Overview of this set: The Main Sequence: Brief review Stellar evolution off the Main Sequence

1. Overview of this set: The Main Sequence: Brief review Stellar evolution off the Main Sequence Cepheid variable stars: Measuring distance

5. Because It gets very dense: T is rising!

6. T-> 100,000,000K-> 3 a Very Fast Degenerate gas P not dependent on T if it did the core would expand and slow reaction 3a

9. Evidence for Stellar Evolution: Variable Stars Some stars show intrinsic brightness variations not caused by eclipsing in binary systems. Most important example: d Cephei Light curve of d Cephei

11. Cepheid Variables: The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. The more luminous it is, the more slowly it pulsates. => Measuring a Cepheid’s period, we can determine its absolute magnitude!

12. Cepheid Distance Measurements Comparing absolute and apparent magnitudes of Cepheids, we can measure their distances (using the 1/d2 law)! The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way! Cepheids are up to ~ 40,000 times more luminous than our sun => can be identified in other galaxies.

13. Pulsating Variables: The Instability Strip For specific combinations of radius and temperature, stars can maintain periodic oscillations. Those combinations correspond to locations in the Instability Strip Cepheids pulsate with radius changes of ~ 5 – 10 %.

16. Pulsating Variables: The Valve Mechanism Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers. => Expansion Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction. Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion

17. Ie:ionization: He+ +photon -> H++ =opacity of radiation R is getting smaller T is going up Opacity holds back radiation Star expanding High T drops opacity Radiation floods out and

18. We get Absolute Magnitude By knowing the Distance of Some Cepheids So get P gives M look at m use distance modulus to find Distance…..Cepheids are visible in other galaxies! SEE HW Examples

20. Period Changes in Variable Stars Periods of some Variables are not constant over time becauseof stellar evolution. Another piece of evidence for stellar evolution.

21. The End of a Star’s Life When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die. High-mass stars will die first, in a gigantic explosion, called a supernova. Less massive stars will die in a less dramatic event, called a nova

22. Red Dwarfs Stars with less than ~ 0.4 solar masses are completely convective. Mass Hydrogen and helium remain well mixed throughout the entire star. No phase of shell “burning” with expansion to giant. Star not hot enough to ignite He burning.

23. Sunlike Stars Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core. Mass Expansion to red giant during H burning shell phase Ignition of He burning in the He core Formation of a degenerate C,O core

24. Mass Loss From Stars Stars like our sun are constantly losing mass in a stellar wind (solar wind). The more massive the star, the stronger its stellar wind. Far-infrared WR 124

25. The Final Breaths of Sun-Like Stars: Planetary Nebulae Remnants of stars with ~ 1 – a few Msun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts)  Less than 10,000 years old Have nothing to do with planets! The Helix Nebula

26. The Formation of Planetary Nebulae Two-stage process: Slow wind from a red giant blows away cool, outer layers of the star The Ring Nebula in Lyra Fast wind from hot, inner layers of the star overtakes the slow wind and excites it => Planetary Nebula

27. The Dumbbell Nebula in Hydrogen and Oxygen Line Emission

28. Planetary Nebulae Often asymmetric, possibly due to • Stellar rotation • Magnetic fields • Dust disks around the stars • binaries The Butterfly Nebula

29. marker

31. ? Photons carry momentum =E/c

32. 7. All H-> He core collapse GP energy to KE to heat H shell Forms H shell ignites * moves to the right now a SUBGIANT. Note energy output increased but outer layers cooling 8. H shell energy intensifies, (Triple alpha starts here) Helium flash, He to C fast, outer expansion to Red Giant. Radius increase s but outer T nearly constant. I.e. star expands cools outer layer but energy coming out Compensates since it is rising. The greater the temp gradient dT/dr) the faster the energy flow. 9.After He flash Energy production slows ( core no longer degenerate) star goes to Horizontal branch. Outer layer weakly held, Mass loss starts 10. All He cor -> C , oore contracts He shell heats up *-> asymptotic Giant Oscillations of energy can now occur. 11. Outer core held loosely _>PN form 10.

34. Deep Sky: The Planetary Nebula (show) Glowing gaseous shrouds shed by dying sun-like stars trying to stabilize as they run out of nuclear fuel.. Typically 1,000 times the size of our solar system These Ten have names like Owl, the Cat's Eye, the Ghost of Jupiter, Ring. This glorious final phase in the life of a star lasts only about 10,000 yrs. Note: The Star Remnant At center of The Planetary Nebulae. Called “Planetary” Because the Resemble planets In a telescope GOOD-BYE PLANETS!

35. PN Formation is still a mystery and can be complex as this image shows

37. Physical conditions… from lines which are sensitive to temperature and density Appears as ring since Line of sight in shell to us Has more material at edges Of shell. Like looking through a Balloon! See this at CSI observatory

38. The Remnants of Sun-Like Stars: White Dwarfs Sunlike stars build up a Carbon-Oxygen (C,O) core, which does not ignite Carbon fusion. He-burning shell keeps dumping C and O onto the core. C,O core collapses and the matter becomes degenerate. Formation of a White Dwarf

39. White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense:1 teaspoon of WD material: mass ≈ 16 tons!!! Chunk of WD material the size of a beach ball would outweigh an ocean liner! White Dwarfs: Mass ~ Msun Temp. ~ 25,000 K Luminosity ~ 0.01 Lsun

40. The Chandrasekhar Limit The more massive a white dwarf, the smaller it is. Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity. WDs with more than ~ 1.4 solar masses can not exist!

41. White Dwarfs (2) Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.

42. HW #14read Chapter 10Questions Problems 1,2,4,6,9,12