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The Life and Death of Stars: From Main Sequence to Neutron Stars

Learn about the stages in the life of a star, from its time on the main sequence to its death as a white dwarf or neutron star. Understand the processes of fusion and the formation of elements, as well as the explosive nature of supernovae.

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The Life and Death of Stars: From Main Sequence to Neutron Stars

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  1. Conversations with the EarthTom Burbinetburbine@framingham.edu

  2. Quiz on Thursday • Sun • Hertzsprung-Russell Diagram • Death of stars

  3. Main Sequence • Is not an evolutionary track • Stars do not evolve on it • Stars stop on the main sequence and spend most of their lives on it

  4. Sun ends it time on the main sequence When the core hydrogen is depleted, nuclear fusion stops The core pressure can no longer resist the crush of gravity Core shrinks

  5. Why does the star expand? The core is made of helium The surrounding layers are made of hydrogen

  6. And .. Gravity shrinks the inert helium core and surrounding shell of hydrogen The shell of hydrogen becomes hot for fusion This is called hydrogen-shell burning

  7. And … The shell becomes so hot that its fusion rate is higher than the original core This energy can not be transported fast enough to surface Thermal pressure builds up and the star expands

  8. And .. More helium is being created Mass of core increases Increases its gravitational pull Increasing the density and pressure of this region

  9. When When helium core reaches 100 million Kelvin, Helium can fuse into a Carbon nucleus

  10. Helium Flash The rising temperature in the core causes the helium fusion rate to rocket upward Creates a lot of new energy

  11. However The core expands Which pushes the hydrogen-burning shell outwards Lowering the hydrogen-burning shell’s temperature

  12. And Less energy is produced Star starts to contract

  13. Now In the core, Helium can fuse to become Carbon (and some Oxygen) Star contracts Helium fusion occurs in a shell surrounding the carbon core Hydrogen shell can fuse above the Helium shell Inner regions become hotter Star expands

  14. http://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.pnghttp://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png

  15. Some carbon fuses with He to form Oxygen 12C + 4He → 16O + gamma ray Harder to fuse Oxygen with Helium to produce Neon

  16. Planetary Nebulae There is a carbon core and outer layers are ejected into space The core is still hot and that ionizes the expanding gas

  17. Planetary Nebulae

  18. White Dwarf The remaining core becomes a white dwarf White dwarfs are usually composed of carbon and oxygen Oxygen-neon-magnesium white dwarfs can also form Helium white dwarfs can also form

  19. High-Mass Stars The importance of high-mass stars is that they make elements heavier than carbon You need really hot temperatures which only occur with the weight of a very high-mass star

  20. Stages of High-Mass Star’s Life Similar to low-mass star’s Except a high-mass star can continue to fuse elements When the fusion ceases, the star becomes a supernova Supernova is a huge explosion

  21. Fusion The temperatures of high-mass stars in its late-stage of life can reach temperatures above 600 million Kelvin Can fuse Carbon and heavier elements Helium Capture can also occur where Helium can be fused into heavy elements

  22. “Deaths” of Stars White Dwarfs Neutron Stars Black Holes

  23. White Dwarfs White Dwarfs is the core left over when a star can no longer undergo fusion Most white dwarfs are composed of carbon and oxygen Very dense Some have densities of 3 million grams per cubic centimeter A teaspoon of a white dwarf would weigh as much as an elephant

  24. White Dwarfs Some white dwarfs have the same mass as the Sun but slightly bigger than the Earth 200,000 times as dense as the earth

  25. White Dwarfs Collapsing due to gravity The collapse is stopped by electron degeneracy pressure

  26. Electron Degeneracy Pressure No two electrons can occupy the same quantum state

  27. The Sun Will end up as a White Dwarf

  28. Neutron Star Neutron stars are usually 10 kilometers acroos But more massive than the Sun Made almost entirely of neutrons Electrons and protons have fused together

  29. How do you make a neutron star? Remnant of a Supernova

  30. Supernova • A supernova is a stellar explosion. • Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.

  31. Type Ia Supernova Type II Supernova

  32. This stops with Iron • Fusion of Iron with another element does not release energy • Fission of Iron does not release energy • So you keep on making Iron

  33. Initially • Gravity keeps on pulling the core together • The core keeps on shrinking • Electron degeneracy keeps the core together for awhile

  34. Then • The iron core becomes too massive and collapses • The iron core becomes neutrons when protons and electrons fuse together

  35. Density of neutron star • You could take everybody on Earth and cram them into a volume the size of sugar cube

  36. Explosion • The collapse of the core releases a huge amount of energy since the rest of the star collapses and then bounces off the neutron core • 1044-46 Joules • Annual energy generation of Sun is 1034 Joules

  37. How do we know there are neutron stars? • The identification of Pulsars • Pulsars give out pulses of radio waves at precise intervals

  38. Pulsars • Pulsars were found at the center of supernovae remnants

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