Almost a thousand years ago the brightest star in the sky was surpassed as the world was filled with wonder and light not seen before in recorded history. This doubtless shocked those living at the time, much as it would anyone today. The new bright object in the sky, which was visible even during the day, lasted for almost two years before fading away. We know this thanks to detailed accounts from Chinese astronomers, and the evidence points toward this “guest star” being the Supernova that formed the Crab Nebula we see today.
A Supernova is, in short, a really big explosion. As in big enough that the one seen in 1054 AD happened 6500 light years away, radiated only around 1% of its energy as visible light and could still be seen burning bright in the night sky all around the world (except maybe the then primitive continent of Europe where no records of it exist). NASA put it a bit more succinctly; a Supernova is “the largest explosion that takes place in space”.
Supernovae are the fate of only the largest stars, those with masses 8 to 15 times that of our Sun, which has a very different fate in store. There are two types of Supernova, with just type two explained here (typically they’re a bigger explosion). When stars run out of Hydrogen fuel they collapse and heat up until the core is hot enough to fuse the Helium left over from Hydrogen fusion. When they then run out of Helium they collapse again, heating up, until the leftover Carbon can be fused and so on, up to Silicon being fused into Iron. The stages past Helium fusion require such high temperatures that only the most massive stars can reach them, with Silicon burning occurring only at temperatures 2000 times greater than at the centre of the Sun. No elements past Iron can be fused in the hearts of stars as to do so requires more energy to be put in than is released, which is known as endothermic.
Each of these stages becomes progressively shorter, from 10 million years of stable Hydrogen burning to just 2 days of Silicon burning. This is for two reasons, firstly there is simply less of the heavier elements (2 Hydrogen nuclei become 1 Helium and so on). Secondly, at the insane temperatures in the later stages, the photons released in fusion actually have enough energy to disintegrate the products of fusion, undoing the work done millions of years before. This is known, unsurprisingly, as photo-disintegration and it is also an endothermic process. The energy removed in photo-disintegration critically weakens the core.
Without enough pressure generated in the core, the star will succumb to crushing force of its own gravity. An exceptionally rapid collapse begins, the star falling inwards at some 70,000 km/s, crushing the star until it is three times as dense as an atomic nucleus. Then the strong nuclear force takes hold, causing an enormous repulsive force, which violently throws the star into a rebound, releasing an enormous shockwave as it does so. This shockwave strikes the outer layers of the star, which causes more photo-disintegration, releasing a truly immense number of neutrinos. These are wraith like particles that very rarely interact with anything (billions are streaming through you right now), but in the extreme conditions of a supernova a lot of them are captured, which releases a ridiculous amount of energy, more than the entire energy produced by a typical galaxy. This accelerates the gas cloud even more, to become the biggest explosion humanity has ever witnessed. The star has gone Supernova.
It is in this last hurrah of a star that the elements beyond Iron are created. When the shockwave strikes the outer layers some of the energy is used in the endothermic fusion processes needed to create all the 75 or so elements heavier than Iron. In the ensuing explosion these are scattered far across the cosmos. We can be sure about this because we ourselves are formed of some of these elements, as is the Earth. Supernovae also leave beautiful nebulae, from which other stars, like our Sun are later born in huge cosmic events. These events of incredible violence lead directly to some of the most stunning scenes in the night sky, as well as the life which witnesses them.
What about the star’s core? Well its fate depends yet again on its mass, but there are two known paths it could take; neutron star or, for the larger cores, a black hole. These degenerate stars are fascinating in their own right and are be explained here.
In a galaxy the size of the Milky Way we expect to see a Supernova once every 100 years or so. The last witnessed was the Kepler Supernova in 1604, so we are a touch overdue for one of these phenomenal events, and you can be sure that you’ll know when it happens.