When dying stars explode into supernovae, the outward shock wave is preceded by a kind of “bounce” — matter and elementary particles collapse toward the star’s core so tightly that they reach a critical threshold of density, such that nuclear forces kick in and push back against that collapse. In the end, all that’s left is a glowing remnant of dust and gas.
Yet much of the physics that occurs during this process is not yet well understood, although scientists believe that at some point during the collapsing stage, the ultra-dense matter organizes itself into unusual shapes dubbed “nuclear pasta.”
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A new computer simulation by physicists at the University of Tennessee in Knoxville, published last month in Physical Review Letters, has identified yet another new pasta shape, or phase, which they hope may one day shed some light on the very complicated physics behind supernova explosions — such as the role of neutrinos during such events.
The pasta shapes formed by nuclear particles when a supernova is developing can be rods, slabs, or bubbles (round holes, or holes shaped like cylinders). It’s a category known among physicists as “frustrated matter,” which also includes ferromagnets, glasses, soft solids, and the like.
This phenomenon occurs when different forces clash within a material system, unable to find a balance, making the material unstable, i.e., “frustrated.”
In the case of a supernova, those competing forces are the nuclear attraction and Coulomb repulsion — the physical law stating that the force of attraction or repulsion between two electrically charged bodies is directly proportional to the strength of the electrical charges (and inversely proportional to the square of the distance between them.)
Basically, you have positively charge nucleons that repel each other, at least until they get too close, at which point the strong nuclear force kicks and the nucleons start to attract each other. The result is the strange pasta shapes predicted by prior computer models. Frustrated matter may account for as much as 15-20 percent of the total matter during the “bounce” stage of a supernova.
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Jirina Stone and her Knoxville student, Helena Pais, decided to build a new computer model from scratch, in hopes of producing a better simulation of the complex physics taking place — and perhaps learning what role this “nuclear pasta” plays during that critical bounce period.
As the density of the matter increases, their model predicted all the shapes previously identified, as well as one new shape, resembling a “cross rod” — or, as Physics Buzz prefers to call it, a “double bow-tie pasta phase.”
Image: Density distributions for several pasta phases. Credit: Helena Pais/Jirina Stone/American Physical Society