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The cataclysm :

supernovaeLet's go back to our journey when all the core - at least the

biggest part of it- of the star has burned into carbon, but nowlet's assume that the initial star has a mass greater than 6 or 7solar masses. The carbon core is able to collapse because of itsown weight, and carbon in the star begins fusion intomagnesium. At this time, the inner temperature is greater than

hundreds of millions of degrees.

The star is becominglike an onion, where thedifferent concentriclayers correspond todifferentfusion

reactions. Theoutermost layer isburning hydrogen (H) toform helium(He), next,it's helium which ischanging into carbon(C),then oxygen(O) isforming, and when wego deeper to the core,we find more and more

heavy elements :neon(Ne), sodium, magnesium(Mg), silicon(Si), sulphur(S),nickel, cobalt and, at last iron(Fe).

http://nrumiano.free.fr/Elexique.html#fusionhttp://nrumiano.free.fr/Elexique.html#fusionhttp://nrumiano.free.fr/Elexique.html#fusionhttp://nrumiano.free.fr/Elexique.html#fusion

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Iron can't change into any other element, simply because thereis no more energy : it accumulates in the core, which at thesame time fills with electron degenerate matter.

Outer layers are contracting, so the mass of the core is getting

bigger and bigger, but it has no more energy to fight againgravity. When its mass reaches the critical Chandrasekharmass, - this name comes from an Indian physicist - whosevalue is around 1.4 solar masses, it suddenly collapses,dragging along the outer layers of the stars.

This collapsegenerates a hugemecanical energywhose transfert to theouter layers results inthe explosion of thestar, producing one ofthe most luminousevents known : thesupernova.

This supernova is called 'type II', as opposed to the 'type I'what we have previously seen.

Supernova SN1999em, situated 25million light years away in thegalaxy NGC1637. It has been veryrecently discovered by the Chandraspace telescope, which works inthe Xray part of the spectrum.

This star is radiating as muchpower as 50,000 suns in the Xray

domain, and 200 million suns in thevisible part of the spectrum.

A supernova can shine like tenbillions of suns, i.e. more than itsharbouring galaxy.

http://nrumiano.free.fr/Elexique.html#mat_degenhttp://nrumiano.free.fr/Elexique.html#sntypehttp://nrumiano.free.fr/Elexique.html#sntypehttp://nrumiano.free.fr/Elexique.html#mat_degenhttp://nrumiano.free.fr/Elexique.html#sntypehttp://nrumiano.free.fr/Elexique.html#sntype

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The end of a massive star is a very fast process : if the fusionof hydrogen, as long as the star is on the main sequence, canlast billions of years, all of the carbon is transformed in 10,000years, all of the neon and the oxygen in one year, and the finaltransformation of silicon to iron requires only one day.

We must say that a supernova is a rare phenomenon : a roughestimation is 0.6 supernova each year for 10 billion solarbrightness, i.e., one supernova every 800 years in the MilkyWay.

This means that, if we want to observe a hundred or sosupernovae each year, we must observe a volume of about 40cubic Megaparsec.

Rfrences:

Presupernova Evolution of Rotating Massive Stars (A. Heger, N. Langer)

The galactic evolution of the supernova rates (E. De Donder and D.

Vanbeveren)

http://arxiv.org/abs/astro-ph/0005110http://arxiv.org/abs/astro-ph/0305462http://arxiv.org/abs/astro-ph/0005110http://arxiv.org/abs/astro-ph/0305462