1.Life Cycle of a star

2. 24.3 The birth of a star Energy from the Bang was first generation energy Light from that original spreading out The light we see from most stars & galaxies is second generation energy Light from concentrated matter 3. 24.3 The birth of a star Begins as a nebulae huge clouds of H & He Gravity causes the nebulae to collapse in on themselves As it collapsed it became more dense and began to spin. This spinning motion created gravity which pulled the gas and dust in tighter creating friction This Friction created heat which caused the Hydrogen atoms to undergo thermo-nuclear fusion creating Helium and huge amounts of energy. This energy is emitted as light- thus a star is born!! 4. Fig. 24.4, p.612 Balance of forces gravity and energy from fusion must exist to maintain the star 5. Fig. 24.5a, p.612 Orion Constellation One of the most Recognizable star formations 6. Fig. 24.5b, p.612 A close-up schematic view of Orions belt region 7. Fig. 24.6, p.613 a close-up view of stellar nurseries near Orions belt. 8. 24.5 Stars: the main sequence Stars initially cataloged just by brightness Apparent brightness luminosity as seen from Earth Brightness as affected by distance Absolute brightness luminosity as if stars were at a fixed distance Brightness at 10 parsec (32.6 light years) Scale is set with our Sun as a value of 1.0 9. 24.5 Stars: the main sequence Hertz-Sprung Russell diagram Main sequence a sinuous line along which 90% of stars lie Luminosity and temperature are in direct relation Larger stars are brighter Greater gravity drives fusion reactions faster Non-main sequence stars are part of the stars life cycle. 10. Fig. 24.11, p.618 11. Steps in the death of a medium mass star For medium mass stars (size of our Sun) 1. H nuclei fuse to He, He cannot fuse to heavier elements at these temperatures 2. As fusions slows, temperatures drop 3. Outward pressure drops and star contracts 4. As the core contracts, it heats up This heat ignites H fusion in outer layers outer layer expands and becomes a red giant 12. Fig. 24.4, p.612 13. Steps in the death of a medium mass star cont 5. Core of red giant shrinks until He fusion starts 6. Helium fuses to carbon and eventually it runs out 7. Further contraction causes heating again 8. At one solar mass, pressure does not cause any more fusion 14. Fig. 24.15, p.621 The Ring Nebula is a sphere of gas and dust expelled as a dying star exploded. Death of a Star 5. Blows gas shell off and creates planetary nebula 6. Contracts to about Earth size and glows from residual heat a white dwarf 7. After tens of billions of years, it will just go dark 15. Fig. 24.13, p.620 16. The death of a massive star Stars with a large mass, >1.44 solar masses Star doesnt go white dwarf Carbon fusion initiates and iron forms Iron fusion doesnt release energy, it absorbs energy This cools the star very fast, rapidly collapses on the order of seconds Massive heat build-up (trillions of K) causes the star to go supernova or explode Shockwave from explosion causes fusion reactions that create the rest of all elements 17. 24.6 The life and death of a star First and second generation stars All original stars had to form from only H & He Some of these old population II stars still exist When a star dies (as nebula or supernova) the dusts and gases form into new stars These contain heavier elements Thus newer, population I stars, contain small bits of heavy elements Our own solar system was born of this recycled material 18. Fig. 24.16, p.621 19. Fig. 24.17, p.622 20. 24.7 Neutron stars, pulsars, and black holes After a supernova, there is often a large mass left, larger than a white dwarf Subatomic particles squeezed together form a neutron star Very dense 1013 kg.cm3 Pulsars Storms on the spinning neutron star put out regularly-spaced radio waves Black-holes stars of >5x solar masses When these die, the leftover core is so massive that even the neutrons collapse Eventually this compresses to a point-mass, a black hole Gravity so strong it bends light around it 21. Fig. 24.30, p.629 22. 24.12 The end of the universe The Big Bang theory supports the expansion of the universe however. Gravity pulls on all object everywhere, gradually bringing things together so How will it end or will it? Closed universe gravity will eventually win and it will all collapse on itself one day Open universe it will fly apart forever and eventually the fusion stops 23. Fig. 24.32a, p.631 24. Fig. 24.32b, p.631