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Ch. 10 Stellar Old Age QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. The Death of a Low-Mass Star: Planetary Nebula Remnants of stars with ~ 1 – a few Msun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts) Lifetime < 10,000 years Have nothing to do with planets! The Helix Nebula The Formation of Planetary Nebulae Two-stage process: The Ring Nebula in Lyra Slow wind from a red giant blows away cool, outer layers of the star Fast wind from hot, inner layers of the star overtakes the slow wind and heats it => Planetary Nebula The Cat Eye Nebula: • Approx 3000 LY away • Central star T = 80,000 K • Spectral class O • Mass ~ 1 Msun • Radius ~ 0.65 Rsun The Cat Eye White Dwarfs are supported by electron degeneracy pressure • in a low-mass star, Fusion stops after He -->C and O • Just cools off and fizzles out Siruis and its white dwraf companion, Sirius B High mass stars : CNO Cycle • H fusion is faster because C, N and O act as catalysts • Same net result: 4 H become 1 He. • No total gain or loss of C, N, O Question: How does energy produced by CNO cycle compare to PP chain? Death of low-mass star: White Dwarf • White dwarfs are the core, once fusion stops • Some H fusion at surface • Electron degeneracy pressure supports them against gravity • Cool and grow dimmer over time More massive white dwarfs are smaller! A white dwarf cannot be more massive than 1.4MSun (Chandrasekhar limit) Nobel Prize 1983 A white dwarf can accrete mass from its companion Nova • The surface of a white dwarf can fuse H to He • Fusion begins suddenly and explosively: nova • The nova star system temporarily appears much brighter Novae The End… Some extra slides follow… Summary: Evolution of a Sun-Like Star Running out of H… ‘Helium flash’ ‘Flash’ occurs because matter is degenerate Core of helium is supported by electron degeneracy pressure When He ‘ignites’, whole core is ready to fuse He into C Helium burning stars are temporarily stable. After the Helium Flash After He fusion stops in the core… Variable stars: RR Lyrae Cepheids Planetary Nebulae • Double-shell burning ends with a pulse that ejects the outer layers into space: planetary nebula • The core left behind is a white dwarf Planetary Nebulae Often asymmetric, possibly due to • Stellar rotation • Magnetic fields • Dust disks around the stars The Butterfly Nebula Earth’s Fate Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth Earth’s Fate • Sun’s radius will grow to near current radius of Earth’s orbit