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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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
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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
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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