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LIFECYCLES OF STARS

LIFECYCLES OF STARS. Option 2601. Stellar Physics. Unit 1 - Observational properties of stars Unit 2 - Stellar Spectra Unit 3 - The Sun Unit 4 - Stellar Structure Unit 5 - Stellar Evolution Unit 6 - Stars of particular interest. Unit 5. Stellar Evolution. Stellar Evolution.

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LIFECYCLES OF STARS

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  1. LIFECYCLES OF STARS Option 2601

  2. Stellar Physics • Unit 1 - Observational properties of stars • Unit 2 - Stellar Spectra • Unit 3 - The Sun • Unit 4 - Stellar Structure • Unit 5 - Stellar Evolution • Unit 6 - Stars of particular interest

  3. Unit 5 Stellar Evolution

  4. Stellar Evolution • Star formation • Main sequence • Stellar clusters (open, globular) • Population I & II stars • Red Giants • Planetary Nebulae • White Dwarfs • Supernovae • Neutron Stars

  5. Sequence • Protostar • Pre-main Sequence (PMS) • Main Sequence • Post-main Sequence

  6. Protostars • Stars born by gravitational contraction of interstellar clouds of gas and dust • Gravitation energy  50% thermal & 50% radiative • Cloud is a Protostar before hydrostatic equilibrium is established

  7. Protostars • Collapse starts in “free fall” • Particles do not collide during collapse • i.e. P=0, gravity is only force involved • Collapse is uneven • Core collapses more rapidly forming a small central condensation • Core then accretes material

  8. Protostars • Low mass objects accrete all (most) of material • High mass objects behave similarly, but • Fusion begins before end of accretion • Some material then blown away by radiation pressure

  9. Effect of Rotation • If angular momentum > 0 • Cloud flattens into a disk • In some cases several central blobs form, which can coalesce into fewer… • Multiple star systems

  10. Cloud Collapse

  11. Star Formation

  12. Star Formation

  13. Star Formation

  14. Pre-main sequence for a solar mass star

  15. Evolution of a high mass star

  16. Star Formation

  17. Star Formation

  18. Stellar Lifecycle

  19. The Main Sequence • Start of nuclear burning  zero-age main sequence • As H  He composition () changes, structure changes • Rates of evolution depend on two things • Initial mass • Composition

  20. The Main Sequence • High mass stars are hotter & more luminous • Use their energy faster, i.e. evolve faster • Spend less time on the main sequence • O & B stars evolve faster than M stars

  21. Mass-luminosity relation: Giving star lifetime: Quantitatively

  22. Eagle Nebula

  23. Eagle Nebula

  24. Rosette Nebula

  25. T

  26. The Pleiades

  27. Population I Stars • Accreting from the ISM now! (i.e. recent past) • Typical stars are young, in galactic spiral arms where gas and dust found • Typically reside in open star clusters • ~2% of mass elements heavier than H or He (ISM enriched by supernovae) • If M* a little > M energy generation is by CNO cycle • Sun is population I

  28. Post main-sequence for a solar mass star

  29. Evolutionary phases of a solar mass star, post main-sequence

  30. End of Main Sequence

  31. Post main-sequence for a solar mass star

  32. Population II Stars • First stars to be formed in Universe • Have only 0.01% heavy elements • Typically found in galactic bulge and globular clusters • Similar sequence of evolution but occupy different region of H-R diagram during core He burning • Significant temperature changes, heating and then cooling

  33. Late in the life of a solar mass star

  34. Red Giant > PN

  35. Evolutionary phases of a solar mass star, post main-sequence

  36. Late in the life of a solar mass star

  37. PN > White Dwarf

  38. White Dwarfs

  39. For a degenerate gas (non-relativistic): Constant For a perfect gas: From hydrostatic equilibrium:  Greater mass, smaller radius Chandrasekhar Limit White dwarfs form from stars with M  8MSun Degenerate gas pressure prevents further gravitational contraction Chrandrasekhar limit: degeneracy pressure can only support M  1.4MSun. Above this limit a neutron star is formed

  40. White dwarf companions e.g. Sirius – companion Sirius B (Alvan Clark, 1862) Procyon – Procyon B (1882) In binaries we can measure the companion’s mass from Kepler’s laws MSirius B = 1.0MSun TSirius A = 10,000K ; MV = -1.5 TSirius B = 25,000K ; MV = 8 From : R  7  10-3RSun   = 3  109kg m-3 3  10-3LSun

  41. Massive Stars • Stars with masses > 7 M • Masses greater than ~ 50 M • Affected by mass loss (i.e. winds) • As mass of star changes so does the structure and luminosity

  42. Evolution of a high mass star

  43. Evolutionary phases of a massive star

  44. Evolution of a high mass star

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