Core Collapse In Giant Stars

When iron is produced in the core of the massive star, the internal processes suddenly change. The iron is so dense, consisting of 26 protons and 30 neutrons, that when it burns no energy can be extracted by combining this to make heavier elements. The result is that when significant quantities of iron accumulate in the core the internal fires suddenly are extinguished, and the equilibrium of the star is destroyed. The temperature in the core is several billion K, but without continued nuclear fusion the force of gravity at this stage begins to overwhelm the outward gas pressure and the star begins a fateful collapse and implosion.

As the star begins to collapse the core temperature shoots up to about 10 billion K, and the photons take on a high energy state, splitting the iron into lighter nuclei, quickly breaking down the elements in a process called photodisintegration so that only protons and neutrons remain. In less than one second all of the heavy elements produced by nuclear fusion in the entire core of the star have broken down into simple protons and neutrons, undoing 10 million years of nuclear reactions. Photodisintegration requires a huge amount of energy from the high-temperature core and transferring this energy from the core to break down these elements cools the core, which reduces the outward fluid pressure that is resisting the inward pull of gravity. The sudden loss of pressure in the core causes the collapse of the core of the star to accelerate rapidly.

As the core continues to shrink, the pressures on the mixture of free electrons, protons, neutrons, and photons, and the rapidly rising pressures crush the protons and electrons together to form neutrons and neutrinos in a process called neutronization. Neutrinos are such high-energy particles that they rarely interact with matter, even matter as dense as that in the core of the collapsing giant star. Most of the neutrinos therefore pass right through the core and escape to space carrying large amounts of energy with them. At this stage the core finds itself in a state where the electrons and neutrinos have escaped, so that the neutrons in the core are coming into contact with each other, at a super high density of 1015 kg/ m3. At this high density the neutrons influence and oppose each other with a very strong force, generating strong pressure called the neutron degeneracy pressure, which acts against the inward pull of gravity. By the time this force is able to oppose the ongoing rapid gravitational collapse of the star, however, the core has gone beyond its point of equilibrium and has reached densities between 1017 to 1018 kg/m3. These forces interact; the rapid gravitational collapse hits the super-dense neutron mass in the core with the strong neutron degeneracy pressure, and the inward collapse bounces backward to produce a rapid expansion. The total time elapsed from the start of the collapse of the core to the beginning of the outward expansion is less than one second. The outward expansion is associated with a tremendously powerful shock wave that moves outward through the star, blasting all of the star's outer layers into space in one of the most powerful events known in the universe, a supernova. These outer layers contain many heavy elements around the core, as well as light elements near the surface, and as the star explodes these are all blown into interstellar space in a bright flash that may last only a few days at its peak. Supernovas are among the brightest events known in the universe, being about a million times brighter than novas, and often being about the same brightness—for a few days—as a galaxy with a trillion stars. The supernova may have an initial flash that is more than a billion times brighter than the Earth's Sun and produces more energy over the time from its initial flash until it completely fades away a few months later than the Sun produces in its entire history.

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