Red supergiants, the biggest stars in the universe, are the stars close to the explosion as they have already consumed all of their hydrogen, helium or carbon in successive nuclear reactions. Their equilibrium broken, the gravitational collapse of the core of the star continues and explosion is imminent. The core internal pressure of the supergiant related to the gravitational force is no longer in equilibrium with the pressure of nuclear radiation. At the same time his core collapses, the outer layers of the envelope swell considerably and cool.
Betelgeuse (α Orionis) red supergiant in the constellation of Orion, is one of the largest known stars after Antares. Betelgeuse is about 600 times larger than the Sun and shines as 100,000 suns. Aged only a few million years, Betelgeuse is already nearing the end of his life. Like all supergiants it consumes a huge amount of matter in a very short time, about the mass of the Sun in only 10,000 years. In a few thousand years, Betelgeuse will explode as a supernova also described as a core-collapse supernova. The men of this time can see from Earth, even in broad daylight. A supernova is a new star, the term comes from the Latin "nova" means "new."
They suddenly appear in the sky and disappear after a few months. Supernovae are rare events in our Milky Way, about one to three per century, contrariwise on the scale of the universe, we observe every day these events. Supernova explosions contribute to the chemical enrichment of the universe. It was during its explosion in supernova than the star frees the chemical elements that it has synthesized during its life and during the explosion itself.
These are the stars at the end of life, especially supernovae which generate all the elements beyond iron. Iron is a stable element and the collapse of the supernova core stops when it is entirely made of iron. Indeed the iron smelting does not produce energy but consumes, then the star out of fuel, can not support the weight of the upper layers, which compress even more the iron, the nuclei disintegrate and the protons capture the electrons to form neutrons. This new core concentrated of neutrons incredibly compact, a few km radius and mass of the star is then able to resist to the pressure of the outer layers, thereby stopping abruptly its collapse.
The effect is cataclysmic, the layers of gas "bounce" then on the core. The energy released by the layers falling towards the center, produces a shock wave which "blows" the outer layers of the star, this is what we call a supernova explosion.
The gaseous envelope that will be projected into space will release considerable energy, as much energy as the Sun for billions of years. This terrible explosion, which projects the upper layers of the star into space will be visible throughout the galaxy. Then a supernova will extend hundreds of billions of Km, seeding the interstellar medium heavy elements produced during the life of the star and during the explosion. These heavy elements are the building blocks of terrestrial planets like our Earth and also the constituents of our body and all objects that surround us.
Tycho's supernova called SN 1572 is a nova appeared in the constellation Cassiopeia in 1572. It was visible to the naked eye and observed by Tycho Brahe 11 November 1572, while brighter than the planet Venus. Tycho Brahe noticed that the position of the object does not move relative to the fixed stars as the planets. He concludes that it was not a planet but a star. From March 1574, it disappeared below the threshold of visibility. This beautiful image of the space telescope Chandra X-Ray, is one of the most important images ever produced with an observatory in Earth orbit. It is also known than the supernova of the Crab Nebula. The Crab Nebula (M1 or NGC 1952) is the result of the explosion of the supernova SN 1054 observed from July 1054 to April 1056 by a Chinese astronomer. In 1758 Charles Messier discovers and makes the first object of its catalog (M1 in the Messier catalog). Supernovae are extremely luminous explosions of stars, so bright they are remarkable in the midst of brilliant stars of a galaxy.
NB: Supernovae of type II, Ib and Ic are stars at the end of life. Type II supernovae have a spectrum that contains hydrogen, while Ib and Ic supernovae type are stars who have already exhausted their hydrogen so that it does not appear on their spectrum. Type Ic supernovae have also exhausted their helium and does not appear in the spectrum.
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Image: Tycho's supernova (SN 1572) nova appeared in the constellation Cassiopeia in 1572. It was visible to the naked eye and observed by Tycho Brahe 11 November 1572, then brighter than Venus. Tycho Brahe noticed that the position of the object does not move relative to the fixed stars as the planets. He concludes that it was not a planet, but a fixed star. From March 1574, it disappeared below the threshold of visibility.
Credit Public Domain Image, Composite image of SN 1572 seen by Chandra X-ray. Chandra X-Ray Observatory, Spitzer Space Telescope, and Calar Alto Observatory.