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<-   Life and Death of a star   ->
Nébuleuse du Crabe
images/crabe.jpg
Ejected matter from a supernova. This event occured in 1054 and was observed by the Chinese : superimposing of an image in X rays (blue) and in the visible (red). The size of the ring is about one light-year.
Copyright : X rays : NASA/CXC/ASU/J. Hester et al. ; visible : NASA/HST/ASU/J. Hester et al.

The life of a star starts by a turbulent stage, called T-Tauri, which lasts about one million years. During this stage, the star throws radiations and particles which strongly disrupt the circumstellar disk.

Then, the star is in the « main sequence », the major stage of its evolution (9 billion years for a star like the Sun). When the hydrogen is exhausted in the core, the contraction resumes and the temperature in the center of the star increases. The following of the scenario depends on the mass of the star :

  • For the low mass stars ( M<0.3*M_soleil), the contraction ceases and the star "turns off".
  • For the more massive stars (M>0.3*M_soleil), the majority of the stars, the temperature in the center reaches 10^8K, and helium fusion starts. In the shell, the increase of temperature enables the return of hydrogen fusion into helium. The luminosity, and thus the radiation pressure and the radius of the star, increase dramatically. At the same time, the shell enlarges, gets colder and the center becomes denser. The star becomes a red giant. In the center, the combustion of helium into carbon and oxygen lasts a short time. Following the exhaustion of helium, the mass of the star still determines its evolution.

If the mass is less than 1.4*M_soleil, the diluted envelope is blown to form a "planetary nebula". The core slowly "turns off", forms a small (R~3000 km) and dense (~10^10 kg/m3 ) white dwarf, which cools down very slowly.

For the stars with a mass greater than 1.4 solar masses, the contraction continues. The fusion of the carbon (C), oxygen (O), silicium (Si), manganese (Mg), neon (Ne), and iron (Fe), is fast and releases little energy. After the iron (the most stable element in thermonuclear reactions), there is no available fuel anymore. The contraction resumes. In the center, the temperature increases, which leads to the fusion of elements heavier than iron. However, these reactions consume a lot of energy, which accelerates the contraction. Then, the fusion of the electrons and protons into neutrons occurs. The stellar core collapses in free-fall, to reach a radius of about 10 km and a density of 10^17 kg/m3. An interior bounce and a shock wave creates a supernova. After the explosion, a very dense central object remains, either a neutron star, or a black hole.