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Ch. 10
Stellar Old Age
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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The Death of a Low-Mass Star: Planetary Nebula
The Helix 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!
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The Formation of Planetary Nebulae
The Ring Nebula in Lyra
Two-stage process:
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
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The Cat Eye Nebula: • Approx 3000 LY away• Central star T = 80,000 K• Spectral class O • Mass ~ 1 Msun
• Radius ~ 0.65 Rsun
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The Cat Eye
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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
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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?
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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
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More massive white dwarfs are smaller!
A white dwarf cannot be more massive than 1.4MSun (Chandrasekhar limit)Nobel Prize 1983
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A white dwarf can accrete mass from its companion
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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
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Novae
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The End… Some extra slides follow…
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Summary: Evolution of a Sun-Like Star
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Running out of H…
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‘Helium flash’
‘Flash’ occurs because matter is degenerateCore of helium is supported by electron degeneracy pressureWhen He ‘ignites’, whole core is ready to fuse He into C
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Helium burning stars are temporarily stable.
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After the Helium Flash
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After He fusion stops in the core…
Variable stars:RR LyraeCepheids
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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
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Planetary NebulaeOften asymmetric, possibly due to
• Stellar rotation
• Magnetic fields
• Dust disks around the stars
The Butterfly Nebula
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Earth’s Fate
Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth
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Earth’s Fate
• Sun’s radius will grow to near current radius of Earth’s orbit