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Understanding the Hertzsprung-Russell Diagram: Stars, Gravity, and Supernovae

This lecture discusses the Hertzsprung-Russell Diagram and its significance in understanding stellar evolution, focusing on the balance between gas pressure and gravitational forces within stars. As stars exhaust their heat sources, they face collapse, leading to phenomena like supernovae, with examples such as SN 1994D and SN 1987A. The lecture explores how stellar evolution reveals the cosmic struggle of pressure against gravity, ultimately producing neutron stars or black holes. It emphasizes that massive stars synthesize heavier elements, reminding us that "we are stardust."

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Understanding the Hertzsprung-Russell Diagram: Stars, Gravity, and Supernovae

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  1. UCL Science Centre ‘Science Lectures for Schools’ 2010 Nov 26 Ian Howarth http://www.star.ucl.ac.uk/~idh/

  2. The Hertzsprung-Russell Diagram: Stars Struggle Against Gravity The Hertzsprung-Russell Diagram: Stars Struggle Against Gravity

  3. What’s this got to do with supernovae? Normal stars are in a state of equilibrium between gas pressure pushing outwards and gravity pulling inwards (just like our atmosphere). However, to maintain the gas pressure we need a heat source. When that source is exhausted, gas pressure is removed, and the star will collapse. A big star will undergo a big collapse: a supernova SN 1994D in NGC 4526

  4. RCW 86: remnant of “Guest Star” from 185 1054, Crab Nebula SN 1006: brightest star ever seen

  5. “Tycho’s Star” (1572) De nova [et nullius aevi memoria prius visa] stella

  6. Kepler’s Star (1604)

  7. SN 1885 in M31

  8. Fritz Zwicky (1898-1974) (coined Supernova)

  9. SN 1937A NGC 4157

  10. Tom Boles

  11. M51

  12. Nuclear ‘burning’: HHe ~1x107K

  13. ~3x107K

  14. Helium burning: ~108K The continuing struggle against gravity... Carbon burning: ~109K

  15. Then what...? Gravity’s victory!

  16. Collapse!! Timescale ~1s Velocities ~1/4 c Cooling by photo- disintegration γ+56Fe↔134He+4n and electron capture p++e-→n+νe Most energy comes out in neutrinos Shock wave propagates out over a day or so  observed SN

  17. SN 1987A (Feb 23)

  18. 25 neutrinos = all extragalactic neutrino astronomy...confirms core-collapse model (and limits neutrino mass)

  19. To recap: Stellar evolution is the struggle of pressure against gravity. Gravity always defeats gas pressure, eventually For solar-type stars, the last defence is electron degeneracy pressure (the sun will end its life as a white dwarf). For more massive stars, the final fate is a neutron star, or a black hole, formed in a supernova explosion On the way, massive stars make pretty much all the elements heavier than oxygen (and quite a lot of the lighter ones): “we are stardust” http://www.star.ucl.ac.uk/~idh/

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