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

Lecture 18: Supernovae

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

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:

Iron Core Collapse

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.

Catastrophic Collapse

• Radius ~ 6000 km (~R_earth)
• Density ~ 10⁸ g/cc

A second later, the properties are:

• Radius ~50 km
• Density ~10¹⁴ g/cc
• Collapse Speed ~0.25 c !

Core Bounce

At this point, the strong nuclear force comes into play!

Inner ~0.7M_sun of the core:

• comes to a screeching halt
• overshoots & springs back a little ("bounces")

Infalling gas hits the bouncing core head-on!

Post-Bounce Shockwave

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 shockwave is regenerated after ~300 milliseconds.

A blastwave smashes outwards through the star:

• Explosive nuclear fusion in wake of shock produces more heavy elements.
• 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 M_sun(?) 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!

• Star brightens to ~10 Billion L_sun 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.

Echoes

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

Historical Supernovae

• Observed by Chinese astronomers (late Song dynasty)
• Visible in daylight for 23 days
• Visible at night for ~6 months
• Left behind the Crab Nebula

• Important supernovae that were influential at the beginnings of modern astronomy.

• Observed by the Sumerians; appears in legends about the god Ea.

Supernova 1987a

Explosion occured on February 23, 1987:

• 15 M_sun Blue Supergiant Star named SK-69^o202 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 Hubble Space Telescope observations of Supernovae

Nucleosynthesis

• Fuse Hydrogen into the light elements up to Iron/Nickel
• These accumulate in the core layers of stars.

Supernova Explosion:

• "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

Supernova Remnants

• 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

Stardust

• 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.