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Main Sequence And Post-Main-Sequence Stellar Evolution

Main Sequence And Post-Main-Sequence Stellar Evolution. Late Stages of Stellar Evolution…continued Stellar Clusters. The Horizontal Branch Red-ward portion. After the most blue-ward point of HB is reached Mean molecular weight of core reaches point where it begins to contract

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Main Sequence And Post-Main-Sequence Stellar Evolution

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  1. Main Sequence And Post-Main-Sequence Stellar Evolution • Late Stages of Stellar Evolution…continued • Stellar Clusters

  2. The Horizontal BranchRed-ward portion After the most blue-ward point of HB is reached Mean molecular weight of core reaches point where it begins to contract Expansion and cooling of star’s envelope Shortly after beginning of the red-ward portion of the HB loop, helium has been exhausted from the core Inert CO core contracts Temperature increase in core. A thick helium burning shell develops outside the CO core. As core contracts. He shell narrows and strengthens Material above shell cools. Hydrogen shell burning turns off temporarily • Instabilities in outer envelopes • Periodic pulsations

  3. The Early Asymptotic Branch • After He core exhausted • When redward HB approximately meets Hayashi track • He shell burning dominates during this phase (Hydrogen buning shell nearly inactive) • Expanding envelope initially absorbs much of the energy produced by the Helium burning shell • As effevtive temperature decreases convective envelope deepens again • Second dredge up brings helium( Carbon and nitrogen as well) from the center outward

  4. The Thermal Pulse Asymptotic Giant Branch • Upper portion of Asymptotic Giant Branch • The once dormant Hydrogen shell re-ignites and again dominates the energy production of the Star • The Narrow helium burning shell turns on and off quasi-periodically • The hydrogen burning shell is dumping helium ash into the helium burning shell region-->intermittent helium shell flashes • As the mass of the helium shell increases, its base becomes slightly degenerate. When temperature at helium shell base becomes sufficient a helium shell flash occurs • Drives hydrogen burning shell outward, causing it to cool. • Star’s luminosity abruptly decreases

  5. The Thermal Pulse Asymptotic Giant Branch

  6. Third Dredge Up and Carbon Stars Convection zone is set up between the helium burning shell and the hydrogen burning shell. For stars with M> 2 M this convection zone will merge with upper convection zone Third Dredge up brings Carbon to surface

  7. S-Process Nucleosynthesis • http://en.wikipedia.org/wiki/S-process • Slow neutron capture

  8. Mass Loss and AGB Evolution • Asymptotic Giant Branch Stars lose mass at a rapid rate M~10-4 M /year • Mass ejected during thermal pulses • Stars with initial masses between 4-8 M most often lose enough mass to get below the Chandrasehkar limit of 1.4 M. • Stars with initial mass M<8 M …will then eventually leave behind a white dwarf • AGB is also source of interstellar dust • Effective temperature cool T ~ 3000K • --> Dust grains can form

  9. Mass Loss and AGB Evolution

  10. Post-Asymptotic Giant Branch • As cloud of expelled material becomes optically thin central star is exposed ..F,G supergiant • Star contracts but maintains luminosity…Effective temperature rises • Hydrogen and helium shells eventually extinguish…Luminosity plummets White Dwarf

  11. Post-Asymptotic Giant Branch

  12. Links • http://aspire.cosmic-ray.org/labs/star_life/support/HR_animated.swf • http://leo.astronomy.cz/sclock/sclock.html • http://www.arm.ac.uk/~csj/astnow.html

  13. Planetary Nebula • Expanding shell of gas around white dwarf progenitor • Ultraviolet photons from central star excite surrounding gases

  14. Stellar Clusters

  15. Population I,II and III Stars • Population III : The original stars (thus far hypothetical) that formed immediately after the big bang. Z=0. • Population II : Metal poor stars. Z>~0. Found well above or below the disk of the galaxy. High relative velocity to the Sun. • Population I : Metal rich stars. Z~0.03. Found predominately in the disk of the mily way with low velocities relative to the Sun.

  16. Globular Clusters and Galactic (Open) Clusters • Stellar clusters thought to have formed from same cloud within a relatively short amount of time. • Size ranging from 10-100,000 stars • Evolutionary states of the stars depend on initial mass • Extreme population II clusters formed when the Galaxy was very young…largest number of members • Population I clusters are younger and smaller …often found as open clusters

  17. Spectroscopic Parallax

  18. Color-Magnitude Diagrams

  19. Color-Magnitude Diagrams

  20. Isochrones and Cluster Ages • Can determine age of cluster from where stars have left main sequence

  21. The Hertzsprung Gap • Absence of stars in regions of H-R diagram just off of main sequence • Lack of representatives due to relatively short time spent at these points on H- R diagram

  22. Blue Stragglers • Tardiness in leaving main-sequence due to some unusual aspect of their evolution…Not completely understood • Mass exchange with binary companion????

