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In a matter of seconds, the explosion releases as much energy as the Sun will produce during it's entire lifespan. Ejecting a small portion of the mass of the stars. As this hot, radioactive mass emits light, it glows with the strength of thousands of classical Novae, thus the name. Novae are smaller explosions that happen when white dwarf stars pull matter from another star to the point of causing a runaway fusion reaction.
Neutron star mergers are also understood now to be the main source of heavy elements like gold and platinum. The explosion forms outflows of hot, dense plasma rich in free neutrons. As these neutrons are quickly captured by the atoms in the plasma, heavier and heavier radioactive elements are formed, which later decay to stable nuclei.
Astronomical estimates indicate that supernovas tend to happen in our galaxy with a frequency of two or more per century. Nonetheless, the last time a supernova was seen with the naked eye on Earth was in 1604. Since them, it was expected that more than 10 of such events to have happened, but none was recorded since then.
One of the main reasons is that the solar system is embeded at the edge of one of the dusty arms of the Milky Way, and the huge dust clouds obscure our observation of these extremely bright explosions, that tend to happen along the galactic plane. For a supernova to be visible to the naked eye, it needs to happen relatively close to Earth and in a place with a clear light of sight.
In 1984, the VLA radio telescope detected a supernova remnant almost 28 thousand light years from Earth in the constellation of Sagittarius, named G1.9+0.3. It's small angular size implied a very young object. Further observations makes us believe its light would have reached Earth sometime between 1890 and 1908. Making it the youngest supernova remnant known!
On 2 March 2018, astronomers discovered a new supernova in the NGC 2146 galaxy, 21 million light years away from Earth. Named SN2018zd, this supernova is believed to be the most compelling evidence for a new type of supernova, an electron-capture supernova.
The two most common types of supernovas are the thermal runaway supernova and the core collapse supernova. Thermal runaway supernovas are formed when white dwarfs increase their mass beyond a critical limit and ignite a runaway fusion reaction that completely disrupts the star, be it by transferring mass from another star, or by the merger of two white dwarfs.
Core collapse supernovas, on the other paw, are created when massive stars, with masses beyond 10 solar masses, start producing iron in their cores, stopping the fusion process that generates the heat that supports the pressure in the core of the star. Quickly, the star falls into itself, compressing the core into a neutron star, and releases insane amounts of energy as neutrinos, which then power the shock wave that disrupts the star.
Core collapse supernovas, on the other paw, are created when massive stars, with masses beyond 10 solar masses, start producing iron in their cores, stopping the fusion process that generates the heat that supports the pressure in the core of the star. Quickly, the star falls into itself, compressing the core into a neutron star, and releases insane amounts of energy as neutrinos, which then power the shock wave that disrupts the star.
As the star ages, it starts producing heavy elements like oxygen, neon and magnesium in its core and these accumulate in their cores, being supported just by the sheer pressure between the electrons compressed to insane densities. When the mass of the core surpasses a critical limit, the magnesium and neon atoms start capturing the electrons and using them to convert protons to neutrons and producing neutrinos. As the electrons are removed, the electron pressure that supports the core drops. As it loses support, the core quickly collapses.
As it collapses, the temperature rises and causes the oxygen in the outer layers to ignite, starting a fusion runway reaction. Which the explosion resulting tearing the star apart. This process makes electron capture supernovas an intermediate type between thermal runaway and core collapse supernovas. While creating a very distinct chemical signature of the new elements formed in the supernova that has just been observed in SN2018zd.
As it collapses, the temperature rises and causes the oxygen in the outer layers to ignite, starting a fusion runway reaction. Which the explosion resulting tearing the star apart. This process makes electron capture supernovas an intermediate type between thermal runaway and core collapse supernovas. While creating a very distinct chemical signature of the new elements formed in the supernova that has just been observed in SN2018zd.