The word “Nova” comes from the Latin word for “new.” Plural is Novae.
Observers initially thought such objects to be a new star, hence the name.
The energy released during core collapse is unimaginable - more energy than is produced by 100 stars like the Sun during their entire lifetimes of more than 10 billion years! Most of the energy released during collapse is carried off into space by neutrinos; however a small fraction of the energy triggers the accompanying Type II supernova explosion.
Supernova remnant Cassiopeia A
observed by the Chandra X-ray Observatory
For a period of about one month, a single star shines brighter than a whole galaxy of a billion stars. In-falling material around the core is bombarded with neutrons, which forms higher elements up to (and maybe beyond) Uranium. The energy from the collapsing core not only creates these heavier elements but also speeds up the outer material of the star and generates a Supernova remnant such as the Crab nebula. This heavy element enriched gas can be incorporated into future generations of stars and planets.
The remaining core has become a neutron star. Because the star is shrinking enormously, the initial rotation speeds up to as much as several rotations per second. The density reaches that of atomic nuclei: one teaspoon of it could weigh a billion tons.
Without supernovae there would be no heavy elements
Indeed, we are made of stardust. When this was discovered by astronomers in the late 1960’s Joni Mitchell wrote the song "Woodstock": “...we are stardust, we are golden, we are billion years old carbon...”
A neutron star with the mass of our Sun will have a diameter of about 30 km. The neutrons resist further compression by neutron degeneracy pressure, in a way that is analogous to electron degeneracy pressure, but much stronger.
Source: Image Tours at http://hubblesite.org/gallery/tours/