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How Stars Evolve

How Stars Evolve. Pressure and temperature Normal gases Degenerate gases The fate of the Sun Red giant phase Horizontal branch Asymptotic branch Planetary nebula White dwarf. Normal gas. Pressure is the force exerted by atoms in a gas Temperature is how fast atoms in a gas move.

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How Stars Evolve

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  1. How Stars Evolve • Pressure and temperature • Normal gases • Degenerate gases • The fate of the Sun • Red giant phase • Horizontal branch • Asymptotic branch • Planetary nebula • White dwarf

  2. Normal gas • Pressure is the force exerted by atoms in a gas • Temperature is how fast atoms in a gas move • Hotter  atoms move faster  higher pressure • Cooler  atoms move slower  lower pressure Pressure balances gravity, keeps stars from collapsing

  3. Degenerate gas • Very high density • Motion of atoms is not due to kinetic energy, but instead due to quantum mechanical motions • Pressure no longer depends on temperature • This type of gas is sometimes found in the cores of stars

  4. Fermi exclusion principle • No two electrons can occupy the same quantum state • Quantum state = energy level + spin • Electron spin = up or down

  5. Electron orbits Only two electrons (one up, one down) can go into each energy level

  6. Electron energy levels • Only two electrons (one up, one down) can go into each energy level. • In a degenerate gas, all low energy levels are filled. • Electrons have energy, and therefore are in motion and exert pressure even if temperature is zero.

  7. Which of the following is a key difference between the pressure in a normal gas and in a degenerate gas? • Degenerate pressure exists whether matter is present or not. • In a degenerate gas pressure varies rapidly with time. • In a degenerate gas, pressure does not depend on temperature. • In a degenerate gas, pressure does not depend on density.

  8. The Fate of the Sun • How will the Sun evolve over time? • What will be its eventual fate?

  9. Sun’s Structure • Core • Where nuclear fusion occurs • Envelope • Supplies gravity to keep core hot and dense

  10. Main Sequence Evolution • Core starts with same fraction of hydrogen as whole star • Fusion changes H  He • Core gradually shrinks and Sun gets hotter and more luminous

  11. Gradual change in size of Sun Now 40% brighter, 6% larger, 5% hotter

  12. Main Sequence Evolution • Fusion changes H  He • Core depletes of H • Eventually there is not enough H to maintain energy generation in the core • Core starts to collapse

  13. Red Giant Phase • He core • No nuclear fusion • Gravitational contraction produces energy • H layer • Nuclear fusion • Envelope • Expands because of increased energy production • Cools because of increased surface area

  14. Sun’s Red Giant Phase

  15. HR diagram Giant phase is when core has been fully converted to Helium

  16. A star moves into the giant phase when: • It eats three magic beans • The core becomes helium and fusion in the core stops. • Fusion begins in the core • The core becomes helium and all fusion in the star stops.

  17. Broken Thermostat • As the core contracts, H begins fusing to He in a shell around the core • Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting

  18. Helium fusion Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

  19. Helium Flash • He core • Eventually the core gets hot enough to fuse Helium into Carbon. • This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. • We don’t see this flash because it’s buried inside the Sun. • H layer • Envelope

  20. Movement on HR diagram

  21. Movement on HR diagram

  22. Helium Flash • He core • Eventually the core gets hot enough to fuse Helium into Carbon. • The Helium in the core is so dense that it becomes a degenerate gas. • H layer • Envelope

  23. Red Giant after Helium Ignition • He burning core • Fusion burns He into C, O • He rich core • No fusion • H burning shell • Fusion burns H into He • Envelope • Expands because of increased energy production

  24. Sun moves onto horizontal branch Sun burns He into Carbon and Oxygen Sun becomes hotter and smaller What happens next?

  25. What happens when the star’s core runs out of helium? • The star explodes • Carbon fusion begins • The core starts cooling off • Helium fuses in a shell around the core

  26. Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate

  27. Sun moves onto Asymptotic Giant Branch (AGB)

  28. Sun looses mass via winds • Creates a “planetary nebula” • Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen • Hydrogen continues to burn

  29. Planetary nebula

  30. Planetary nebula

  31. Planetary nebula

  32. Hourglass nebula

  33. When on the horizontal branch, a solar-mass star • Burns H in its core. • Burns He in its core. • Burns C and O in its core. • Burns He in a shell around the core.

  34. White dwarf • Star burns up rest of hydrogen • Nothing remains but degenerate core of Oxygen and Carbon • “White dwarf” cools but does not contract because core is degenerate • No energy from fusion, no energy from gravitational contraction • White dwarf slowly fades away…

  35. Evolution on HR diagram

  36. Time line for Sun’s evolution

  37. In which order will a single star of one solar mass progress through the various stages of stellar evolution? • Planetary nebula, main-sequence star, white dwarf, black hole • Proto-star, main-sequence star, planetary nebula, white dwarf • Proto-star, red giant, supernova, planetary nebula • Proto-star, red giant, supernova, black hole

  38. Death of stars • Final evolution of the Sun • Determining the age of a star cluster • Evolution of high mass stars • Where were the elements in your body made?

  39. Higher mass protostars contract faster Hotter

  40. Higher mass stars spend less time on the main sequence

  41. Determining the age of a star cluster • Imagine we have a cluster of stars that were all formed at the same time, but have a variety of different masses • Using what we know about stellar evolution is there a way to determine the age of the star cluster?

  42. Turn-off point of cluster reveals age

  43. The HR diagram for a cluster of stars shows stars with spectral types A through K on the main sequence and stars of type O and B on the (super) giant branch. What is the approximate age of the cluster? • 1 Myr • 10 Myr • 100 Myr • 1 Gyr

  44. Higher mass stars do not have helium flash

  45. Nuclear burning continues past Helium 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core

  46. Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells

  47. Why does fusion stop at Iron?

  48. Fusion versus Fission

  49. Advanced reactions in stars make elements like Si, S, Ca, Fe

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