Ch. 10

Stellar Old Age

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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!

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

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 degenerateCore of helium is supported by electron degeneracy pressureWhen 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 LyraeCepheids

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 NebulaeOften 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