Kicks of the White Dwarf

As Sun‑like stars near the end of their lives, they expand into red giants before evolving into white dwarfs. During the red giant phase, material from the star’s outer shell is expelled in random directions. Each ejection imparts a small recoil, or “kick,” to the star in the opposite direction.

Red Giant Star (Image Credit: Wikipedia)

(Image Credit: Jim Fuller/Caltech)

This phenomenon was proposed over forty years ago as a possible explanation for the unexpectedly low number of white dwarfs observed in open clusters. Michael Fellhauer at the University of California, Santa Cruz, showed that if enough of these small kicks occur, white dwarfs could be nudged out of their host clusters. Supporting evidence came in 2007, when astronomers using the Hubble Space Telescope discovered young white dwarfs at the edge of the globular cluster NGC 6397. This was surprising, since heavier, younger white dwarfs were expected to remain near the cluster’s center. Their presence at the outskirts strengthened the case that white dwarfs receive kicks during the red giant stage.

Credit: NASA, ESA, and H. Richer (University of British Columbia)

Jim Fuller, professor of theoretical astrophysics at Caltech, developed a model explaining how these kicks occur as matter is ejected. His work, presented at the 248th meeting of the American Astronomical Society in Pasadena, suggests that a star may experience up to 10,000 small kicks over hundreds of thousands of years, each randomly altering its position. The model also predicts that in binary systems, repeated kicks can shrink the orbital separation, potentially driving the stars into collision and triggering a catastrophic explosion. Future observations may provide direct physical evidence of this process.

Computer simulation of a red giant star near the end of its life. Color shading shows density, with white showing the high-density stellar core, and dim orange showing the stellar wind. The plume of material to the bottom right is a mass ejection event that imparts a kick to the star.

Credit: Bernd Freytag, Uppsala University.

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