Supernova explosions are rather rare events so the last one seen by the naked eye in our galaxy was described by Johannes Kepler in 1604. For several months, that was the brightest star in the sky so one can imagine how mysterious that looked. Since then we have learned there are two basic scenario for a star to explode. The first one happens when a carbon-oxygen white dwarf gradually increases its mass via the capture of matter from a neighboring star. Thereby the gravitational pressure grows and finally ignite the carbon fusion throughout the star in a very short time. The explosive fusion starts when the star’s mass approaches the Chandrasekhar limit of about 1.4 solar masses. The second scenario is the collapse of an old star when its nuclear fuel is exhausted so that the radiation pressure becomes unable to prevent the gravitational contraction. In this case the explosion energy actually is the released gravitational energy of the star. The peak supernova’s luminosity is billions of that of the Sun so we can see them in very distant galaxies. Now, given that a supernova is an instantaneous explosion, a question arises why is this visible for that long. The reason is the radioactivity of gases ejected in the explosion e.g. the decay of a Cobalt isotope of about 80-day half-life. For the second type of supernova the fading of luminosity is longer because of gradual recombination of ionized hydrogen ejected by the supernova progenitor.