Pedagoguery

Over the past several years, there have been several new searches for supernovae. Supernovae are used to answer many questions astronomers have about the wider universe, so it is no surprise that these surveys have been undertaken. What they have revealed, however, is something else. They have revealed new types of supernovae, types that are 10 to even 100 times brighter than standard core collapse supernovae.

We are all familiar with the mechanics of a core collapse supernova. When a massive star nears the end of its life, it fuses successively heavier elements until it reaches iron. Iron cannot be fused without taking, rather than releasing energy, so the core of the star collapses. This causes a shock wave which blows out the outer layers of the star. The nuclear reactions also produce large amounts of nickel 56, which is radioactive. It decays into cobalt 56 with a half life of just over 6 days. Cobalt 56 decays into iron 56 with a half life of over 77 days. It is the decay of these two elements that provides the long, slow decline in luminosity for most supernovae.

However, in 2006, a supernova was observed that did not neatly fit this mold. It was significantly brighter than a standard core collapse supernova. A clue was found when it was discovered that in 2004, what appeared to be a supernova was at the exact spot in the galaxy as this same one. It was theorized that a very massive star had blown off its outer layers during that earlier outburst, which was called a supernova impostor. Then, two years later when the star actually did go supernova, the shock wave of the supernova caught up with the earlier cast-off stellar layers. The impact of the shock wave with this earlier gas caused that large gas shell to heat up, causing the higher-than-normal brightness.

What happens if the original star is even larger? Also in 2006, a supernova was observed that was over 100 times brighter than a typical core collapse supernova. In addition, there didn't appear to be enough radioactive nickel to cause so much brightness. There are three different potential explanations for this. The first is a scaled-up version of the shell collision mechanism described above. The second involves a magnetar, and the third involves pair production instability.

In the magentar model, it is not the explosion itself which causes most of the fireworks, but what happens after. A magnetar is a neutron star with an unusually high magnetic field. Given that a typical neutron star has a magnetic field a million times stronger than the Earth's, this is saying quite a lot. Since during a core collapse supernova, most of the angular momentum of the star resided in the collapsed core, newly born neutron stars spin incredibly quickly – hundreds of times per second. However, the extremely strong magnetic field interacts with the surrounding plasma to transfer some of the star's angular momentum to that plasma. This can cause the plasma to generate a prolonged additional pulse of light.

Pair production instability comes in two forms. The first is called pulsational pair instability. When the temperatures in the core of a star get to be high enough, the gamma rays produced by stellar fusion approach energies twice that of the rest mass of the electron. When that threshold is reached, the gamma rays can spontaneously convert into electron-positron pairs. These particles do not contribute to the radiation pressure holding the star up against gravity. This causes the star to collapse. The heat generated by that collapse triggers a round of explosive fusion reactions, blowing off the outer layers of the star. This event will probably appear like a supernova impostor, and it can happen multiple times. The ejecta of each subsequent explosion will hit the previous one, triggering yet another supernova impostor. When the star does finally explode as a supernova, the shock wave will plow through the shells of gas cast off by the previous explosions and triggering a very bright event.

The second type of pair production instability is even more dramatic. It starts the same way, with the sudden collapse of the core. But, if the star is massive enough, it can be blown apart in runaway nuclear reactions. One such event may have been observed in 2007, where a very bright supernova in a dwarf galaxy lasted for over 600 days. Observations indicated the production of between 3 and 10 solar masses of nickel 56 – an amount indicative of a progenitor star of between 130 and 260 solar masses. The helium core of the star itself probably had about 100 solar masses. Such hugely massive stars are very rare in today's universe, but were probably much more common during the early universe, where star formation tended to favor such massive stars because of the lack of elements heavier than helium.

Next time, dark matter in the Milky Way galaxy.