It is believed in a galaxy there is a supernova, that is the catastrophic explosion of a big mass star, about twice in every century. The last supernova in our galaxy visible from Earth was observed in 1604, five years before the telescope started to  observe the sky. Since with the large telescopes we are able to see thousands of galaxies from Earth, on average we should observe a supernova every few months. So there are astronomers in observatories in the whole world who survey the sky and are waiting for the appearance of these stellar bursts in the distant galaxies. Explosions that for some weeks can exceed in brightness all the other billions of stars of the belonging galaxy.
The first supernova, and also the brightest one observed in an outer galaxy, was visible in the 1885. In the August of that year a new star suddenly appeared in the central zone of the nearest galaxy to us, the Big Nebula in Andromeda. The star quickly increased its brilliance  to achieve the seventh magnitude. At that time it was not yet known if the Andromeda Nebula and the other similar nebulas were objects belonging to our galaxy or star systems out of it. The ordinary novas are common enough stellar explosion where a star increases its own luminosity of a factor that can be 10.000 times. These  were well known in 1885 and therefore it was supposed that the new star in the Andromeda Nebula was a normal nova, and on the basis of this consideration was estimated a distance of the nebula within the borders of our galaxy.
The Edwin P. Hubble study in 1920 of the variable stars in the Andromeda Nebula and in other spiral nebulas showed these systems are in a great distance from our galaxy: only then was the true nature of the new stars observed in those systems recognized. Once discovered that the distance of the Andromeda Nebula was no longer thousands of light years but hundreds of thousands, the extraordinary brightness of the 1885 supernova was made evident: it was not 10.000 times brighter of an ordinary star, but even 10 billions of times.
These wonders of the sky are objects carefully studied since the thirties, when Fritz Zwicky in the Mount Wilson observatory began a classification dividing them in two separate groups. The most common of them are called Type I supernovae and have an -18 or -19 absolute magnitude, later fading away quickly enough after the explosion. If one of them should explode at the same distance of Vega, it would equal the brightness of hundreds of full Moons. The Type I supernovae show peculiar spectra, whose fundamental characteristic is the total absence of the hydrogen lines. The expansion velocities of the nebular remnants are measured by the P Cygni lines and can touch the 10.000 Km/s, 3% of  light speed. The Type II events are about two magnitudes weaker, showing generally a plateau in their light curve, that is they are stabilized for some time on a constant luminosity and moreover they have the hydrogen lines; the expansion speed is approximately half of the Type I supernova one.
The source zones of the two types of supernovae are different. The Type I events are produced in the galactic disks, where the majority of the stars resides, but also in the elliptic galaxies and in the central swelling or in the halos of the spiral galaxies, so that it is believed they originate from small mass stars. The Type II explosions instead stay confined in the galactic disks and in the arms of the spiral galaxies, the only places where very massive stars are found. If we consider the phenomenons more in detail, we are in need of adding new types and subtypes to the classification. The most important subdivision concerns the Type I, separated in the Ia one, the class originally defined by Zwicky, and in the Ib one in which form the supernovae that, though arise in the galactic arms, neverthless don't show the hydrogen lines in their spectra.
The researches of an astronomers and physicists generation have finally allowed to understand how thesestellar bombs detonate. The Type II explosion is the natural end of a massive star. With the proceeding of the nuclear reactions, the atomic nuclei of the inner core of the star are more and more compressed among them. Every following stage of the nuclear fusion can always supply a quantity of small energy, and consequently it will last for a briefer and briefer time. It needs about one million years before the helium of a red supergiant of 20 solar masses fuses in carbon and less than 100.000 years before the fusion of the carbon is concluded in neon and magnesium. When also carbon ends, the nucleus starts to run hot until it begins to melt also the oxygen, that is transmuted in silicon and sulphur:
this phase lasts less than 20 years! At this point, during only a week, the silicon changes into iron.The temperature exceeds 3 x 10⁹ K and now the reactions produce energy in the form of more neutrini than photons. The inside of the supergiant shows a structure of shells, (like an onion) since every stage of the nuclear fusion is moved outwards in following shells that surround an iron nucleus of about 1,4 solar masses.
Among all the atomic nuclei, iron is more firmly tied up. Energy cannot be produced by its fusion. When the silicon fusion is over, the nucleus, that has about the size of the Earth, is near the Chandrasekhar limit and for brief time it is sustained by the degenerate electrons. At this point the iron nuclei suffer the first attack. Density is elevated so the electrons begin to combine with them, making manganese; the heat generated is so intense as to emit gamma rays extremely energetic, able to penetrate in the manganese nuclei and divide them in nuclei of helium. As soon as the support provided with the degeneration of the electrons and the pressure of the gamma rays diminishes, the nucleus is compressed more and more quickly finishing to collapse in a catastrophic way. The star has lived 10 million years, yet in less than a tenth of second the iron nucleus falls down in itself with a speed equal to a fourth of the light one, up to become a sphere of only 100 kms in diameter. The gravitational energy released isbeyond all description. In a flash the star wastes 10⁴⁶ Js, more than 99% in the form of neutrini. The energy produced is 100 times greater than the energy the Sun has burnt during its whole existence.