  23. Stellar Pulsation • Observations of Pulsating Stars • The Physics of Stellar Pulsation • Modeling Stellar Pulsation • Non-radial Stellar Pulsation • Helioseismology and Asteroseismology

  24. RR Lyrae variables in the globular cluster M3 (one night’s observation) http://www.astro.princeton.edu/~jhartman/M3_movies.html

  25. Observations of Pulsating Variables • First noticed in 1595 by David Fabricius 2nd magnitude at its brightest…would “vanish”… • O-Ceti-->Mira • Believed to be due to dark splotches on rotating star…

  26. Delta-Cephei Prototype of the classical cepheid variable star John Goodricke discovered in 1784 that the brightness of Delta-Cephei was variable with a period of about 5 days!!!! magnitude varies from 3.4 to 4.3, luminosity changes by factor of100(Dm/5) = 100(0.9/5) = 2.3

  27. Period-Luminosity Relation Stars of the “Classical Cepheid Variable” type in the Small Magellanic Cloud were observed…and found to have a strong correlation between Period and apparent magnitude… • Henrietta Swan Leavitt discovered and classified ~2400 classical cepheid variable stars • Periods 1-50 days • She plotted luminosity vs. period for a set of cepheids from the Small Magellanic Cloud and found the Period-Luminosity relation • Ability to measure Distances !!!!! Henrietta Swan Leavitt (1868-1921)

  28. Period-Luminosity Relation Notice that there is scatter…

  29. Calibration of Cepheids The nearest Cepheid is Polaris (~200 pc), too far for trigonometric parallax. d (pc) = 1/p (in arcsec) In 1913, Ejnar Hertzsprung of Denmark used least squares mean parallax to determine the average magnitude M = -2.3 for a Cepheid with P = 6.6 days. d (pc) = 4.16/slope (in arcsec/yr)(4.16 AU/yr is the Sun’s motion) www.cnrt.scsu.edu/~dms/cosmology/DistanceABCs/distance.htm

  30. Calibration of Cepheids • Relation between average V band absolute magnitude and Period

  31. Cepheid Calibration-Infrared • Improved calibration at infrared wavelengths that suffer less from extinction • Adding a color term gives further improvement

  32. How to Find the Distance to aPulsating Star • Find the star’s apparent magnitude m (just by looking) • Measure the star’s period (bright-dim-bright) • Use the Period-Luminosity relation to find the stars absolute magnitude M • Calculate the star’s distance (in parsecs) using d (pc) = 10(m-M+5)/5

  33. Pulsation Hypothesis for Brightness Variation • Shapley proposed that the observed variation in brightness and temperature caused by radial pulsations of single stars. • Rhythmically “breathing” in and out! • R varies-->causes Luminosity and temperature to vary • Sir Arthur Eddington provided theoretical framework that could explain the variations in Brightness, temperature, radius and surface velocity • Delta-Cephei: supergiant star. Radius varies by 5%-10% (~1 R). F5(hottest)-G2(coolest) • Star is brightest when its surface is expanding outward most rapidly, after it has passed through its minimum radius…phase lag Delta-Cephei

  34. The Instability Strip • Majority of pulsating stars lie in the instability strip on the H-R diagram • As stars evolve along these tracks they begin to pulsate as they enter the instability strip and cease oscillations once they leave it. DT ~ 600 – 1100 K

  35. Classes of Pulsating Stars

  36. The Physics of Stellar Pulsation • The radial oscillations of a pulsating star are the result of sound waves resonating in the stars interior • Pulsation period can be roughly estimated from how long it takes a sound wave to cross the star’s diameter • Sound speed; • Pressure: • Period: Standing sound waves in an organ pipe. Radial modes for a pulsating star

  37. The Period – Mean Density Relation Period –luminosity relationship density period incr incr

  38. Eddington’s Thermodynamic Heat Engine • Mechanism that powers these standing waves are powered by the layers of gas that expand and contract • It the net positive work done in a cycle is positive the oscillations will be driven…but how • A layer becomes more opaque during compression. Dam up energy flowing toward surface and push layers outward • As layer expands becomes transparent would fall back down to repeat cycle…

  39. But this does not work for most stellar material! Why? The opacity is more sensitive to the temperature than to the density, so the opacity usually decreases with compression (heat leaks out). But in a partial ionization zone, the energy of compression ionizes the stellar material rather than raising its temperature! In a partial ionization zone, the opacity usually increases with compression! Partial ionization zones are the direct cause of stellar pulsation.

  40. Opacity Effects • Partial ionization zones have increased opacity under compression • Layer will trap energy and be lifted

  41. hydrogen ionization zone (H H+ and He He+) T = (1 – 1.5) x 104 K • helium II ionization zone (He+ He++) T = 4 x 104 K C C If the star is too hot, the ionization zones will be too near the surface to drive the oscillations. This accounts for the “blue edge” of the instability strip. The “red edge” is probably due to the onset of convection. f u n d a m e n t a l 1 s t o v e r t o n e n o p u l s a t I o n

  42. www.univie.ac.at/tops/dsn/texts/nonradialpuls.html

  43. Nonradial Oscillations Pulsational corrections df to equilibrium model scalar quantities f0 go as (the real part of) l = 0 radial m > 0 retrograde m < 0 prograde m = 0 standing http://gong.nso.edu/gallery/images/harmonics

  44. Smith, The Astrophysical Journal, 240, 149, 1980 to Earth In a rotating star, frequencies are rotationally split (~ Zeeman). Si III l = 2, m = 0, -1, -2

  45. Two Types of Nonradial Modes www.astro.uwo.ca/~jlandstr/planets/webfigs/earth/slide1.html

  46. p modes a surface gravity wave

  47. g modes

  48. Seismology and Helioseismology 5-minute p15 mode with l = 20 and m = 16 www.geophysik.uni-muenchen.de /research/seismology Courtesy NOAO

  49. GONG (Global Oscillation Network Group) a six-station network of extremely sensitive and stable velocity imagers located around the Earth to obtain nearly continuous observations of the Sun's "five-minute" oscillations

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