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1 Lec 17: 13 MAR 2012 ASTR 130 - Introductory Astronomy II (Chapter 19) LAST TIME - Main Sequence Evolution TODAY Evolution of Low-Mass Stars Red Giants Planetary Nebulae White Dwarfs NEXT - Evolution of High-Mass Stars Supernovae and Supernova Remnants Nucleosynthesis EXAM #2 – Next Tuesday (20 March) Zero-Age Main Sequence Evolution of 1 M Stars Hydrogen --> Helium in core: He goes up; H goes down Sun now ~50:50 H:He in the core He “ash” settles in core; but not (yet) hot enough to “ignite” core contracts & heats up --> rate of fusion increases Luminosity, Temperature, Radius (of the envelope) slowly increase once core hydrogen fraction gets down to a few percent (~90% of total lifetime), fusion cannot continue as it has been doing steadily for 10 billion years that’s when the fun begins! • He core contracts • as soon as T>10 7 K, H shell burning starts • energy generation up! • envelope expands to red giant • L, R up; T down • He ash falls onto core core becomes degenerate
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TODAY – Evolution of Low-Mass Stars Red Giants Planetary Nebulae White ...neffj.people.cofc.edu/ASTR130/Notes/lec17.pdf · TODAY – Evolution of Low-Mass Stars! • Red Giants!

May 08, 2020

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Page 1: TODAY – Evolution of Low-Mass Stars Red Giants Planetary Nebulae White ...neffj.people.cofc.edu/ASTR130/Notes/lec17.pdf · TODAY – Evolution of Low-Mass Stars! • Red Giants!

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Lec 17: 13 MAR 2012 ASTR 130 - Introductory Astronomy II (Chapter 19) LAST TIME - Main Sequence Evolution

TODAY – Evolution of Low-Mass Stars •  Red Giants •  Planetary Nebulae •  White Dwarfs

NEXT - Evolution of High-Mass Stars –  Supernovae and Supernova Remnants –  Nucleosynthesis

EXAM #2 – Next Tuesday (20 March)

Zero-Age Main Sequence

Evolution of 1 M Stars •  Hydrogen --> Helium in core:

He goes up; H goes down •  Sun now ~50:50 H:He in the core •  He “ash” settles in core; but not

(yet) hot enough to “ignite” •  core contracts & heats up --> rate

of fusion increases •  Luminosity, Temperature, Radius

(of the envelope) slowly increase •  once core hydrogen fraction gets

down to a few percent (~90% of total lifetime), fusion cannot continue as it has been doing steadily for 10 billion years

•  that’s when the fun begins!

•  He core contracts

•  as soon as T>107 K, H shell burning starts

•  energy generation up!

•  envelope expands to red giant

•  L, R up; T down

•  He ash falls onto core

•  core becomes degenerate

Page 2: TODAY – Evolution of Low-Mass Stars Red Giants Planetary Nebulae White ...neffj.people.cofc.edu/ASTR130/Notes/lec17.pdf · TODAY – Evolution of Low-Mass Stars! • Red Giants!

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Helium Fusion: The Triple Alpha Process

42He + 42He → 84Be (−92 keV)

84Be + 42He → 12

6C + γ (+7.367 MeV)

Net energy release is 7.275 MeV

(α particle = helium nucleus) The Helium Flash and Core Helium Burning

•  when T > 108 K, He starts fusing (via triple-alpha process), which increases Temp more without increasing Pressure (degeneracy) --> runaway fusion

•  Helium-Core FLASH within seconds! •  L ~ 1011 Lsun !!! (but not for long) •  rapid increase in T removes degeneracy •  pressure increases and core expands •  T goes up; R and L go down

a)  red giant “clump” if metal-rich (Pop I) b)  “horizontal branch” if metal-poor star (Pop II)

•  remains in this state most of remaining 10% of its life

Page 3: TODAY – Evolution of Low-Mass Stars Red Giants Planetary Nebulae White ...neffj.people.cofc.edu/ASTR130/Notes/lec17.pdf · TODAY – Evolution of Low-Mass Stars! • Red Giants!

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Core H burning on MS

Shell H burning -> RGB

He core

•  flash (M < 1.8 M)

•  burn (M > 1.8 M)

He shell burning -> AGB

C/O core - M < 1.44 M

Evolution of Low and Intermediate-Mass Stars

Pulsating Stars and Mass Loss •  ultimate fate depends on initial mass & how

much mass is lost in penultimate stages •  low surface gravity and high radiation pressure

on massive stars drives substantial mass loss •  thermal pulses can drive semi-regular variability

some (like Mira) vary in regular, predictable ways

factor of 10 in brightness! not due to change in fusion

“Planetary Nebulae”

Evolution of Very Low-Mass Stars •  M Stars (M > 0.08 M)

– PP chain –  fully convective –  gradually He enriched; never hot enough to ignite –  contracts to He-rich white dwarf

•  Brown Dwarfs (10 MJ < MBD< 0.08 M) –  fusion never ignites – Kelvin-Helmholz contraction/cooling –  fully convective –  end life as “black dwarf”?

White Dwarfs some are mostly Helium (from low mass stars)

most are pure Carbon, or mixture of Carbon & Oxygen

upper limit to their mass is 1.44 Msun