Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 18: Supernovae
Readings: Ch 22, sections 22-6 & 22-7
- 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
- Supernova Explosion:
- Explosive envelope ejection
- Creation of elements heavier than Hydrogen & Helium in stars.
Last Days of a Massive Star
Star burns through a succession of nuclear fusion fuels:
- Hydrogen burning: 10 Myr
- Helium burning: 1 Myr
- Carbon burning: 1000 years
- Neon burning: ~10 years
- Oxygen burning: ~1 year
- Silicon burning: ~1 day
Finally builds up an inert Iron core in the center.
Massive Star Core at the Brink:
Iron Core Collapse
Iron core grows until its mas is about 1.2-1.4 Msun
- Collapses & begins to heat up
Core temperature reaches T>10 Billion K & density ~1010 g/cc
At these temperatures, two important energy consuming processes
Both processes rob the core of energy, hastening its collapse.
- High-energy photons hit heavy the nuclei, which disintegrate into
He, protons & neutrons
- This effectively "reverses" the previous fusion, draining
energy (in the form of high-energy photons) out of the system.
- Free protons & electrons fuse into neutrons & neutrinos.
- A neutron has more mass than a proton+electron, so this takes
- The neutrinos escape, carrying even more energy away from the star.
At the start of Iron Core collapse, the core properties are:
- Radius ~ 6000 km (~Rearth)
- Density ~ 108 g/cc
A second later, the properties are:
- Radius ~50 km
- Density ~1014 g/cc
- Collapse Speed ~0.25 c !
Core collapses until its density hits ~2.4x1014 g/cc,
which is about density of an atomic nucleus!
At this point, the strong nuclear force comes into play!
Inner ~0.7Msun of the core:
- comes to a screeching halt
- overshoots & springs back a little ("bounces")
Infalling gas hits the bouncing core head-on!
Core bounce creates a supersonic shockwave that blasts out into the
About 24-40 milliseconds later, the amount of matter swept up by the
shockwave equals the amount of matter in the shock itself.
- Kinetic Energy is ~1051 ergs!
- Traffic jam between in falling & outflowing gas.
- Shock Stalls out
Meanwhile, neutrinos are pouring out of the host central core:
- neutrinos get trapped by the dense surrounding gas.
- this leads to rapid heating of the gas.
- this in turn leads to violent convection above the core.
New, Improved Shockwave
The violent convection from trapped neutrinos breaks the traffic jam.
The shockwave is regenerated after ~300 milliseconds.
A blastwave smashes outwards through the star:
- Explosive nuclear fusion in wake of shock produces more heavy
- Shock violently heats and accelerates the stellar envelope.
In a few hours, the shock breaks out of the surface moving at
a speed of about 1/10th the speed of light.
Seen from a distance, the star explodes...
Post- and Pre-explosion images of
Supernova 1987a in the Large Magellanic Cloud (LMC). On the right,
the arrow points to the 15 Msun(?) blue supergiant star SK-69
202 many years prior to explosion, and on the left the star shines
brightly as it explodes as a supernova following collapse and bounce of
its Iron core. This explosion was totally unpredicted, but we have
"before" images because this part of the LMC is very well
Credit: © Anglo-Australian Observatory
(reproduced with the permission of the AAO)
At shock breakout:
- Star brightens to ~10 Billion Lsun in minutes.
- Can outshine an entire galaxy of stars!
Outer envelope is blasted off:
- accelerated to a few x 10,000 km/sec
- gas expands & cools off
Only the core remains behind.
After its initial brilliance, the Supernova fades out after a few months.
The fade-out is slower than it could be because of extra energy from
gamma rays released by the decay of radioactive elements (primarily
Nickel and Cobalt) created in the final wave of explosive nuclear
burning before breakout.
How fast a supernova fades depends on how much Nickel was created
by the explosion.
- More nickel created = slower fade out
1054 AD:"Guest Star" in Taurus
1572: Tycho Brahe's Supernova
- Observed by Chinese astronomers (late Song dynasty)
- Visible in daylight for 23 days
- Visible at night for ~6 months
- Left behind the Crab Nebula
1604: Johannes Kepler's Supernova
6000-8000BC: Vela supernova
- Important supernovae that were influential at the beginnings
of modern astronomy.
- Observed by the Sumerians; appears in legends about the god Ea.
Nearest naked-eye visible supernova seen since 1604.
Explosion occured on February 23, 1987:
- 15 Msun Blue Supergiant Star named SK-69o202
exploded in the Large Magellanic Cloud (a satellite galaxy of our
Milky Way located some 50,000 pc away).
- Particle experiments on Earth recorded a pulse of neutrinos
arriving just before the burst of light from shock breakout.
- Astronomers have continued to follow its development over
the last 15 years.
SN1987a has provided us with a great wealth of information about
supernova physics, and help to largely experimentally confirm the basic
predictions of the core-bounce picture (although with good data, many
details still remain murky).
Read about various
Space Telescope observations of Supernovae
Start with Hydrogen & Helium:
- Fuse Hydrogen into the light elements up to Iron/Nickel
- These accumulate in the core layers of stars.
- "explosive" nuclear fusion builds more light elements
up to Iron & Nickel.
- fast & slow neutron reactions build Iron &
Nickel into heavy elements up to 254Cf
Supernova explosions are responsible for creating nearly all of the
heavy elements seen in nature, with a few important exceptions. The
universe starts out with only Hydrogen (75%), Helium (~25%), and a
smattering of light metals like Lithium, Boron, and Beryllium. Most
other elements are forged by nuclear reactions occurring inside of stars
or in the final moments of supernova explosions.
Top Ten Most Abundant Elements
- 10) Sulphur
- 9) Magnesium
- 8) Iron
- 7) Silicon
- 6) Nitrogen
- 5) Neon
- 4) Carbon
- 3) Oxygen
- 2) Helium
- 1) Hydrogen
What happens to the envelope?
- Fusion-enriched with metals in the explosion
- Expands at a few x10,000 km/sec
Supernova Blast Wave:
- Plows up the surrounding interstellar gas
- Heats & stirs up the interstellar medium
- Hot enough to shine as ionized nebulae up to a few thousand
years after the explosion
Metal-enriched supernova ejecta mixes with interstellar gas.
- Next generation of stars includes these metals.
- Successive generations are more metal rich.
Sun & planets (& us):
- Contain many metals (iron, silicon, etc.)
- Only ~5 Gyr old
The solar system formed from gas enriched by a previous generation
of massive stars.
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Updated: 2006 October 18
Copyright © Richard W. Pogge, All Rights