Chapter 12: The Life Cycle of Stars (contʼd)

How are stars born, and how do they die? 3/18/09 Habbal Astro 110-01 Lecture 24 1 • Star formation movie (starslife) http://hubblesite.org/gallery/movie_theater

3/18/09 Habbal Astro 110-01 Lecture 24 2 Very massive Mass ranges of stars stars are rare.

Low-mass

Luminosity stars are common.

3/18/09 TemperatureHabbal Astro 110-01 Lecture 24 3 Stars more massive Mass ranges of stars than 100 MSun would blow themselves apart.

Stars less

Luminosity massive than

0.08 MSun cannot sustain core fusion.

3/18/09 TemperatureHabbal Astro 110-01 Lecture 24 4 High-Mass Stars

> 8 MSun Intermediate -Mass Stars

(~2-8 Msun) mass Low-Mass Stars

< 2 MSun

Brown Dwarfs (no H-burning)

< 0.08 MSun

3/18/09 Habbal Astro 110-01 Lecture 24 5 Brown dwarfs: Between planets and stars Brown dwarfs Planets Stars

Mass = 15-80 Mjup start hot, get cold Mass < 15 Mjup (“failed stars”) Mass > 80 Mjup cold objects hot objects

3/18/09 Habbal Astro 110-01 Lecture 24 6 Life & death of 1 a low-mass star 2 (like the Sun) 1 Main sequence star 3 2 Red giant star (expands) 3 Helium-burning star (contracts) 4 Double-shell burning star (expands) 4 5 Planetary nebula (expands and explodes) 6 White dwarf (very 5 small and hot) 6 3/18/09 Habbal Astro 110-01 Lecture 24 7 Main Sequence to Red giant phase Hydrogen-burning core to Hydrogen-burning shell

Core is not hot enough for Helium burning

Main sequence star Red giant star (core H burning) (H-shell burning) 3/18/09 Habbal Astro 110-01 Lecture 24 8 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).

3/18/09 Habbal Astro 110-01 Lecture 24 9 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.

3/18/09 Habbal Astro 110-01 Lecture 24 10 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.

3/18/09 Habbal Astro 110-01 Lecture 24 11 Phase #5: Planetary nebula (misleading name!) A low-mass star sheds its outer layers as it dies.

Ring Nebula

3/18/09 Habbal Astro 110-01 Lecture 24 12 • Remaining core is a white dwarf

• Very dense Sunʼs mass in the size of the Earth. • Very hot ~20,000 K • But no internal energy generation.

Eskimo Nebula 3/18/09 Habbal Astro 110-01 Lecture 24 13 A white dwarf is about the same size as Earth (with 300,000x the mass of the Earth)

3/18/09 Habbal Astro 110-01 Lecture 24 14 Very high temperature of the white dwarf leads to highly ionized gas shells Example 1: Spirograph Nebula

3/18/09 Habbal Astro 110-01 Lecture 24 15 Very high temperature of the white dwarf leads to highly ionized gas shells Example 2: Hourglass Nebula

3/18/09 Habbal Astro 110-01 Lecture 24 16 Life stages of a low-mass star like the Sun (external view)

Hubble movie hm_helix_twist

3/18/09 Habbal Astro 110-01 Lecture 24 17 Life of a low-mass star (<2 MSun) as viewed on the H-R diagram

4

3

2

3/18/09 Habbal Astro 110-01 Lecture 24 18 Phase 1: Main sequence (core hydrogen burning in helium)

4

3 1

2

3/18/09 Habbal Astro 110-01 Lecture 24 19 Phase 2: Red giant (inert helium core, hydrogen burning shell) 4

3 1

2

3/18/09 Habbal Astro 110-01 Lecture 24 20 Phase 3: Core helium burning (core burns helium into carbon, also H-burning shell) 4

3 1

2

3/18/09 Habbal Astro 110-01 Lecture 24 21 Phase 4: Double shell burning (inert C core, H and He-burning shells) 4

3 1

2

3/18/09 Habbal Astro 110-01 Lecture 24 22 Phase 5: Planetary nebula (heavy mass loss of outer layers) 4 5

3 1

2

3/18/09 Habbal Astro 110-01 Lecture 24 23 Phase 6: White dwarf (inert carbon core, no energy generation) 4 5

3 1

2 6

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

3/18/09 Habbal Astro 110-01 Lecture 24 25 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.

3/18/09 Habbal Astro 110-01 Lecture 24 26 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. 3/18/09 Habbal Astro 110-01 Lecture 24 27 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

3/18/09 Habbal Astro 110-01 Lecture 24 28 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. 3/18/09 Habbal Astro 110-01 Lecture 24 29 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

3/18/09 Habbal Astro 110-01 Lecture 24 30 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. 3/18/09 Habbal Astro 110-01 Lecture 24 31