ASTR 1102-0022008 Fall Semester

Joel E. Tohline, Alumni ProfessorOffice: 247 Nicholson Hall

[Slides from Lecture13]

Chapter 20: Stellar Evolution:The Deaths of Stars

and

Chapter 21: Neutron Stars

Low-, Moderately Low-, & High-Mass 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 high-mass 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 high-mass 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 high-mass 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 high-mass 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 high-mass 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-, & 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 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 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 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 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 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 ofHigh-mass Star’s Core

Fig

ure

20-1

3

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!

Excerpt from §21-1

On the morning of July 4, 1054, Yang Wei-T’e (imperial astronomer to the Chinese court) made a startling discovery. Just a few minutes before sunrise, a new and dazzling object ascended above the eastern horizon.

This “guest star” was so brilliant that it could easily be seen during broad daylight for the rest of July!

This “guest star” was visible in the night sky (to the naked eye) for 21 months.

Excerpt from §21-1

On the morning of July 4, 1054, Yang Wei-T’e (imperial astronomer to the Chinese court) made a startling discovery. Just a few minutes before sunrise, a new and dazzling object ascended above the eastern horizon.

This “guest star” was so brilliant that it could easily be seen during broad daylight for the rest of July!

This “guest star” was visible in the night sky (to the naked eye) for 21 months.

Excerpt from §21-1

On the morning of July 4, 1054, Yang Wei-T’e (imperial astronomer to the Chinese court) made a startling discovery. Just a few minutes before sunrise, a new and dazzling object ascended above the eastern horizon.

This “guest star” was so brilliant that it could easily be seen during broad daylight for the rest of July!

This “guest star” was visible in the night sky (to the naked eye) for 21 months.

Crab Nebula

Today, if we look at the location on the sky where Yang Wei-T’e discovered his “guest star” nearly 1000 years ago, we see a glowing gaseous nebula that we call the “Crab Nebula”: – The gaseous debris is expanding away from

its center at a rapid rate; – projecting this expansion rate backward in

time, we conclude that the nebula originated from a “point-like explosion” approximately 1000 years ago

Crab Nebula

Crab Nebula

Today, if we look at the location on the sky where Yang Wei-T’e discovered his “guest star” nearly 1000 years ago, we see a glowing gaseous nebula that we call the “Crab Nebula”: – The gaseous debris is expanding away from its center

at a rapid rate;

– projecting this expansion rate backward in time, we conclude that the nebula originated from a “point-like explosion” approximately 1000 years ago

Crab Nebula

• At the center of the crab nebula, astronomers have identified a peculiar, compact star (a “pulsar”) that …– At visible wavelengths is difficult to see;– At radio wavelengths is a powerful “light-house”

beacon that flashes on and off 33 times every second!

Crab Nebula

• Astronomers are convinced that the gas making up the Crab Nebula is (what is left of) the outermost layers of a massive star that died violently (a “supernova explosion”) in the year 1054, and that its central pulsar is a rapidly rotating neutron star – a compact stellar remnant, which was once the “core” of the highly evolved, massive star.

• This illustrates how massive stars die!

Crab Nebula

• Astronomers are convinced that the gas making up the Crab Nebula is (what is left of) the outermost layers of a massive star that died violently (a “supernova explosion”) in the year 1054, and that its central pulsar is a rapidly rotating neutron star – a compact stellar remnant, which was once the “core” of the highly evolved, massive star.

• This illustrates how massive stars die! The “disaster” alluded to earlier results in an explosion of cataclysmic proportion.

Analogy Between SNe and PNe

• The outer layers of …– moderately-low-mass stars are ejected (nonviolently)

to form a planetary nebula;– high-mass stars are ejected explosively to form a

gaseous supernova remnant.

• The compact stellar remnant that remains is …– A white dwarf, if the MS star is moderately low-mass;– A neutron star, if the MS star is high-mass

• Maximum mass of compact stellar remnant is …– 1.4 M⊙(Chandraskhar mass) for a white dwarf;– Approximately 3 M⊙ for a neutron star.

Analogy Between SNe and PNe

• The outer layers of …– moderately-low-mass stars are ejected (nonviolently)

to form a planetary nebula;– high-mass stars are ejected explosively to form a

gaseous supernova remnant.

• The compact stellar remnant that remains is …– A white dwarf, if the MS star is moderately low-mass;– A neutron star, if the MS star is high-mass

• Maximum mass of compact stellar remnant is …– 1.4 M⊙(Chandraskhar mass) for a white dwarf;– Approximately 3 M⊙ for a neutron star.

Analogy Between SNe and PNe

• The outer layers of …– moderately-low-mass stars are ejected (nonviolently)

to form a planetary nebula;– high-mass stars are ejected explosively to form a

gaseous supernova remnant.

• The compact stellar remnant that remains is …– A white dwarf, if the MS star is moderately low-mass;– A neutron star, if the MS star is high-mass

• Maximum mass of compact stellar remnant is …– 1.4 M⊙(Chandraskhar mass) for a white dwarf;– Approximately 3 M⊙ for a neutron star.

Summary of Stellar Evolution

Supernovae

• Easily (and now frequently) detected in other galaxies. (Statistically, every galaxy should display 1-3 supernovae every 100 yrs.)

• The light display from each SN generally can be categorized as one of several standard “types”:– Type Ia– Type Ib, Ic– Type II

Supernovae

• Easily (and now frequently) detected in other galaxies. (Statistically, every galaxy should display 1-3 supernovae every 100 yrs.)

• The light display from each SN generally can be categorized as one of several standard “types”:– Type Ia– Type Ib, Ic– Type II

What About SNe in Our Galaxy?

• We’ve already discussed the Crab SN, which exploded in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE: Well over a thousand (!) pulsars have

been catalogued in our Milky Way Galaxy.

What About SNe in Our Galaxy?• We’ve already discussed the Crab SN, which exploded

in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE:

– Dozens of gaseous SN remnants are identifiable in our Galaxy– Well over a thousand (!) pulsars have been catalogued in our

Galaxy.

What About SNe in Our Galaxy?• We’ve already discussed the Crab SN, which exploded

in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE:

– Dozens of gaseous SN remnants are identifiable in our Galaxy– Well over a thousand (!) pulsars have been catalogued in our

Galaxy.

What About SNe in Our Galaxy?• We’ve already discussed the Crab SN, which exploded

in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE:

– Dozens of gaseous SN remnants are identifiable in our Galaxy– Well over a thousand (!) pulsars have been catalogued in our

Galaxy.

What About SNe in Our Galaxy?• We’ve already discussed the Crab SN, which exploded

in 1054; our distance from the Crab nebula is about 2000 parsecs, and it is approximately 4 pc in diameter.

• Over the past 1000 years, written records indicate that only 5 SN explosions have been seen (by humans) in our “Milky Way” Galaxy– Years 1006, 1054 (Crab), 1181, 1572, 1604

• We’re overdue!• NOTE:

– Dozens of gaseous SN remnants are identifiable in our Galaxy– Well over a thousand (!) pulsars have been catalogued in our

Galaxy.

SN 1987A