Types of Supernova

Collapsars (or Hypernovae
It is possible that, if the star started with more than about 40 or 50 solar masses, the "bounce" off the neutron core is skipped, avoiding the supernova stage, and the star collapses immediately into a black hole, absorbing most of the in falling matter.  The remaining matter forms an accretion disk.  This process produces an extraordinary release of energy as it emits two jets of intense gamma rays, which may explain the phenomenon of gamma ray bursts.  These used to be called Hypernovae, but are now more usually called collapsars.  Here is a detailed description of a proposed collapsar model

Pair-Instability Supernovae
This scenario applies only to extremely massive stars with at least 130, and up to (very approximately) 250, solar masses.  It occurs only in stars with fairly low amounts of elements heavier than hydrogen and helium.  In stars with more than about 100 solar masses, it is gamma ray pressure in the core that supports the outer layers against gravitational collapse.  As the temperature rises, the gamma rays become more energetic, and start to produce electron-positron pairs.  If the mass is less than about 130 solar masses, the production rate is not sufficient to cause instability.  These annihilate and produce more gamma rays.  When it is more than 130, as pair production increases, the temperature rises producing ever more energetic gamma rays that produce more pair particles in a feedback loop. 

The process collapses the core, and a runaway fusion reaction destroys it in a thermonuclear explosion within a few seconds.   The release of energy is so great that it overcomes the stars' gravitational binding energy.  The star is destroyed completely; there is no neutron star or black hole left behind. 

Quark Novae
The supernova 2006gy was extraordinarily powerful; perhaps one hundred times brighter than usual for a supernova.  It is, in fact, second only to supernova SN 2005ap in terms of brightness.  Some observers suggest that these may have been "Quark Novae" that produced a quark star.  Their idea is that the star went supernova in the usual way, blew away its outer layers, and became a neutron star.  Due to its extreme magnetism, the rate of rotation slowed rapidly so that centrifugal force was no longer sufficient to help support the core of the neutron star.  More of the outer layers are blown off at even higher energies; possibly very close to light speed.  This second layer of matter catches up with the first layer from the original supernova creating the intense energies observed.  The core is now so compressed that the neutrons experience quark deconfinement, and break up into a soup of quarks. 

Then other (perhaps most?) observers say that there are many less complex and less exotic explanations, like a Pair-Instability Supernova, discussed above, and that there is no solid mathematical or theoretical basis for quark stars.  So the question remains open, for now.  Supernova 2006gy happened in the galaxy NGC 1260, which is approximately 72 megaparsecs away, and the progenitor star probably had a mass of around 150 solar masses.  Supernova SN 2005ap was much further away; nearly 1.5 giga parsecs, and its progenitor was probably even heavier. 

The Quark Nova Project is a group of scientists based in the University of Calgary in Canada, and they are "dedicated to the investigation of quark novae".  This is probably the best place to track progress about this hypothesis. 

Astronomy & Cosmology -

Stars - Life & Death of Stars