In the center of the star the density is now so great the protons and the electrons of the nucleus begin to glue each other, generating neutrons condensed in a sphere of 10 or 20 kms in the final diameter. At the beginning the central temperature is around 2 x 10¹¹ K. The sudden implosion of the nucleus produces a shock wave bouncing towards the outside. The tangle is so dense that even the neutrini, normally able to filter through a wall of lead 1 light year thick, have difficulty to emerge: so they perform a pressure that, adding to the shock wave one, blows away the remnants of the star. Only the inner nucleus remains, a collapsed star sustained by the pression of the degenerate neutrons.
The nuclear fury  creates a big number of neutrons quickly combining each other, giving rise to highly radioactive isotopes. This r-process (r means rapid) can produce isotopes more heavy than those generated by the s-process occuring in the giant stars. Different solar masses of stellar material, enriched of heavy elements the star has created in the supergiant stage and during the supernova explosion, are suddendly released to the speed of thousand of kilometers each second and returned to the interstellar space. A nebular shell in expansion is produced.
And what can be said about the some more bright Type I supernovae? Since they appear in the old galactic halos, they cannot be originated by massive stars. It is rather believed these events represent the final act of the evolution of white dwarfes belonging to the binary systems. There are two possibilities. An ordinary nova happens when a main sequence companion, deformed by the tide forces, loses matter falling down the surface of a white dwarf where, increasing, causes an explosion followed by a return to the normality. If however the colapsed star is close to the limit of Chandrasekar, it is possible that before the superficial explosion happens, the mass accumulation of the falling gas forces the star above that limit. At this point the degeneration pressure is not enough to sustain the weight of the star, the white dwarf collapses and a reaction of explosive fusion primes in the whole stellar body. After the explosion neither an iron nucleus nor a nucleus of neutrons will survive: the star is literally annihilated.
The other possibility concerns a binary system of two white dwarfes. The two stars had already been near when, during the evolution, they had in the red giant state: the one expanded ball reached to touch the other one. Now the two near orbiting white dwarfes will emit gravitational waves, that are perturbations of the respective gravitational fields giving out from the system (the gravitational waves, foretold by the theory of the relativity, till now have not been ever directly observed). This emission is the cause of a loss of energy so that the two stars getting near more and more, with a spiral approaching motion. At the end, because of the tide interactions, the two stars are melt in one, overcoming the limit of Chandrasekhar and exploding (as in the first model). Probably both the star of Tycho and the Keplero one were Type I supernovae.
The supernovae are responsible for the synthesis of the elements above the helium and they are the only engines producing the heavy elements existing in the Universe.
 
 

by Pio Passalacqua  
translated into english by Salvo Marino