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.