How fast does a supernova actually happen?

Answer

The last phase before the supernova really starts is the conversion to iron of mainly silicon. Exactly how quickly this process happens depends on the mass of the star, because the more mass, the more luminosity, and the faster the nuclear fusion has to happen: the phase is therefore shorter as the mass of the star increases. It’s about time scales of days only.

Then the actual start of the explosion, namely the collapse of the star. This occurs on what is called the dynamic time scale of the core, the time it takes for a sound wave to propagate through the core. And that’s on the order of a second here. Thus, as the star falls, its size decreases and its density increases, and both aspects cause the dynamic timescale of the formed neutron star to be on the order of a millisecond. (It is not entirely correct to say that the incident outer layers bounce off the neutron star. Within the second in which the neutron star is formed, the outer layers have not had time to ‘fall’. It is the energy released by the formation of the neutron star which – through complicated processes – blows away the outer layers.)

The collapse happens deep inside the star, an outside observer only sees something when the signal from the explosion reaches the outside. The shock wave propagates at the speed of sound from the star’s outer layers, typically taking a day or so to surface. Then one sees a UV or X-ray flash. At least, if you’re lucky, because supernovae are usually discovered in visible light, and then the flash is over. But if our galaxy ever erupts again, there’s hope we’ll “see” them early enough, namely at the core collapse itself. Because then a lot of neutrinos are released, and they fly through the star like that; so it is quite possible that the next galactic supernova will be detected by a neutrino detector.

The visible spectacle lasts longer. Much of the energy of the shock wave goes into (further) ionizing the star layers above, and the recombination of especially hydrogen (the most abundant element) releases that energy afterwards. As the outer layers are expanding, the surface radiating towards us gets bigger and bigger, and the visible brightness increases. It can sometimes take weeks before the maximum brightness is reached. Once the process of recombinations has reached its peak, the further course of the brightness is determined by radioactive decay. The enormous energy released in the eruption also triggers new nucleosynthesis, greatly favoring the formation of iron (the most stable element). But iron-56 contains not 28 protons and 28 neutrons, but 26 protons and 30 neutrons: to form iron from mainly elements such as C, O, …, Si, which have an equal and even number of protons and neutrons, a detour is necessary, namely via the unstable element nickel-56 (28+28): which decays on a time scale of one week into cobalt-56 (27+29), which in turn decays on a time scale of three months in cobalt-56 (27+29). iron-56. It is the energy released in that decay that determines the brightness of the supernova during the later phases and gives the characteristic exponential tail to the light curve.