The cataclysm : supernovae

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 Let'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 now let's assume that the initial star has a mass greater than 6 or 7 solar masses. The carbon core is able to collapse because of its own weight, and carbon in the star begins fusion into magnesium. At this time, the inner temperature is greater than hundreds of millions of degrees.
Contrary to what happens in a type Ia supernova, that we have previously seen, the dilatation and the heating of the star are able to regulate this fusion, and so to avoid an explosion.

 

'Onion' structure of a giant star
The star is becoming like an onion, where the different concentric layers correspond to different fusion reactions.
The outermost layer is burning hydrogen (H) to form helium(He), next, it's helium which is changing into carbon(C), then oxygen(O) is forming, and when we go 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).

Iron can't change into any other element, because it would need energy to transform, and there is not enough energy for this : it accumulates in the core, which at the same 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 again gravity. When its mass reaches the critical Chandrasekhar mass, - this name comes from an Indian physicist - whose value is around 1.4 solar masses, it suddenly collapses, dragging along the outer layers of the stars.

the supernova phenomenon
This collapse generates a huge mecanical energy whose transfert to the outer layers results in the explosion of the star, producing one of the most luminous events known : the supernova.

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

supernova SN1999em
Supernova SN1999em, situated 25 million light years away in the galaxy NGC1637.
It has been recently discovered by the Chandra space telescope, which operates in the Xray part of the spectrum.
This star is radiating as much power as 50,000 suns in the Xray domain, and 200 million suns in the visible part of the spectrum.
A supernova can shine like ten billions of suns, i.e. more than its harbouring galaxy.
Source NASA / Lick Observatory


The fragments of the star are ejected at a speed which can be faster than 10.000 kilometres by second. They form a splendid nebula around the remnant of the star.

Crab nebula
In the year 1054, a supernova shone in daylight for a few weeks.
Il has given us this beautiful nebula, known as the 'Crab nebula'.
Viewed from the Earth, the supernova was brighter than Venus and is located more than 7000 light years away.


Such an explosion is powerful enough to briefly allow new fusion reactions in the iron core, permitting the generation of elements heavier than iron.
All the elements that you can find on Earth, except hydrogen and helium, come from supernovae explosions.

Compared evolution of stars of different masses
Compared evolutions of stars of one, ten and thirty solar masses.

Massive stars go through a 'red supergiant' stage, and disapear in a supernova explosion.


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

We must say that a type II supernova is a rare phenomenon : a rough estimation is 0.6 supernova each year for 10 billion solar brightness, i.e., one supernova every 800 years in the Milky Way.
This means that, if we want to observe a hundred or so supernovae each year, we must observe a volume of about 40 cubic Megaparsec.

 


References :
Presupernova Evolution of Rotating Massive Stars (A. Heger, N. Langer)
The galactic evolution of the supernova rates (E. De Donder and D. Vanbeveren)