Supernova

Explosions

• Stars may explode cataclysmically.

– Large energy release (103 – 106 L)

– Short time period (few days)

• These explosions used to be classified as novas or supernovas.

– Based on absolute magnitude

• They are now all called supernovas.

Hydrogen Lines

• Supernovas are classified by their emission spectra.

– Historical classification

– Not related to mechanism

• The initial classification is based on hydrogen.

• Secondary classification is based on other elements.

– Silicon absorption

– Helium emission

Mass Relations

• Stars on the HR diagram line up according to mass.

• The time on the main sequence is spent burning hydrogen.

– Massive stars burn faster

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O B A F G K MSpectral Type

10 M

3 M

0.02 M

0.5 M

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Giants

• When core hydrogen is exhausted helium burning begins.

– Degenerate gas core 108 K

• Helium fusion through triple alpha causes a helium flash.

– Rapid expansion 100 x R

AldebaranCapella

giants

Degenerate electrons

• The nuclei from fusion are separated from their electrons.

– Filled fermi states with degenerate electrons

– Provides opposing force to gravity

• The energy of contraction blows off outer layers of star.

inward force of gravity

outward force of electrons

Dwarves

• Giants that exhaust their core helium become white dwarves.

– Planetary nebulas

• Isolated white dwarves slowly cool due to lack of further fusion.

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white dwarves

giants

Binary Dwarves

• White dwarves can occur in binary stars.

– One star ages faster

– Original detection

• White dwarves continue gravitational pull on companion.

– Tidal forces

Sirius image from Chandra - NASA

Binary Explosions

• A binary can transfer gas from a giant to a white dwarf.

• If the white dwarf exceeds MCH, gravity will exceed electron repulsion.

• It will explode into a type I supernova.

– Star-sized fusion bomb

giant star

gas pulled to partner

white dwarf supernova

Binary Life Cycle

• Close binary stars will evolve at different times.

• The massive star will form a white dwarf first.

• The second star goes giant and engulfs white dwarf.

– Material from the second star is also blown away

supernova

1-3 M 4-9 M

1-3 M 1.5 M

Core Fusion

• For high mass stars fusion continues beyond helium fusion.

• Each fusion stage requires higher temperatures and pressures and takes place in deeper layers.

• Fusion steps

– Hydrogen to helium

– Helium to carbon

– Carbon to oxygen

– Oxygen to neon

– Neon to silicon

– Silicon to iron

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O B A F G K MSpectral Type

Supergiants

• Massive stars can sustain helium burning and that are brighter than expected are large and are called supergiants.

– M > 5-8 M

Rigel

Betelgeuse

supergiants

Gravitational Binding

• The change in gravitational energy is released during collapse.

– From 1 M, r = 1000 km

– To r = 10 km

• The estimate is an order of magnitude greater than the amount needed for nuclear changes.

– 90% available for release

rM

M

R

GME

sun

km10J103

2

462

Total Energy

• The energy released by the collapse of a core is great.

– Optical: 1042 J in weeks

– About 1010 times the Sun

– Equal to some galaxies

Death of Supergiants

• A supergiant with more than 8 M will oscillate in temperature becoming more luminous.

• Eventually the core is so collapsed by gravity that the electrons cannot hold the core apart.

• A star like this will become a type II supernova.

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supernovae

Neutrino Production

• The core can cool by producing neutrinos.

– Plasma at 1011 K

– Opaque to photons

• Neutrinos can carry kinetic energy.

– Hot enough for all three types

– Pair production dominates

Neutrino Observation

Stellar Explosion

• When gravitational force exceeds the electron repulsion, the core collapses immediately.

• The energy is released as photons and mostly neutrinos.

• The outward energy hits collapsing material and the star explodes.

Supernova Remnants

• The supernova core collapse is at 200 billion K.

• The photons are energetic enough to break up iron nuclei.

• The particles from the broken nuclei fuse with iron to create heavy elements.

• This matter goes to form new stars and planets.

Nuclear Force

• Neutron stars forms when the core mass exceeds the Chandrasekar mass: 1.5 M.

– Photodisintegration: 1.4 x 1045 J

– Electron capture: 1.6 x 1045 J

• Nuclear forces stop further collapse.

– Reach nuclear density

310 ArR

r0 = 1.2 x 10-15 m

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Am NNnuc

nuc = 2.3 x 1017 kg/m3

Pulsars

• Neutron stars create very large magnetic fields.

– Spin faster with collapse

– Up to 30 Hz

• They can be observed as repeating flashes of light as the magnetic poles point towards us.

Rotation Time

• Minimum period is found by balancing gravity and centripetal force.

– Fast rotation from high density

• The period decreases with time.

– Magnetic dipole radiation

– Predict 1200 years for Crab pulsar

213

maxmin 2

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GM

R

M

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cm

h

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ms6.0112

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)sin(43

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d

X-rays

• The surface gravity creates tremendous accelerations.

– Accelerating electrons radiate photons

– Radiate as x-rays

• X-ray telescopes in orbit can spot neutron stars in supernova remnants.

X-ray Pulsars

• Pulsars also emit x-rays.

– Blink at characteristic period

– Crab nebula period 33 ms

Crab nebula off Crab nebula on