The Origin of Elements and the Mystery of Supernova Explosions

Naoki Yamamoto (Associate Professor, Department of Physics)

The matter around us is made of various elements. For example, our bodies are composed of elements such as hydrogen, carbon, nitrogen, and oxygen. But where, when, and how were these elements created in the course of the universe's evolution? It is believed that light elements like hydrogen and helium were synthesized shortly after the Big Bang in the early universe, while carbon and heavier elements were synthesized through nuclear fusion reactions inside stars.

Stars like the Sun exist in a stable state by generating energy through internal nuclear fusion, which counteracts the star's own gravity. In particular, stars with more than about 10 times the mass of the Sun form an iron core at the final stage of their evolution. Since iron is the most stable of all atomic nuclei, it cannot produce energy through nuclear fusion. As a result, the star collapses under its own gravity. When this gravitational collapse makes the core's density sufficiently high, matter falling in from the outer layers bounces off the core, ultimately causing an explosion. This phenomenon is called a (core-collapse) supernova explosion. Supernova explosions scatter the elements created inside the star into space, and these elements become the "ingredients" that make up our bodies. Carl Sagan expressed this by saying, "We are made of star-stuff."

However, the mechanism of these supernova explosions has not yet been fully understood. In fact, current three-dimensional numerical simulations of supernovae have not been able to successfully reproduce the explosion. The key to triggering the explosion is thought to be a subatomic particle called the neutrino. The vast majority of the energy released by the star's gravitational collapse is carried away from the star's interior by the massive number of neutrinos produced there. Therefore, the essential question is whether neutrinos can transfer enough energy to the surrounding matter to cause an explosion.

Incidentally, these neutrinos are also constantly produced by the nuclear fusion reactions in the Sun, with more than ten trillion of them passing through a human body every second. However, because neutrinos interact very weakly with matter and pass through it almost completely, we do not perceive their existence in our daily lives. The very existence of such a "ghost particle" is a profound mystery. Even more mysteriously, neutrinos have a property called left-handed chirality, and no right-handed ones have ever been found. Most physical laws, such as Newtonian mechanics and electromagnetism, have left-right symmetry, meaning the same physical laws apply in a mirrored world. For some reason, however, neutrinos break this left-right symmetry with this property.

Recent theoretical research has revealed that the chirality of neutrinos may play a crucial role in supernova explosions. By breaking left-right symmetry, neutrinos enable a form of efficient, dissipationless energy transport within a supernova—a phenomenon not seen in everyday matter like air or metal. It is truly astonishing that "the properties of microscopic subatomic particles and macroscopic celestial phenomena, which exist on completely different scales, are so closely intertwined." Discovering such non-trivial relationships between the micro and the macro is one of the great joys of physics.

Will we be able to completely explain supernova explosions—the "final piece" of the puzzle of the origin of heavy elements—in the near future?