Origin and Evolution of Black Holes
What causes black holes?
Black holes are the end result of gravitational collapse when there is insufficient energy to keep matter from falling into a singularity and then outside of the known laws of physics for this universe. This can happen because atoms are mostly empty space. If you could condense the atoms in your body so that there was no space between sub-atomic particles, you would collapse into and object so small, not even someone using a scanning electron microscope would be able to see you. This is one of the principle ideas behind the formation of black holes, but there are certain limits that must be overcome before this can actually happen. Black holes have a few origins according to cosmologists. The usual cause is when a massive star about three times the mass of the sun expends its fuel and collapses dramatically during a supernova explosion-implosion event. Cosmology tells us that during the beginning moments of the cosmos, many black holes of all sizes including mini black holes were formed in the primordial density of the early fireball of the big bang event. Not all cosmologists accept the big bang as the beginning of the universe. The third cause is when two neutron stars merge.
Every star in the cosmos exists in a state of delicate balance between total gravitational collapse into a singularity and pressure exerted by electrostatic repulsion and energy created as a result of fusion that tends to explosion. As a star condenses from a dark nebular cloud into a compact rotating object, the force of gravity helps to condense it to the point where temperatures and pressures are high enough to cause fusion of simple hydrogen into helium 3 and helium 4. In the process, the condensing plasma is kept from collapsing further due to the repulsion of positively charged nucleons and the generation of gamma radiation due to the matter and anti-matter annihilation of electrons and positrons that are the by product of fusion. Stars come in all sizes and have different fates. A star will not collapse beyond certain thresholds provided the energy can be derived from fusion of low mass elements and isotopes into more massive ones.
Red dwarfs will glow for a long time as they fuse elements slowly. They are expected to burn for hundreds of billions of years. As mass of stars increase, so does the rate of fusion. Life expectancy on the other hand decreases with mass increase in an inverse proportion. Stars that can fuse elements like carbon, oxygen, nitrogen, neon but not beyond will end as white dwarfs. They will slough off mass during various flashes when fusion exhausts each supply of more massive nucleons in turn. Our sun is in this category. It will not become a black hole or even a neutron star. The sun will end as a white dwarf and be stopped at the Carbon stage above the Chandrasekhar Limit, which is the stage of electron pressure when a core collapses into a neutron star. Thus these types of stars end as white dwarfs because the core is less than 1.1 solar masses.
Stars that can fuse nucleons into iron have a different fate. They have cores that are more massive than 1.1 solar masses and can fuse elements by a complex process up to iron. If the core is between 1.1 to 3 solar masses, the collapse will end in a neutron star when electrons fuse with protons to create neutrons at he Chandrasekhar limit. Before this, these stars evolve into layered objects with successively lighter elements toward the surface (excluding turbulence). When all possible fusion reactions have converted the core to iron, a catastrophe results. The core shrinks dramatically. The overlying regions collapse and bounce off the iron core and create an enormous outpouring of energy the results in a super-nova. As a result, the core collapses into a "neutron" that is five to fifteen kilometers in diameter and glows with extreme brightness. The star losses 90 percent of its mass, leaving only a naked super hot "neutron" core behind.
Any star over three solar masses has the fate of becoming a black hole. Once gravity becomes too great for even he known laws of physics to support, then the core shrinks to the Schwarzschild radius, which is related to the gravitational constant and the speed of light. Simply put, gravity becomes so intense, not even light can achieve escape velocity; hence the term black hole. Once the core shrinks down below the Schwarzschild radius to a singularity according to theory, a phenomenon called the event horizon at the Schwarzschild radius becomes the boundary between the laws of physics as we see them on this side and a different reality inside that exists in a state outside of our known laws. Everything that approaches the event horizon is transformed, by being torn to atoms, sub-atomic particles and then once absorbed into the event horizon, disappears from our cosmos never again to be retrieved by ordinary processes within the laws of our known physics. This of course does not take into account Hawking radiation.
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Black holes that originate from such implosions are small and will feed on anything venturing too close. They are also spinning at an extreme rate, even faster than millisecond pulsars. Due to contraction or the original spinning core, the rate of spin increases dramatically. This fact has been proven with observation of millisecond pulsars; neutron stars that are the remnants of less massive stars. It follows by extension that black holes also have a spin and are rotating even faster due to their more compact size. As a result, they create an oblate form and something called an ergosphere, which is an extension of the event horizon, oblate in form and intersecting the event horizon at the spin axis. In the equatorial region there is intense frame dragging as space-time is distorted. This is a region of intense energy creation as atoms are torn apart, releasing copious amounts of gamma radiation.
There is some debate as to what occurs at the event horizon. Both in falling matter and energy falls to the singularity (in the non rotating case, which is an ideal) or the ring singularity in the case of the spinning black hole or a Kerr black hole, due to conditions of time slowing, matter and energy virtually comes to a halt once inside the event horizon according to theory. Since we cannot venture inside such an object, we don’t really know. We do know that a smaller black hole is more dangerous to approach than a billion solar mass black hole as the intense gravity is more spread out. Thus, if conditions were right, we may be able to approach a billion solar mass black hole more successfully than one that is only a few city blocks in diameter.
If matter and energy are nothing more than constructs of vibrations and empty space, then a black hole is the ultimate demise of matter and energy as a singularity, a point of infinite smallness, infinite temperature and infinite density. It becomes an issue of absolute relativity. There is much that we still don’t actually know.
References and Further reading
Pickover, Clifford (1998), Black Holes: A Traveler's Guide , Wiley, John & Sons, Inc, ISBN 0-471-19704-1 .