"Normal" Black Holes

   There are several theories to how black holes form.  The Essential Cosmic Perspective helps explain the formation of black holes.  The beginning of the life of a black hole starts with a supernova, or a massive star collapsing in on itself.  The iron core succumbs to gravity and the mass of the star compacts into a dense sphere of neutrons.  This is how a neutron star forms.  However, there is a neutron star limit.
     When the supernova occurs, it blows away the star’s upper layers.  If the explosion did not fully blow away the layers, the matter can fall back into the core, causing what would be a neutron star to be denser than it can handle.  Once the mass of the star reaches beyond this limit, the neutron degeneracy pressure cannot escape the intense gravity.  This causes the core to collapse again.  At this point, there is nothing that can escape the gravitational pull of a black hole.  All of the thermal energy and pressure from the exploding star are additions to the gravity, making it even stronger.  “The more the star collapses, the stronger the gravity gets,” (Cosmic Perspective 367).  Once the gravity gets this great, there is nothing that can stop the star from collapsing into a black hole.
      A similar theory explained by Dr. Sten Odenwald uses the Schwarzchild radius and goes into more detail about the core of a massive star.  When a core implodes, the gas can reach a point where it can’t resist the pull of gravity.  Stars large enough can reach a point where the fusion reactions inside the core drastically changing its neutrino production.  The high production of neutrinos release energy from the core, and the core gets even hotter and collapses.  The reactions in the core start to occur at hotter temperatures, and produce even more neutrinos.  As the core continues to force itself inward, the neutrinos being produced get trapped under the density of the core.  The energy from the neutrinos builds up until the core erupts.  This explosion compacts the core to such a high density that it cannot support itself. This inevitably turns into a black hole.

To See a how a Black hole is born click here

Supermassive Black holes

     To this day, astronomers are not sure how supermassive black holes form.  However, there are many theories.  These monstrous mysteries may form from the merging of other black holes, the growth of galaxies, or by the collapse of large gas clouds.
     Supermassive black holes form by galaxies colliding or merging together, which combines the black holes in the center of each of the galaxies.  The black holes get bigger when another is merged with it.   Supermassive black holes are anywhere from 106 to 109 solar masses.  The Formation of Ellipticals, Black Holes, and Active Galactic Nuclei: A Theoretical Perspective describes that elliptical galaxies that have formed recently should have Supermassive black holes with smaller masses then other galaxies as a whole, because they were formed in events that lacked abundant amounts of gas.  This is testable from observation.  On the other hand, ellipticals at the center of gas rich bulges and clusters with a large disc will have comparatively immense black holes for how bright they are due to the formation being at high redshifts. As the galaxies merged, they maintained their ration of black hole mass to mass of all of the stars and gas (0.15 percent).
     Another theory involves the galaxies growing with the age of the universe. The Birth of Supermassive Black Holes: Battle in the Bulge describes black holes as starting small, and growing larger as galaxies formed around them.  This process releases large amounts of energy back into the galaxy.  This energy controls the galaxy’s ability to make stars, and pushes away clouds of gas and dust.  This cuts off the supermassive black hole’s fuel supply.  When this occurs, the size of the black hole and galaxy itself are regulated.  This happens when the ratio is 0.15 percent.
Observations within the last decade seem to support the second theory.  The idea that black holes started very small and grew into supermassive black holes is favored.

Sources:
The Formation of Ellipticals, Black Holes and Active Galactic Nuclei: A Theoretical Perspective
Author(s): Guinevere Kauffmann, Stephane Charlot, Martin Haehnelt
Source: Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 358, No. 1772, The Formulation of Galaxies (Jul. 15, 2000), pp. 2121-2132
Published by: The Royal Society
Stable URL: http://www.jstor.org/stable/2666898

The Essential Cosmic Perspective, Fourth Edition, pp 367-368
Author(s): Jeffery Bennett, Megan Donahue, Nicholas Schneider, and Mark Voit
Published by: Pearson Education Inc.

Black Hole Encyclopedia
How does a Black Hole Form?
and Birth of Supermassive Black Holes: Battle in the Bulge
Reasercher: Karl Gebhardt
Copyright 2008 University of Texas
Stable URL: http://blackholes.stardate.org/resources/faqs/faq.php?id=7

Ask the Astronomer: How do Black Holes Form?
Author: Dr. Sten Olden Wald
Stable URL: http://www.astronomycafe.net/