ASTR Fall Semester Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture12]
Chapter 20: Stellar Evolution: The Deaths of Stars
Main Sequence (MS)
Stellar Masses along the MS Masses obtained from Fig and Table 19-1
Low-, Moderately Low-, & High- Mass Stars along the MS Terminology used throughout Chapter 20
Main-sequence Lifetimes Lifetimes obtained from Table 19-1
Low-, Moderately Low-, & High- Mass Stars along the MS Terminology used throughout Chapter 20
Summary of Evolution Low-Mass, “red dwarf” Stars (0.08 M sun M * 0.4 M sun ) –Never leaves the main sequence –Fully convective all of the star’s hydrogen is eventually brought into the core for “burning” –Over hundreds of billions of years, evolves into an inert ball of helium –Boring!
Summary of Evolution Low-Mass, “red dwarf” Stars (0.08 M sun M * 0.4 M sun ) –Never leaves the main sequence –Fully convective all of the star’s hydrogen is eventually brought into the core for “burning” –Over hundreds of billions of years, evolves into an inert ball of helium –Boring!
Summary of Evolution Low-Mass, “red dwarf” Stars (0.08 M sun M * 0.4 M sun ) –Never leaves the main sequence –Fully convective all of the star’s hydrogen is eventually brought into the core for “burning” –Over hundreds of billions of years, evolves into an inert ball of helium –Boring!
Summary of Evolution Low-Mass, “red dwarf” Stars (0.08 M sun M * 0.4 M sun ) –Never leaves the main sequence –Fully convective all of the star’s hydrogen is eventually brought into the core for “burning” –Over hundreds of billions of years, evolves into an inert ball of helium –Boring!
Summary of Evolution Low-Mass, “red dwarf” Stars (0.08 M sun M * 0.4 M sun ) –Never leaves the main sequence –Fully convective all of the star’s hydrogen is eventually brought into the core for “burning” –Over hundreds of billions of years, evolves into an inert ball of helium –Boring!
Low-, Moderately Low-, & High- Mass Stars along the MS Terminology used throughout Chapter 20
Main-sequence Lifetimes Lifetimes obtained from Table 19-1
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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”
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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”
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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”
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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”
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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 M sun M * 4 M sun ) –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
Planetary Nebulae (PN) Figure 20-6c Infrared Image of PN “NGC 7027”
Planetary Nebulae (PN) A planetary nebula located inside globular cluster M15 Figure 20-6a
Planetary Nebulae (PN) For more images of various planetary nebulae, see –
AGB PN white dwarf
Summary of Evolution Moderately Low-Mass Stars (like the Sun) (0.4 M sun M * 4 M sun ) –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
Low-, Moderately Low-, & High- Mass 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 M sun 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 M sun 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 M sun 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 M sun 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 M sun 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
“Onion-skin” Structure of High-mass Star’s Core Figure 20-13
Summary of Evolution High-Mass Stars (cont.) –Successive stages of nuclear fusion ignition proceed until elements in the “iron-nickel group” are formed –Any attempt by the star to fuse elements in the iron- nickel group into heavier elements is a disaster!
Summary of Evolution High-Mass Stars (cont.) –Successive stages of nuclear fusion ignition proceed until elements in the “iron-nickel group” are formed –Any attempt by the star to fuse elements in the iron- nickel group into heavier elements is a disaster!
Summary of Evolution High-Mass Stars (cont.) –Successive stages of nuclear fusion ignition proceed until elements in the “iron-nickel group” are formed –Any attempt by the star to fuse elements in the iron- nickel group into heavier elements proves to be a disaster!