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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 18: Supernovae

Readings: Ch 22, sections 22-6 & 22-7

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

Supernova Explosion:
Explosive envelope ejection

Nucleosynthesis
Creation of elements heavier than Hydrogen & Helium in stars.

Last Days of a Massive Star

Star burns through a succession of nuclear fusion fuels:
  1. Hydrogen burning: 10 Myr
  2. Helium burning: 1 Myr
  3. Carbon burning: 1000 years
  4. Neon burning: ~10 years
  5. Oxygen burning: ~1 year
  6. Silicon burning: ~1 day

Finally builds up an inert Iron core in the center.

Massive Star Core at the Brink:

A Massive Star on 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 kick in:

Photodisintegration:
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.

Neutronization:
Free protons & electrons fuse into neutrons & neutrinos.
A neutron has more mass than a proton+electron, so this takes energy.
The neutrinos escape, carrying even more energy away from the star.
Both processes rob the core of energy, hastening its collapse.

Catastrophic Collapse

At the start of Iron Core collapse, the core properties are:

A second later, the properties are:


Core Bounce

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:

Infalling gas hits the bouncing core head-on!


Post-Bounce Shockwave

Core bounce creates a supersonic shockwave that blasts out into the star:
Kinetic Energy is ~1051 ergs!
About 24-40 milliseconds later, the amount of matter swept up by the shockwave equals the amount of matter in the shock itself.
Traffic jam between in falling & outflowing gas.
Shock Stalls out

Meanwhile, neutrinos are pouring out of the host central 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:

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 photographed.
Credit: © Anglo-Australian Observatory (reproduced with the permission of the AAO)


Supernova!

At shock breakout:

Outer envelope is blasted off:

Only the core remains behind.


Echoes

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.


Historical Supernovae

1054 AD:"Guest Star" in Taurus 1572: Tycho Brahe's Supernova
1604: Johannes Kepler's Supernova 6000-8000BC: Vela supernova

Supernova 1987a

Nearest naked-eye visible supernova seen since 1604.

Explosion occured on February 23, 1987:

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 Hubble Space Telescope observations of Supernovae

Nucleosynthesis

Start with Hydrogen & Helium:

Supernova Explosion:

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

Supernova Remnants

What happens to the envelope?

Supernova Blast Wave:


Stardust

Metal-enriched supernova ejecta mixes with interstellar gas.

Sun & planets (& us):

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 Reserved.