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Stellar Death

Astronomy 315 Professor Lee Carkner Lecture 14. “I am glad we do not have to try to kill the stars. … Imagine if a man each day should have to try to kill the sun? We were born lucky” --Earnest Hemingway, The Old Man and the Sea. Stellar Death. Death Defined.

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Stellar Death

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  1. Astronomy 315 Professor Lee Carkner Lecture 14 “I am glad we do not have to try to kill the stars. … Imagine if a man each day should have to try to kill the sun? We were born lucky” --Earnest Hemingway, The Old Man and the Sea Stellar Death

  2. Death Defined • The star can no longer support itself by internal thermal pressure and so: • The details depend on mass

  3. Very Low Mass • Red dwarfs (M < 0.4 Msun) burn their fuel very slowly • Take a very long time (10’s of billions of years) to use up all hydrogen • Red dwarfs will fade away as they run out of fuel • Never become giants since they produce no helium core

  4. Solar Type • Stars with between 0.4 and 4 Msun go through the following phases: • Hydrogen and helium shell burning (asymptotic giant branch) • What happens next?

  5. Evolution of 1 Solar Mass Star

  6. Mass Loss • All stars lose mass • Mass loss is very low for main sequence stars • Giants have higher mass loss rates, due to: • Thermal pulses: changes in the core that cause bursts of energy which can push the outer layers away

  7. Separation • Core gets denser, outer layers get less dense • If the core is hot enough, its radiation will make the ejected outer layers glow

  8. Planetary Nebulae • These glowing ejecta are known as planetary nebulae • Have nothing to do with planets • Composition: low density gas producing emission lines

  9. IC3568 --HST

  10. Mz3 -- HST

  11. Ring Nebula -- HST

  12. Structure of Planetary Nebulae • We would expect planetary nebulae to be spherical • How does spherical star eject mater into a non-spherical shape? • Blocked by companion stars or planets? • Different waves of ejecta interacting?

  13. White Dwarf • The leftover core of the star becomes a white dwarf • There is no fusion going on in a white dwarf so it slowly cools • What supports a white dwarf?

  14. Degeneracy • Electrons obey the laws of quantum physics including the Pauli Exclusion Principle: • Due to its high pressure the core becomes degenerate • Degenerate gas resists compression because electrons cannot be forced any closer together due to the Pauli exclusion principle

  15. White Dwarf Properties • White dwarfs are very dense • Start out hot and then cool • White dwarfs obey the Chandrasekhar Limit • Must be less than 1.4 Msun, or they cannot be supported by electron degeneracy pressure

  16. Sirius A and B

  17. High Mass Stars • Star will become a supergiant with a huge radius (up to 5 AU) but most of its mass in a small earth-sized core of layered elements

  18. Evolutionary Paths

  19. Core Collapse • In a short time (million years or less) the star burns through all elements up to iron • There is no more thermal energy to support the very dense core • Energy from the collapsing core rebounds to produce a supernova

  20. Supernova • A nova is a generic term for a sudden brightening of a star • An exploding massive star is technically known as a Type II supernova • Explosion is almost a billion times more luminous than the sun • Leaves behind a supernova remnant

  21. Supernova 1987a -- Before & After

  22. Supernova 1987a -- Remnant

  23. Anasazi Depiction of 1054 SN?

  24. Crab Nebula -- Optical & X-ray

  25. Post Main Sequence Paths

  26. Stellar Corpses • After a supernova (or the planetary nebula phase) the core of the star gets left behind • Low and medium mass stars leave white dwarfs • Higher mass stars produce neutron stars • Very high mass stars produce black holes

  27. Next Time • Read Chapter 22.1-22.4

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