Download Supernova

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
-20
• Stars on the HR diagram line up
according to mass.
-15
Abs. Magnitude
-10
-5
10 M
0
3 M
0.5 M
5
10
0.02 M
15
20
O B A F G K M
Spectral Type
• The time on the main sequence
is spent burning hydrogen.
– Massive stars burn faster
Giants
-20
-15
Abs. Magnitude
-10
-5
0
Capella
giants
Aldebaran
• Helium fusion through triple
alpha causes a helium flash.
– Rapid expansion 100 x R
5
10
15
20
• When core hydrogen is
exhausted helium burning
begins.
– Degenerate gas core 108 K
O B A F G K M
Spectral Type
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
-20
• Giants that exhaust their core
helium become white dwarves.
– Planetary nebulas
-15
Abs. Magnitude
-10
-5
0
giants
5
10
15
20
white
dwarves
O B A F G K M
Spectral Type
• Isolated white dwarves slowly
cool due to lack of further
fusion.
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
white dwarf
giant star
gas pulled
to partner
supernova
Binary Life Cycle
1-3 M
4-9 M
• Close binary stars will evolve at
different times.
• The massive star will form a
white dwarf first.
1-3 M
1.5 M
supernova
• The second star goes giant and
engulfs white dwarf.
– Material from the second
star is also blown away
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
Supergiants
-20
-15
Abs. Magnitude
-10 Rigel
-5
0
supergiants
Betelgeuse
5
10
15
20
O B A F G K M
Spectral Type
• Massive stars can sustain
helium burning and that are
brighter than expected are large
and are called supergiants.
– M > 5-8 M
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
2
 M   10 km 
GM 2
 
E
 3 10 46 J 

R
M
r

 sun  


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
-20
supernovae
-15
Abs. Magnitude
-10
• A supergiant with more than 8
M will oscillate in temperature
becoming more luminous.
-5
0
5
Sun
10
15
20
O B A F G K M
Spectral Type
• 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.
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
R  r0 A1 3
r0 = 1.2 x 10-15 m
nuc 
3 AmN 3mN

3
3
4R
4r0
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
 min 
2
max
 min  11
• The period decreases with time.
– Magnetic dipole radiation
– Predict 1200 years for Crab
pulsar
 R3 
 2 

 GM 
12
M  1 2 h
M


G

0
.
6
ms
M
mN c 2
M
dErot
d
2 
 I
 3 0 (m 2 sin  ) 2
dt
dt 3c 4
d
 C 3
dt
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