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Astr 2310 Thurs. Mar. 30, 2017 Today’s Topics

•  Chapter 17: Stellar Evolution •  Birth of Stars and Pre Main Sequence Evolution •  Evolution on and off the Main Sequence

–  Solar Mass Stars –  Massive Stars –  Low Mass Stars

•  Chemical Composition and Evolution – H-R Diagrams of Star Clusters – Synthesis of Heavy Elements in Stars

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Chapter 17 Homework

Chapter 17: #1, 2, 3, 4, 6 (Due Thurs. Apr. 7)

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How does mass affect collapse? •  More massive protostars have

stronger gravity •  Collapse speed will be much

faster than for smaller protostars

•  Fast collapse and short lifetime means massive stars can reach end of lifetime while low mass stars in cloud are still forming –  Supernova shocks from earlier

generation of stars may compress clouds and trigger collapse.

•  Sequential Star Formation

–  Energy from supernova and other effects eventually disrupts cloud – prevents further collapse.

From our text: Horizons, by Sees

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Post Main-Sequence Evolution – Moderate Mass

•  As Hydrogen is exhausted in the cores the stars evolve off the main sequence. Core burning slows and Hydrogen shell burning begins

–  As shell burns outward the star’s envelope rapidly expands

–  Luminosity and radius increase as temp. decreases.

–  Core shrinks and becomes degenerate (electrons strongly interact). Equation of state is no longer an ideal gas.

•  Helium burning begins when core temp. ~ 108 K.

–  Entire core flashes to C (Helium flash) –  Star rapidly restructures onto a

Horizontal Branch burning Helium –  Star expands again as Helium is

exhausted and double shell burning begins (Asymptotic giant branch)

–  Pulsational instabilities produce rapid mass loss

–  Central pressure drops, fusion halts and hot core is revealed (planetary nebula)

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Post Main-Sequence Evolution – High Mass

•  As Hydrogen is exhausted in the cores the stars evolve off the main sequence. Hydrogen shell burning rapidly begins

–  Star’s envelope rapidly expands –  Luminosity remain approx. constant

and radius increases rapidly. Star becomes a supergiant.

•  Helium burning begins when core temp. ~ 108 K.

–  No core degeneracy so no flash –  Helium rapidly exhausted and

Carbon burning begins producing Magnesium

–  Elements capture He nuclei to build up even numbered nuclei

–  Star develops a multi-shell “onion-skin” structure.

–  Heavy elements rapidly built up but little energy released (recall curve of binding energy)

–  Massive (1 solar mass) core of Iron is eventually formed.

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Testing Stellar Evolution via Star Clusters

•  Observations of a variety of star clusters allow comparison of their H-R diagrams – H-R diagrams of most clusters are devoid of

massive, hot stars. – Main-sequence lifetime is short for high

mass stars – Most massive and hottest stars on the main

sequence can be used to age-date a star cluster

– Location of evolved stars in H-R diagram can be fit with model evolutionary tracks.

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Observations of Youngest Star Clusters

•  Young cluster “NGC 2264” –  Few million years old

•  High mass stars have reached main sequence

•  Lower mass stars are still approaching main sequence –  Locus of Deuterium burning –  Variable stars known as T

Tauri stars

•  Earlier stages hidden by dust –  Infrared observations reveal

hot cores (protostars). –  Accreting material in rotating

disk –  Gaseous outflows along poles

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Observations of Moderately-Young Clusters

•  Praesepe star cluster –  Few million years old

•  Higher mass stars on the main sequence

•  A few Red Giants present too. •  Low mass stars have reached

main sequence –  Entire main sequence populated

•  Evidence of young age –  Stars rapidly rotating –  Strong magnetic fields –  High variability of low-mass stars Note the contamination by stars along

the line of sight

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Observations of Oldest Star Clusters •  Old cluster “47 Tucanae”

–  About 12 Billion years old

•  Stars more massive than the Sun have evolved off the main sequence

•  Horizontal Branch stars are evident

•  Asymptotic Giant Branch stars evident as well

•  Note that the stars at the Tip of the Red Giant Branch are bright but not extremely bright.

•  Note the accuracy of the stellar models and the age dating.

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Evolutionary States of Stars in H-R Diagram

•  Evolution state of individual stars can now be evaluated given their location in the H-R diagram –  Evolutionary tracks “fit” to a star to estimate mass

and age. –  Observational properties of these post-main-

sequence stars can then provide context to their evolutionary state.

•  Variability (pulsations) •  Rotational velocity •  Stellar winds (mass loss, dust production) •  Atmospheric compositional differences

(dredge-up of enriched material)

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Chapter 17 Homework

Chapter 17: #1, 2, 3, 4, 6 (Due Thurs. Apr. 7)