core planetary nebula This remnant core cools to become a white dwarf Structure of an AGB Star Summary of Evolution Moderately Low Mass Stars like the Sun 0 4 Msun M 4 Msun Helium may ignite via a helium flash In red giant phase core helium fusion converts helium into carbon oxygen hydrogen fusion continues in a surrounding shell After core no longer contains helium star may enter asymptotic giant branch AGB phase helium continues to burn in a shell that surrounds an inert C O core As AGB star star s radius is 1 AU or larger Outer envelope ejected nonviolently to reveal the hot inner core planetary nebula This remnant core cools to become a white dwarf Planetary Nebulae PN PN Abell 39 Figure 20 6b Summary of Evolution Moderately Low Mass Stars like the Sun 0 4 Msun M 4 Msun Helium may ignite via a helium flash In red giant phase core helium fusion converts helium into carbon oxygen hydrogen fusion continues in a surrounding shell After core no longer contains helium star may enter asymptotic giant branch AGB phase helium continues to burn in a shell that surrounds an inert C O core As AGB star star s radius is 1 AU or larger Outer envelope ejected nonviolently to reveal the hot inner core planetary nebula This remnant core cools to become a white dwarf AGB PN white dwarf Comments pt 1 Before moving on to discuss the fate of highmass stars a few comments about Planetary Nebulae and White Dwarfs are in order The shell of gas that is visible in each planetary nebula illustrates that stars have a way of returning material to the interstellar medium that has undergone nuclear processing Over time the hot central star of a PN cools to become a white dwarf Approximately 1 M of material squeezed into a spherical ball the size of the Earth Density of material about 1 million times the density of water Comments pt 1 Before moving on to discuss the fate of highmass stars a few comments about Planetary Nebulae and White Dwarfs are in order The shell of gas that is visible in each planetary nebula illustrates that stars have a way of returning material to the interstellar medium that has undergone nuclear processing Over time the hot central star of a PN cools to become a white dwarf Approximately 1 M of material squeezed into a spherical ball the size of the Earth Density of material about 1 million times the density of water Comments pt 1 Before moving on to discuss the fate of highmass stars a few comments about Planetary Nebulae and White Dwarfs are in order The shell of gas that is visible in each planetary nebula illustrates that stars have a way of returning material to the interstellar medium that has undergone nuclear processing Over time the hot central star of a PN cools to become a white dwarf Approximately 1 M of material squeezed into a spherical ball the size of the Earth Density of material about 1 million times the density of water Comments pt 1 Before moving on to discuss the fate of highmass stars a few comments about Planetary Nebulae and White Dwarfs are in order The shell of gas that is visible in each planetary nebula illustrates that stars have a way of returning material to the interstellar medium that has undergone nuclear processing Over time the hot central star of a PN cools to become a white dwarf Approximately 1 M of material squeezed into a spherical ball the size of the Earth Density of material about 1 million times the density of water Comments pt 1 Before moving on to discuss the fate of highmass stars a few comments about Planetary Nebulae and White Dwarfs are in order The shell of gas that is visible in each planetary nebula illustrates that stars have a way of returning material to the interstellar medium that has undergone nuclear processing Over time the hot central star of a PN cools to become a white dwarf Approximately 1 M of material squeezed into a spherical ball the size of the Earth Density of material about 1 million times the density of water Comments pt 2 As a WD continues to cool gravity usually is unable to squeeze it into an even smaller volume because of electron degeneracy pressure which distinct from ordinary gas pressure arises due to the quantum mechanical nature of matter can resist further gravitational compression even if the gas temperature falls to zero S Chandrasekhar showed however that degeneracy pressure is unable to beat the force of gravity if a white dwarf has a mass greater than 1 4 M Chandrasekhar mass 1 4 M Comments pt 2 As a WD continues to cool gravity usually is unable to squeeze it into an even smaller volume because of electron degeneracy pressure which distinct from ordinary gas pressure arises due to the quantum mechanical nature of matter can resist further gravitational compression even if the gas temperature falls to zero S Chandrasekhar showed however that degeneracy pressure is unable to beat the force of gravity if a white dwarf has a mass greater than 1 4 M Chandrasekhar mass 1 4 M Low Moderately Low HighMass Stars along the MS Terminology used throughout Chapter 20 Main sequence Lifetimes Lifetimes obtained from Table 19 1 Summary of Evolution High Mass Stars 4 Msun M Evolution begins as in lower mass stars through the fusion of He into C O and into the AGB phase But gravity is strong enough because of the star s larger mass for succeeding stages of nuclear burning to be triggered When the star exhausts a given variety of nuclear fuel in its core the ash of the previous fusion stage is ignited The star s core develops an onion skin structure with various layers of burning shells separated by inert shells of various elements Summary of Evolution High Mass Stars 4 Msun M Evolution begins as in lower mass stars through the fusion of He into C O and