Transcript Supernovae: Heavy Elements

Supernova
Star Formation
• Nebula - large clouds
comprised mostly of hydrogen
• Protostar - a massive
collection of gas within the
nebula that begins the process
of star formation
• Star - as the protostar gathers
more mass, it is compressed
under it’s own gravitation and
eventually is compressed
enough to ignite nuclear fusion
and a star is born
Types of Stars
• The Hertzsprung-Russell
Diagram - catagorizes stars by
spectral class and luminosity.
Some stars can be
distinguished from one
spectral class to another with
the naked eye.
• Main Sequence stars generally run from lower right
(low temperature and
luminosity) to upper left (high
temperature and luminosity)
• Exceptions - Secondary band
of very cool, yet very luminous
stars known as Giants
Upper Main Sequence Stars
• Upper Main
Sequence stars are
the most massive and
luminous stars
• Range from 8 solar
masses to over 100
solar masses
The Beginning of the End
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Upper main sequence stars burn all of the
hydrogen in their core much more quickly
than intermediate dwarfs like the Sun - To
illustrate this, the lifecycle of a 20 solar mass star
will be used in all subsequent examples
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Initial hydrogen burning completes in only 8
million years
After the hydrogen in the solar core is spent,
fusion ends briefly and the remaining core,
now comprised of helium, begins to contract
During each stage of contraction, fusion
continues in the outer shell of the star,
causing the star to expand
As the helium core contracts it’s temperature
rises
Once the core contracts to a density of ~1
million g/cm3 , approximately 1 million years
after the hydrogen was spent, the
temperature has risen to over 100 million K,
fusion begins again converting helium into
carbon
However, this type of fusion is not easy!
Triple Alpha Process
• 2 helium particles
collide and form
beryllium
• Problem - beryllium
begins to fall apart in
10-16 seconds
• If a third helium
particle hits the
beryllium before it
disintegrates, carbon
is formed
Things are Starting to Speed Up
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Carbon fusion can only be
sustained for 100 thousand years
before the core begins to contract
again
At 1 billion K, the carbon core
contraction comes to a halt and
begins to fuse into oxygen, neon,
and magnesium
The O/Ne/Mg core burns away in
20 years and the core contracts
further
At 2 billion K the solar core comes
alive again and is converted into
silicon and sulfur
The silicon/sulfur core fuses into
iron in only 1 week at a
temperature of over 3 billion K
Gravity Wins…Again
• The star is now left with a solar core
comprised of iron
• Because iron requires energy to be fused
into heavier elements fusion is no longer
possible and the core begins to heat and
contract one final time
• This final, catastrophic collapse happens
at incredible speeds
• Freaky, incredible speeds*
*Freaky, incredible speeds are approximately ¼ the speed of light
Uh-Oh
• The diameter of the remaining core can go from 2,000
km to less than 20 km in a few seconds
• Lacking any radiative support the outer shell begins to
collapse as well
• The core now has a temperature of over 200 billion K
and has reached a density of 1012 g/cm3
• Under these conditions, protons and electrons merge
into neutrons and even thought their charge cancels out,
the energy is conserved and released in the form of
neutrinos
• Neutrinos - extremely small, nearly mass less forms of
energy
A Different Kind of Big Bang
• The pressure of these neutrinos causes part of the
imploding core to rebound
• This rebound comes in the form of a shockwave, which
rips through the still collapsing outer core
• The collapsing outer shell is met by the shockwave and
then by a rarefaction wave and is ejected back into
space in a catastrophic explosion
• The amount of energy released is unimaginable…at the
moment of collapse the energy output is 1046 joules and
equal to that of all the other stars in the observable
Universe combined
What Does the Death of a Star
Have to do With Life on Earth?
• The tremendous energy of the
shockwave causes nuclear
reactions to go berserk
• Heavy elements up to,
including, and perhaps beyond
plutonium are created and
ejected back into space as the
star goes supernova
• Judging from the historical
records, supernova explosions
have been detected once
every 200 years
• However, many have been
obscured by interstellar dust
• Best estimates state that there
is 1 supernova explosion every
25 years
That’s a lot of Stuff
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This means that 250 million
supernova have occurred in the
history of our galaxy
On average each explosion sends
10 solar masses of heavy
elements back into space
So, over 1 billion solar masses or
1% of all stellar mass is from
supernova explosions
Supernova explosions could easily
be responsible for all of the iron
and other heavy elements found in
the galaxy
Our sun, our planets, the silicon in
our rocks, the change in our
pockets, and the metal in the little
green men’s spaceships, are all
the result of supernova explosions