Dying stars may kick themselves through space, disrupting binary systems

Jim Fuller, a theoretical astrophysicist at Caltech, has spent months working through a deceptively simple question: what happens when a dying star sheds its outer layers? The conventional answer is that it fades away in an orderly fashion, gradually transforming into a white dwarf. But Fuller's calculations suggest something messier is actually happening—something that could tear apart distant stellar pairs or, in rare cases, send two stars hurtling toward each other.

The mechanism Fuller proposes hinges on asymmetry. As a red giant star nears the end of its life, blobs of gas erupt from its swollen surface in chaotic, uneven bursts. Each eruption sends material flying outward, and by Newton's third law, the star itself recoils in the opposite direction. Individually, these pushes are tiny—each one moves the star at only a few meters per second, roughly the pace of a slow jog. But they accumulate. Over several hundred thousand years, a dying star may experience roughly ten thousand of these small kicks, each one nudging it in a slightly different direction.

What makes this process consequential is that the kicks do not cancel each other out. Even though they occur randomly, they follow a mathematical pattern known as a random walk—the same principle that governs a coin flip deciding which way to step. Flip a coin enough times, and you will eventually end up some distance from where you started. Fuller's model suggests that after thousands of random kicks, a dying star could acquire a net velocity of about one kilometer per second, moving in a direction determined by the accumulated history of its chaotic eruptions.

The implications for binary star systems are significant. Kareem El-Badry, an assistant professor of astronomy at Caltech, had observed something puzzling: widely separated pairs of stars become noticeably less common after one member of the pair transforms into a white dwarf. The new model offers an explanation. If a dying star receives a kick of one kilometer per second, and the two stars in the binary are orbiting each other at a slower speed than that, the kick can be strong enough to break them apart gravitationally. The pair, once bound together, simply drifts away into separate trajectories through space.

Fuller developed his model by combining El-Badry's observational data with computer simulations of the churning convection that occurs inside aging red giants. Those simulations revealed that material near the surface does not escape in a smooth, balanced pattern. Instead, it flows away unevenly, creating the asymmetric eruptions that Fuller's theory requires. The work represents the first direct connection between many randomly directed ejection events and the motion that astronomers have long suspected white dwarfs receive.

The model also makes a striking prediction. In some binary systems, the accumulated kicks to a dying red giant could alter its orbit enough to send it crashing into its companion star. Such a collision would produce an explosion—a violent merger that leaves behind observable signatures. This prediction offers astronomers a way to test whether Fuller's model accurately describes the final stages of Sun-like stars. If researchers can identify these merger events in the sky, they will have evidence that dying stars are not quietly fading away, but rather being kicked through space by their own chaotic death throes.

Fuller presented his findings at the 248th meeting of the American Astronomical Society in Pasadena, and the study has been submitted to the Publications of the Astronomical Society of the Pacific. El-Badry, whose observations helped inspire the model, expressed satisfaction with having a physical explanation for a phenomenon that had puzzled him for years. The work was funded by Caltech.