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 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 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/cm 3, 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 seconds If a third helium particle hits the beryllium before it disintegrates, carbon is formed

Things are Starting to Speed Up 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 g/cm 3 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 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 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