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Detailed look at late stages of the Sun’s life:

Detailed look at late stages of the Sun’s life:

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Detailed look at late stages of the Sun’s life:

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  1. Detailed look at late stages of the Sun’s life: • from Schröder et al 2001 (Astronomy & Geophysics Vol 42): • radius of Sun as it ascends RGB, then AGB (with inner planet orbits marked!) PHYS1005 – 2003/4

  2. but mass-loss in red-giant wind could be important: • as is the likely temperature on Earth! PHYS1005 – 2003/4

  3. Lecture 17: Stellar Structure and Evolution – II • Objectives: • Understand differences in evolution of low and high M stars • Importance of degeneracy pressure • Understand the Helium Flash H-R diagram showing evolutionary tracks followed by both young and old clusters: Note the gap between the Main Sequence and RGB in young clusters Additional reading: Kaufmann (chap. 21-22), Zeilik (chap. 16) PHYS1005 – 2003/4

  4. Evolution of 5MO Star: • on exhaustion of H in core (at 2) •  crisis point for high M stars • cores convective  well-mixed •  H runs out over large central volume at same time! •  must contract radically to ignite H shell • i.e. large change on thermal (short) timescale •  moves rapidly (23) to RGB • explains Hertzsprung Gap in young clusters • cf Low Mass stars: • cores are radiative  more stable  change much more gradual •  continuous and extended RGB in old clusters PHYS1005 – 2003/4

  5. Helium Flash and Degeneracy Pressure in low M stars • ignition of He at tip of RGB is crisis point for low M stars • due to new form of P which has been supporting the He core • comes from Pauli Exclusion Principle: • can estimate it as follows: • electron density of n per unit volume •  each electron occupies box of side • to avoid “overlap”, need de Broglie λ ~ size of box i.e. • therefore “degeneracy” energy • which is significant when Ed ~ kT i.e. • N.B. low m low n  electrons degenerate first! No two electrons within certain volume can occupy same quantum state • which links v and n (m = electron mass) PHYS1005 – 2003/4

  6. Electron degeneracy pressure • For case of ideal gas, where P = nkT • and since n αρ, then • which is independent of T ! • What happens when fusion begins in degenerate gas? • energy generated by fusion •  T rises •  fusion rate rises • but P stays the same  no expansion, no cooling ! • i.e. normal “safety valve” doesn’t work! •  disastrous runaway process until T very high • Helium Flash !L can rise to 1010 LO within ~ minutes PHYS1005 – 2003/4

  7. Evolutionary model for Sun: Binding Energy (first 31 elements) Speed of evolution: • evolution speeds up with age, because • L is higher (and timescale α M / L) • higher neutrino losses • less fusion energy from heavier elements • most stable nucleus is iron (56Fe) • H  Fe fusion converts 0.89% mass  energy, but • H  He fusion converts 0.71% ! • so later phases give little in total PHYS1005 – 2003/4