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

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

5. 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

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

8. 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

9. 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

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

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

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

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

16. And Less energy is produced Star starts to contract

17. 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

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

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

20. 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

21. Planetary Nebulae

22. 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

25. 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

26. 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

27. 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

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

31. 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

32. 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

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

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

35. The Sun Will end up as a White Dwarf

36. 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

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

38. 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.

40. Type Ia Supernova Type II Supernova

42. 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

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

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

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

48. 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

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

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