supernova. explosions stars may explode cataclysmically. –large energy release (10 3 – 10 6 l )...
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Supernova
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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.
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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
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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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Abs. M
agnitude
O B A F G K MSpectral Type
10 M
3 M
0.02 M
0.5 M
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Abs. M
agnitude
O B A F G K MSpectral Type
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
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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
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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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Abs. M
agnitude
O B A F G K MSpectral Type
white dwarves
giants
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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
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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
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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
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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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Abs. M
agnitude
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
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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
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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
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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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Abs. M
agnitude
O B A F G K MSpectral Type
Sun
supernovae
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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
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Neutrino Observation
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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.
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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.
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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
30
3 4
3
4
3
r
m
R
Am NNnuc
nuc = 2.3 x 1017 kg/m3
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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.
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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
2
GM
R
M
M
cm
h
M
M
NG
ms6.0112
21min
2203
)sin(43
2
mcdt
dI
dt
dErot
3C
dt
d
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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.
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X-ray Pulsars
• Pulsars also emit x-rays.
– Blink at characteristic period
– Crab nebula period 33 ms
Crab nebula off Crab nebula on