Lecture 18: Supernovae
Readings: 21-6, 22-6, 22-7,
22-9, 22-10 (22-8)
Key
Ideas
End
of the Life of a Massive Star:
Burn
H through Si in successive cores
Finally
build a massive Iron core
Iron
core collapse & core bounce
Explosive
envelope ejection
Nucleosynthesis
Creation
of elements heavier than H & He in stars
Last
Days of a Massive Star
Burns
a succession of nuclear fuels:
Hydrogen
burning: 10 Myr
Helium:
1 Myr
Carbon
burning: 1000 years
Neon
burning ~10 years
Oxygen
burning ~1 year
Silicon
burning ~1 day
Builds
up an inert iron core in the center.
Iron
Core Collapse
Iron
core grows to a mass of 1.2-1.4 MSun
Collapses
and begins to heat up
T>
10 Billion K
Density
~108 g/cc
Two
energy consuming processes kick
in:
Nuclei
photodisintegrate into He, p, & n
Protons & electrons combine into neutrons and
neutrinos, neutrinos escape and carry off energy
Makes
the core collapse faster, as the insufficient pressure is decreased further
Neutronization
Because
a neutron has more mass than an electron + proton, there has to be extra energy
in this reaction to make it happen. That energy comes from the kinetic (=energy
of motion) of the very hot proton and electron. The neutrino flies out of the
star, taking energy with it.
Note
that electron degeneracy pressure will not be important source of pressure in
this situation because 1) at these densities, the electrons are approaching
their maximum speeds=maximum pressure and 2) now they are disappearing.
Catastrophic
Collapse
Start
of Iron Core collapse
Radius~6000
km (~RSun)
Density
~108 g/cc
1
second laterÉ
Radius
~50 km
Density
~1014 g/cc
Collapse
Speed ~0.25 of the speed of light
Core
Bounce
Core
collapses until its density is ~2.4x1014 g/cc, the density of an
atomic nucleus!
Then
the strong nuclear force comes into play!
Inner
0.7 MSun of the core
comes
to a screeching halt
overshoots
& springs back a bit (bounces)
Infalling
gas hits the bouncing core head-on
Post-Bounce
Shockwave
Shockwave
blasts out into the star:
Kinetic
energy is 1051 ergs
After
25-40 milliseconds
Traffic
jam between infalling and outflowing gas
Shockwave
stalls
Meanwhile
neutrinos pour out of the core (newly created neutron star):
Get
trapped by the dense surrounding gas
This
leads to rapid heating of the gas
This
leads to violent convection
New,
Improved Shockwave
Violent
convection breaks the traffic jam.
Shockwave regenerates after 300 millisec
Blastwave
smashes out through the star:
Explosive
nuclear fusion in its wake produces more heavy elements
Heats
up and accelerates the envelope
Shock
breakout a few hours laster
Breakout
speed ~10% the speed of sound
Supernova!
At
shock breakout
Brightens
by 10 billion LSun in minutes
Outshines
an entire galaxy of billions of stars!
Outer
envelope is blasted off:
Accelerated
to a few x 10,000 km/sec
Gas
expands and cools off
Only
the core remains behind
Echoes
Supernova
fades after a few months.
Fading
slows at late times
Extra
energy from gamma rays emitted by radioactive nickel and cobalt
Fading
rate depends on the amount of Ni created
More
nickel=slower fade
Example:
Supernova 1987a (by the way, SN are names by the year of their discovery +
letters of the alphabet. Exceptions are the historical SN).
Historical
Supernovae
1054
AD: ÒGuest StarÓ in Taurus
Observed
by the Chinese (Song dynasty)
Visible
in daylight for 23 days
1572
AD: Tycho BraheÕs Supernova
1604
AD: Johannes KeplerÕs Supernova
6000-8000BC:
Vela supernova
Observed
by the Sumerians, appears in legends about the god Ea.
Crab
Nebula: (aka M1) remnant of Supernova in 1054
Supernova
1987a
Nearest
visible SN since 1054
February
23, 1987
15MSun Blue Supergiant Star: SK-69o202
exploded in the Large Magellanic Cloud
Saw
a pulse of neutrinos, then the blast
Continued
to observe it since then
Wealth
of information on SN physics
Nucleosynthesis
Start
with Hydrogen & Helium
Fuse
H into elements up to Iron and Nicket
Accumulate
in the core layers of stars
Supernova
Explosion
ÒExplosiveÓ
nuclear fusion builds more light elements up to Iron & Nickel
Fast
neutron reaction build Iron & Nickel into heavy elements up to 254Cf
Of
the Top Ten Most Abundant Elements
10)
Sulfur
9)
Magnesium
8)
Iron
7)
Silicon
6)
Nitrogen
5)
Neon
4)
Carbon
3)
Oxygen
2)
Helium
are
all made in explosions of massive stars. Note that helium and carbon are made
in the low-mass asymptotic giant branch stars as well
1)
Hydrogen
not
made in SN.
Supernova
Remnants
What
happens to the envelope
Enriched
with metals in the explosion
Expands
at a few x 10,000 km/s
Supernova
Blast Wave
Plows
up the surrounding interstellar gas
Heats
& stirs up the interstellar medium (that is, the gas between stars)
Hot enough to shine as ionized nebulae up to a few
thousand years after the explosion
Stardust
Metal-enriched
gas mixes with interstellar gas
Goes
into the next generation of stars
Successive
generations are metal-rich
Sun
& planets (& us):
Contain
many metals (iron, silicon, etc)
Only ~5 Gyr old, so lots of stars had time to die and
contribute to our stock of carbon, etc.
The
Solar System formed from gas enriched by previous generations of massive stars.