It is an accepted fact that supernovas can be categorized in two types:  Type I and Type II.  The supernovas with the highest magnitude are called Type I.  They generally reach a magnitude of -18.6, or 1/100th the luminosity of our galaxy.  "If such a supernova were at the distance of Alpha Centauri, it would appear, at peak brilliance, about one-seveth as bright as the Sun."  (Isaac Asimov, Exploding Suns page 124).   Type II supernovas tend to be a bit darker, reaching a luminosity of only one billion times that of our Sun.   A second distinguishing trait is that Type I supernovas decline in brightness in very regular fashion, after they have reached their peak brilliance.  Type II supernovas decline in brightness much more irregularly.

        From a study of the light spectra in supernovas we see that there is a third difference.  This difference is the amount of hydrogen present in them.  Type I supernovas almost no hydrogen whatsoever while Type II's have a great amount.  The location of these supernovas presents yet another difference between the two.  Type I supernovas are very general in their locations among a galaxy.  They explode not only in the arms of spiral galaxies but also in the center of both spiral and elliptical galaxies!

        The difference in the location of these supernovas tells us something very important.  The stars in elliptical galaxies are small, just a little larger than our Sun, and have existed for most of the life of the dust-free galaxy.  The same is true of the center of spiral galaxies.  The dusty arms of a spiral galaxy are the home of young and massive stars(Exploding Suns page 125).  This means that Type I supernovas are stars that possess the same mass as the Sun or a little more.  Type II stars then involve stars that are at least three times as massive as our Sun maybe considerably more.
 
        The two types of supernovas are equally common in our Universe.  This means that not every small star will end up as a Type I supernova; in fact, only a small minority will.  In order to have a Type I supernova you not only need a Sun sized star but also a special type of star this size.  This is where the chemical difference between the two types of supernovas comes in.  Remember that Type I supernovas have almost no hydrogen in them, which means they are at the end of there growth cycle.  Since they may be rich in carbon, oxygen, and neon they are classified as white dwarfs.  So many scientists concluded that Type I supernovas must represent exploding white dwarfs.
 
        White dwarfs do not explode on their own(Exploding Suns page 127).  However, they are commonly part of a close binary star system.  The companion star of the white dwarf, when it reaches the end of its life and swells to a red giant, matter will spill over into an accretion disk that periodically adds mass to the  white dwarf..  When the matter added to the accretion disk is compressed and heated enough it ignites into fusion.  This creates an explosion that throws the remaining accretion disk away and increases the luminosity of the white dwarf by several times(Exploding Suns page 127).  Seen from earth we would call it a nova.  This episode will repeat itself in shorter or longer intervals.  There is some mass held on to by the white dwarf during each nova, gradually increasing its mass.  If this white dwarf were already massive for its type, maybe about 1.3 times as much as our sun, or its companion star grew to a larger than normal, it would cause it to gain mass rapidly enough to push it over the Chanderasekhar's limit(1.44 times the mass of our Sun)(Exploding Suns page 127).  The white dwarf cannot maintain itself at this mass.  The white dwarf then collapses in on itself.  Isaac Asimov's describes it:  " It compresses very rapidly and slams the nuclei of carbon and oxygen together with great force.  All of it undergoes fusion at once, producing so much energy so rapidly that the result is a vast explosion that radiates as much energy in a few weeks as our Sun will produce in all its multi-billion-year lifetime.  In short, the collapse of the white dwarf and the fusion of its substance produces not just a nova but a Type I supernova."  A Type I supernova like that tears the star apart and leaves no collapsed star of any kind behind, only an expanding gas and dust cloud.  Tycho's nova of 1572 and Kepler's nova of 1604 were both Type I supernovas.  No neutron star was found in either site, only a nebula.  It is the Type I supernova that scientists look at for a clue as to how the Universe is expanding and whether it will collapse back in on itself.