For as long as we have looked at the stars, we’ve known that sometimes a new star appears from nowhere. This has always been a big deal since it was well understood that the stars were eternally fixed in the sky, and could neither move nor change (with the obvious exceptions of 5 bright wandering stars that the Greeks called Planets). A new star was direct intervention from the gods so it was vital that the astrologers found the corresponding event on Earth. Of course, there’s always a war breaking out somewhere, or a new prince being born and news traveled slowly in olden times, so people weren’t concerned with the precise timing of an event. This made it easy to match things up to these new stars, or Novae (from the Latin for “New”).
But every now and then, a Nova would appear like no other. It would be so bright that it would cast shadows at night and shine visibly during daylight. A normal Nova would likely be missed by anybody who wasn’t paying attention, but these special Supernovae were really obvious. Then, to add to their mystery, they would fade away and vanish after a month or so. Such a sight was very rare, happening perhaps once every few centuries and most of them have been captured in art or astrological records. But what exactly is going on to create a new star and what happens that makes a supernova so bright and ephemeral?
Today we know that when a star nears the end of its life, it’s reserves of hydrogen fuel run low and the fires begin to go out. As the nuclear fusion reactions die down, the pressure drops. The core becomes less able to resist the stars immense weight and begins collapsing. The further it compresses, the higher the temperature and pressure get. Eventually the core becomes so hot (around one hundred million degrees Celsius) that a new fusion reaction can begin: The helium ash created by the fusion of hydrogen can begin fusing into newer, heavier elements. This reaction is far more powerful than hydrogen fusion and counters the gravitational collapse so strongly that the entire star inflates like a balloon. It grows to such an extent that it will likely be bigger than the orbits of its inner planets; they will be swallowed up by the star. A star at this stage of its life is called a Red Giant and our own Sun is due for this change in a little over four billion years time. But this is a gradual gentle process – it can take millions of years for the process to complete.
A Red Giant burns helium far faster than it used to burn hydrogen, so this phase doesn’t last anywhere near as long as the previous Hydrogen Burning stage. Before long (although we’re still counting in millions of years) the helium will also be exhausted. Once the Helium is gone from the core, the star again begins to shrink, getting hotter and denser until eventually the pressure is high enough to halt the compression. At this point the star is reduced to a white dwarf. It is now so small and hot that it glows an intense white, but it is not hot enough to burn the ash from the Helium reactions so it gradually radiates its heat away until, at some unimaginably distant time in the future, it will cool enough to be a completely dark and inert body. But if it somehow manages to pick up some more fuel, it might re-ignite – more on this later!
Now stars come in all sizes. The larger the star, the greater the pressure at the core and the faster (and hotter) it burns. A star the size of our Sun burns for about ten billion years, and the smallest stars (known as Red Dwarfs, which many astronomers believe are actually the most common sort) can last for hundreds or thousands of billions of years. But a truly massive star will burn its fuel at such a prodigious rate that it runs out of Hydrogen after only a few million years. When the Helium runs out, there is still such a vast mass left over that it is able to fire off a third sequence of reactions, building heavier and heavier elements until finally it is left with a core of Iron. Now iron marks a boundary on the periodic table of elements. Every fusion reaction up to now has been exothermic, but from iron onwards the reactions become endothermic. This means that instead of generating heat and energy, the core suddenly sucks it back in. All that power holding up the enormous mass of the star is suddenly removed and the core implodes violently with such force that the very atoms themselves are crushed. When an atom’s electron shell is forced into contact with the nucleus, a subatomic reaction occurs, and protons and electrons combine to form neutrons and photons. The enormous pressure is suddenly released and the energy behind it is carried away by photons as heat. So much energy is liberated that the majority of the star is blasted away at incredible speeds. The expanding shell of ejected gas and plasma glows so fiercely that for several weeks it shines brighter than the billions of stars making up the rest of the galaxy combined. As the gases dissipate, they cool and darken, revealing the neutron star which is all that remains. This is called a Type II supernova.
But there is another type of supernova, the Type I, and it only occurs when we have two stars orbiting each other (double stars are actually very common). If our two stars are close enough to each other, and if one of them becomes a white dwarf while its partner is still a red giant, the surface of the giant can get very close to the dwarf. Under these circumstances, the gravity from the white dwarf can actually siphon away material from the surface of the red giant in what must be an astonishing sight: Fiery hot plasma streaming away from one huge dull red star into a tiny hot white one. Over time this material builds up and makes the white dwarf heavier and heavier, and the core pressure builds up higher and higher. Those billions of tonnes of hydrogen plasma eventually pile up to the point where the pressure at the surface is enough to begin a new hydrogen fusion reaction. For a brief while, the surface of the white dwarf blazes with the ferocity of a stellar core, bright enough to appear in a distant observer’s sky as a new star – a type of cataclysmic variable star called a Nova. Of course, the fuel doesn’t last long, and soon only helium is left, but the white dwarf is still a little heavier than it was before. The cycle begins agani and repeatrs until it is about 1.4 times the mass of the Sun, and at this point the internal pressure is high enough to crush the atoms into neutrons, just like in a Type II supernova. Although the exact processes involved are still not fully understood, it is believed that this sudden transformation causes a massive release of energy, which rips outwards through the white dwarf and blasts its outer layers with unimaginable fury. Of course, since a white dwarf is still larger than planet Earth, this explosion takes place over several hours. This type of supernova, incidentally, packs quite a bigger punch than the Type 2.
Thanks to Jim Caffey for identifying several factual errors in the original text of this article.