In the scorching core of a star, two helium nuclei approach each other like two dancers on a crowded stage. Their dance is alpha scattering, the basis of stellar helium burning. How smoothly they twirl determines how quickly the star burns its fuel.
Previously, this dance was described in simplified terms. Now, physicists have for the first time used the most accurate description of nuclear forces — the so-called N3LO interaction from chiral effective field theory. The calculation was performed on a very fine spatial lattice using the adiabatic projection method. But a difficulty arose: the mathematical matrix describing the pair of particles became nearly singular, like a badly out-of-tune musical instrument on which it is impossible to play a clear note.
To fix the discord, the scientists applied Tikhonov regularization — a method that carefully adjusts the instrument, removing noise. The result was verified by an independent approach — truncated singular value decomposition. Both methods gave the same phase shifts for S- and D-waves, and they matched experimental data exactly. This means that the latest theory of nuclear forces works not only for describing stationary nuclei but also for their collisions.
Now astrophysicists have a reliable tool to peer into the very heart of a star and see exactly how elements heavier than helium are born. The alpha-particle dance is no longer a mystery.
The temperature in the core of a star during helium burning reaches 100 million degrees — six times hotter than the center of the Sun.
In Stanisław Lem's novel "Solaris," the ocean models extremely complex nuclear processes — perhaps the writer's intuition anticipated such calculations.