by ACROBAT

Created on: September 26, 2008

A black hole originates from the collapse of the iron core which forms in the late stages of evolution of a massive star, before the star finally goes supernova.

You will remember that any star with a very large starting mass - perhaps more than about ten times our sun, will ultimately undergo a supernova explosion, blowing most of the residual mass out into space. The collapsed core which is left behind following the explosion is normally a neutron star, and we related elsewhere how these stars are very compact and spin at high speed.

However, theoretical models show that the most massive stars may not succeed in blowing all of their outer layers away in the explosion. If enough matter falls back onto the core, raising its mass above the neutron star limit, then the essence of what holds a neutron star together - a property called the Neutron Degeneracy Pressure - will be insufficient to counteract the effects of gravity.

A core whose mass exceeds the neutron star limit will continue to collapse catastrophically, and this time, instead of the heat and pressure generated by the collapse working to overcome the effects of gravity, they actually strengthen the gravity in the core. This is because of the famous Einstein equation which you will all remember from your school days says that E=mc2.
What this equation really means is that energy is equivalent to mass, and although in most normal cases the gravity of pure energy is negligible, in a collapsing neutron star this is no longer the case. Here, the energy associated with the temperature and pressure concentrated in the tiny core acts like additional mass, hastening the collapse. To the best of our current understanding nothing can halt the crush of gravity, and a black hole is formed

These days, they form from massive stars that become supernova. Their core regions implode, and no known force can withstand the force of gravity once the imploding mass gets close to its eventual size as a black hole; a size determined by the masses so-called Schwarzschild radius.

The cores of stars more than about 6 times the mass of the sun reach a condition late in their lives where the fusion reactions in their cores are producing such a high flux of neutrinos that the neutrinos carry away badly needed energy from the core. This causes the core to collapse and heat up as the reactions 'burn' at higher temperatures. This produces more neutrinos and the process runs away.

As the core collapses, the density increases so high that suddenly the neutrinos can't escape, and within a few hours the bottled up energy causes the star to detonate. This produces a reversed shock that compresses the core matter beyond its mechanical ability to support itself. When it reaches a size equal to the black hole horizon size for that mass, it becomes a black hole as it explosively sheds the rest of its mass in a supernova

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