Transcript Ch.10 Stellar old age

Ch. 10
Stellar Old Age
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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