Chapter 17: Stellar Evolution

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1 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 1

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

3 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 From our text: Horizons, by Sees Energy from supernova and other effects eventually disrupts cloud prevents further collapse. 3

4 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. ~ 10 8 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) 4

5 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. ~ 10 8 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 onionskin 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. 5

6 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. 6

7 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 7

8 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 8

9 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. 9

10 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-mainsequence 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) 10

11 Chapter 17 Homework Chapter 17: #1, 2, 3, 4, 6 (Due Thurs. Apr. 7) 11

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