chapter 12: the life cycle of stars (contʼd) 12: the life cycle of stars ... • star formation...

3/18/09 Habbal Astro 110-01 Lecture 24 1 How are stars born, and how do they die?

Chapter 12: The Life Cycle of Stars

(contʼd)

3/18/09 Habbal Astro 110-01 Lecture 24 2

•  Star formation movie (starslife) http://hubblesite.org/gallery/movie_theater

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Mass ranges of stars

Temperature

Lum

inos

ity

Very massive stars are rare.

Low-mass stars are common.

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Mass ranges of stars

Temperature

Lum

inos

ity

Stars more massive than 100 MSun would blow themselves apart.

Stars less massive than 0.08 MSun cannot sustain core fusion.

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High-Mass Stars > 8 MSun

Low-Mass Stars

< 2 MSun

Intermediate-Mass Stars (~2-8 Msun)

Brown Dwarfs(no H-burning)

< 0.08 MSun

mass

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Brown dwarfs: Between planets and stars

Stars

Mass > 80 Mjup hot objects

Planets

Mass < 15 Mjup cold objects

Brown dwarfs

Mass = 15-80 Mjup�start hot, get cold�

(“failed stars”)

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Life & death of a low-mass star (like the Sun)1 Main sequence star 2 Red giant star

(expands) 3 Helium-burning star

(contracts) 4 Double-shell burning

star (expands) 5 Planetary nebula

(expands and explodes)

6 White dwarf (very small and hot)

1 2

3

4

5 6

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Main Sequence to Red giant phaseHydrogen-burning core to Hydrogen-burning shell

Main sequence star(core H burning)

Red giant star(H-shell burning)

Core is not hot enough for Helium burning

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After red giant stage : Helium fusion begins when

core becomes hot again

Helium fusion requires higher temperatures than hydrogen fusion because greater electrical charge leads to greater repulsion (100 million K, compared to 15 million K).

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Helium-fusion/burning stage •  Helium burning in core makes carbon with a surrounding

hydrogen-burning shell. •  Energy generation in this phase is steady, because the

internal thermostat is (temporarily) fixed. •  Star becomes smaller and hotter.

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Double-shell burning star: Expansion and Contraction phases

Expansion •  Similar to the red giant phase

–  Inert core of carbon –  Surrounding shells: burning H and He

•  Star becomes very cool & luminous, expanding in size.

Contraction •  Gravity is very weak at stellar surface, leading to

mass loss through a strong stellar wind (Phase #5 Nebula).

•  Continuing contraction of the core leads to greater & greater luminosity.

•  But: never gets hot enough to burn carbon core: end of the line

•  Star collapses to a dense, small, hot object: white dwarf.

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Phase #5: Planetary nebula (misleading name!) A low-mass star sheds its outer layers as it dies.

Ring Nebula

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•  Remaining core is a white dwarf

•  Very denseSunʼs mass in the size of the Earth.

•  Very hot ~20,000 K

•  But no internal energy generation.

Eskimo Nebula

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A white dwarf is about the same size as Earth(with 300,000x the mass of the Earth)

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Very high temperature of the white dwarfleads to highly ionized gas shellsExample 1: Spirograph Nebula

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Very high temperature of the white dwarfleads to highly ionized gas shells Example 2: Hourglass Nebula

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Life stages of a low-mass star like the Sun (external view)

Hubble movie hm_helix_twist

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Life of a low-mass star (<2 MSun) as viewed on the H-R diagram

2

3

4

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Phase 1: Main sequence (core hydrogen burning in helium)

2

3

4

1

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Phase 2: Red giant(inert helium core, hydrogen burning shell)

2

3

4

1

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Phase 3: Core helium burning(core burns helium into carbon, also H-burning shell)

2

3

4

1

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Phase 4: Double shell burning(inert C core, H and He-burning shells)

2

3

4

1

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Phase 5: Planetary nebula(heavy mass loss of outer layers)

2

3

4 5

1

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Phase 6: White dwarf(inert carbon core, no energy generation)

2

3

4 5

6

1

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Summary: Life stages of a low-mass star (<2 MSun) 1.  Main sequence: core burns H in to He. 2.  Red giant: inert He core, H-burning shell. 3.  He-core burning star: core burns He into C. Also H-burning shell. 4.  Double-shell burning: inert C core, H and He-burning shells. 5.  Planetary nebula: heavy mass loss. 6.  White dwarf: inert C core, no energy generation

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QUESTION: What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools off. B. Core shrinks and heats up. C. Core expands and heats up. D. Helium fusion immediately begins.

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QUESTION: What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools off. B. Core shrinks and heats up. C. Core expands and heats up. D. Helium fusion immediately begins.

Remember gravitational equilibrium: outward pressure from core energy generation balances the inward push of gravity. W/o the energy generation, the core will be compressed and will heat up.

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QUESTION: What happens as a starʼs inert helium core starts to shrink?

A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D. Core pressure sharply drops

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QUESTION: What happens as a starʼs inert helium core starts to shrink?

A. Hydrogen fuses in shell around core B. Helium fusion slowly begins C. Helium fusion rate rapidly rises D.  Core pressure sharply drops The pressure from the outer portion of the star

compresses the interior enough that H-burning starts in a narrow shell around the (inert) helium core.

H-burning shell generates much more energy than during the main sequence. The star swells up due to this energy generation.

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QUESTION: What happens when the starʼs 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

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QUESTION: What happens when the starʼs 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

The core is not hot enough to burn the produced carbon. So analogous to the red giant phase, the core shrinks and the surrounding layers get denser. Get double-shell burning: (1) H-burning outer shell, and (2) He-burning inner shell.