Pedagoguery

Stars are generally paragons of stability, spending billions of years quietly converting hydrogen into helium, and radiating the excess energy produced. However, there are certain stars that explode violently, and until recently astrophysicists have been unable to explain why they explode in precisely the way that they do. They know the broad outlines, but detailed models failed to reproduce explosions observed in nature. That is now changing because of the introduction of a new factor into those models: turbulence.

Supernovae basically come in two different varieties: thermonuclear and core-collapse. A thermonuclear supernovae happens when there are two stars in a close orbit around each other. As they age, the more massive one eventually becomes a white dwarf. Eventually, the other star becomes a red giant, and in so doing, it starts dumping mass onto its companion. If the white dwarf accumulates enough mass, it crosses a critical threshold and starts fusing carbon and oxygen into nickel in its core. This reaction expands outward from the core, eventually consuming the entire star and leaving radioactive nickel in its wake. It is the decay of this radioactive nickel that produces the afterglow of the supernovae.

Those are the broad outlines, but the detailed models didn't work. What they showed was that as the wavefront of nuclear reaction spread outward, the nickel “ash” that was left in its wake was less dense than the surrounding material, making it buoyant. That, combined with the heat generated by the reaction, would cause the star to expand and cool, preventing the reaction from consuming the whole star. The reason for this is that the earlier models had to make simplifying assumptions because computing power was limited. One of the most common simplifying assumption was to assume spherical symmetry. Recently, however, computing power has increased to the point where the models can do away with that assumption, and can also include the possibility of turbulence, which is extremely difficult to model. When this was included, the models predicted that the wavefront that traveled through the star was very irregular, with a very frothy structure, in the sense that it is composed of many bubbles, all interacting and mixing throughout the star. This allows the wavefront to propagate very quickly, far more quickly than the star can react to. The end result is a rapidly expanding cloud of radioactive nickel.

The second type of supernova is the core-collapse supernova. This occurs when a massive star – at least 8 times more massive than the sun – nears the end of its life. At this point, it takes on an onion-like structure with a shell of hydrogen, surrounding a shell of helium, surrounding a shell of carbon, surrounding a shell of oxygen, surrounding a shell of silicon, surrounding a core of iron. At the interaction of each shell is nuclear fusion. However, since you cannot get energy from iron by fusing it, as the core accumulates, it cools, since there is no energy being produced there. As it cools, it contracts. When it reaches a critical point, the collapse becomes catastrophic as it collapses into a neutron star. Surrounding material rushes into the space that the core used to occupy, until it hits the surface of the neutron star and rebounds. In addition, the conversion of all of the protons and electrons that were in the core into neutrons generates tremendous numbers of neutrinos. These neutrinos, which are normally the most aloof of particles, hardly interacting at all, are present in such numbers and energy that they heat the surrounding material, generating a shock wave that drives that material outwards.

Here is where the models used to break down. If the solution is spherically symmetric, the shock wave stalls since it reaches a point where the speed of the shock wave going outwards is matched by the speed of infalling material. If this were true, the end result would not be an explosion – or at least not one as powerful as we observe. However, if you abandon the assumption, something interesting happens. It appears that such explosions are asymmetric. This allows the shock wave to propagate outwards while infalling material is channeled into certain regions. This mixes the interior of the star, allowing many of the heavy elements to be ejected. It also explains some puzzling aspects of some observed neutron stars. Some neutron stars are observed traveling through space at a high velocity – 1600 km per second in at least one case. An asymmetric explosion could easily explain this, as the explosion generates a rocket-like kick that pushes the star in one direction.

As the models grow in sophistication, more and more observed phenomena will be better understood.

Next time, the dark age of the universe.