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Nucleosynthesis and stellar lifecycles

Nucleosynthesis and stellar lifecycles. Outline: What nucleosynthesis is, and where it occurs Molecular clouds YSO & protoplanetary disk phase Main Sequence phase Old age & death of low mass stars Old age & death of high mass stars Nucleosynthesis & pre-solar grains. Stellar lifecycles.

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Nucleosynthesis and stellar lifecycles

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  1. Nucleosynthesis and stellar lifecycles

  2. Outline: • What nucleosynthesis is, and where it occurs • Molecular clouds • YSO & protoplanetary disk phase • Main Sequence phase • Old age & death of low mass stars • Old age & death of high mass stars • Nucleosynthesis & pre-solar grains Stellar lifecycles

  3. What nucleosynthesis is, and where it occurs

  4. Nucleosynthesis formation of elements Except for H, He (created in Big Bang), all other elements created by fusion processes in stars Relative abundance

  5. Stellar Nucleosynthesis Some H destroyed; all elements with Z > 2 produced Various processes, depend on (1) star mass (determines T) (2) age (determines starting composition) Z = no. protons, determines element

  6. Beta Stability Valley. Nucleons with right mix of neutrons (n) to protons (p) are stable. Those that lie outside of this mix are radioactive. p > n >

  7. Beta Stability Valley. Too many n: beta particle (electron) emitted, n converted to p. (Beta Decay) e.g. 26Al -> 26Mg + beta e.g. 53Mn -> 53Cr + beta Some stellar nucleosynthesis resulted in n-rich nucleons that are short-lived nuclides. too many n p > n >

  8. Beta Stability Valley. Too many p: electron captured by nucleus, p converted to n. e.g., 41Ca + electron -> 41K Other stellar nucleosynthesis produced short-lived p-rich nucleons. too many p p > n >

  9. Stellar lifecycles: from birth to death low mass star (< 5 Msun) high mass star (> 5 Msun)

  10. Stellar lifecycles: low mass stars Stellar nucleosynthesis 2. Main Seq. 3. Red Giant low mass star (< 5 Msun) 1 & 5. molecular cloud 4. Planetary nebula 4. White dwarf Nucleosynthesis possible if white dwarf in binary system (during nova or supernova)

  11. Stellar lifecycles: high mass stars Stellar nucleosynthesis 2. Main Seq. (luminous) 3. Red Giant/ Supergiant 1 & 6. molecular cloud high mass star (>5 Msun) 5. Neutron star 4. Supernova 5. Black hole

  12. Track stellar evolution on H-R diagram of T vs luminosity Luminosity: energy / time

  13. Distribution of stars on H-R diagram. When corrected for intrinsic brightness, there are MANY more cool Main Sequence stars than hot.

  14. On main sequence, luminosity depends on mass L ~ M3.5

  15. Molecular clouds: Where it begins & ends molecular cloud

  16. Molecular clouds cold, dense areas in interstellar medium (ISM) Horsehead Nebula Mainly molecular H2, also dust, T ~ 10s of K

  17. Famous Eagle Nebula image. Cool dark clouds are close to hot stars that are causing them to evaporate.

  18. Dust in ISM consists of: -- ices, organic molecules, silicates, metal, graphite, etc. -- some of these preserved as pre-solar grains & organic components in meteorites

  19. A larger Interplanetary Dust Particle (IDP)

  20. Molecules in ISM as of 12 / 2004 Note many C-compounds HF H2D+, HD2+ O2 ? All molecules have been detected (also) by rotational spectroscopy in the radiofrequency to far-infrared regions unless indicated otherwise. * indicates molecules that have been detected by their rotation-vibration spectrum,** those detected by electronic spectroscopy only. http://www.ph1.uni-koeln.de/vorhersagen/molecules/main_molecules.html

  21. Photochemistry can occur in icy mantles to create complex hydrocarbons from simple molecules

  22. Gravity in molecular clouds helps promote collapse of cloud …and sometimes is assisted by a trigger

  23. Young stellar objects (YSOs) & protoplanetary disks (proplyds) YSOs

  24. YSOs & Proplyds: Molecular cloud fragments that have collapsed– no fusion yet < Protoplanetary disk around glowing YSO in Orion Solar nebula: the Protoplanetary disk out of which our solar system formed

  25. Herbig-Haro • Objects-- • YSOs with • disks & bipolar • outflows

  26. Magnetic fields around YSOs can create polar jets and X winds

  27. Collapse of molecular cloud fragments occurs rapidly ~105 to 107 yrs, depending on mass Protostellar disk phase lasts ~106 yrs

  28. Single collapsing molecular cloud produces many fragments, each of which can produce a star

  29. Main Sequence phase: Middle age Main sequence

  30. Star “turns on” when nuclear fusion occurs main sequence star – either proton-proton chain or CNO cycle nucleosynthesis P-P chain net: 4 H to 1 He

  31. CNO cycle – more efficient method, but requires higher internal temperature, so only for stars with mass higher than 1.1 solar masses 12C + p -> 13N 13N -> 13C 13C + p -> 14N 14N + p -> 15O 15O -> 15N 15N + p -> 12C + 4He CNO cycle net reaction : 4 H to 1 He

  32. Star stays on main sequence in stable condition– so long as H remains in the core A more massive star must produce more energy to support its own weight – reason there is a correlation of mass and luminosity on main sequence

  33. But– eventually the H runs out Lifetime on main sequence = fuel / rate of consumption ~ M / L ~ M / M3.5 lifetime ~ 1/M2.5 So a 4 solar mass star will have a main sequence lifetime 1/32 as long as our sun

  34. So, what happens when the core runs out of hydrogen? • Star begins to collapse, heats up • Core contains He, continues to collapse • But H fuses to He in shell– greatly inflating star •  RED GIANT (low mass) • or SUPERGIANT (high mass)

  35. What happens next depends on stellar mass

  36. Old age and death of low mass stars Red Giant Planetary nebula White dwarf

  37. There are different types of Red Giant Stars • RGB (Red Giant Branch) • Horizontal branch • AGB (Asymptotic Giant Branch) • These differ in position on H-R diagram and in • interior structure

  38. Red Giant (RGB) star: H burning in shell

  39. Red Giant (Horizontal branch) star: He fusion in core Red Giant (AGB) star: He burning in shell AGB star

  40. Convective dredge-ups bring products of fusion to surface Red Giant includes: s-process nucleosynthesis

  41. s-process nucleosynthesis: slow neutron addition beta decay keeps pace with n addition No. protons (Z)

  42. An AGB can lose its outer layers— Ultimately a planetary nebula forms, leaving a white dwarf in the center Planetary nebula White dwarf

  43. Planetary nebulas Note: planetary nebula have nothing to do with planets!

  44. Nuclear fusion stops when the star becomes a white dwarf— It gradually cools down

  45. Old age & death of high mass stars Super Giant Neutron star Supernova Black hole

  46. High-mass stars: Progressive core fusion of elements heavier than C

  47. Includes: s-process nucleosynthesis as Supergiant, r-process nucleosynthesis during core collapse

  48. No. protons (Z) r-process nucleosynthesis: rapid neutron addition beta decay does not keep pace with n addition

  49. End for high mass star comes as it tries to fuse core Fe into heavier elements– and finds this absorbs energy STAR COLLAPSES & EXPLODES AS SUPERNOVA

  50. --Fe core turns into dense neutrons --Supernova forms because overlying star falls onto dense core & bounces off of it

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