A Star Sneezes and Drifts Away express
A balloon that lets out air in bursts zips around the room — the jet blasts first one way, then another. A dying star behaves the same way as it sheds its bloated shell. Only instead of air, it's superheated gas, and instead of a balloon, it's a future white dwarf (the dense stellar remnant about the size of Earth).
Recent observations have shown that white dwarfs receive a kick of about one kilometer per second. The authors of the new study built a simple model: the shell doesn't come off in a smooth flow, but in discrete chunks: each about one ten-thousandth of a solar mass at a time. Each such "sneeze" gives the star a weak shove — about five meters per second. But over the course of shedding the shell, about ten thousand such events occur, and their random directions add up to a net kick. This is like a drunkard's walk: each step is small, but in the end, he staggers far from the lamppost.
For more massive white dwarfs, the kick should be noticeably stronger. And if the star isn't alone but is part of a wide binary system (with a separation greater than a thousand astronomical units, i.e., a thousand times farther than Earth is from the Sun), then such shoves can tear the pair apart. The orbit stretches to the limit, and the stars either fly apart forever or collide. A collision produces a brilliant flash, a cosmic firework that can be spotted from Earth.
Original abstract
as written by the authors arXiv:2608.07455v1
Recent observations have found evidence that white dwarf (WD) stars receive a kick of $$\sim$$1 km/s as the envelopes of their red giant progenitors are expelled. We show that these kicks can arise from asymmetric and episodic mass loss of red giant stars. Based on simple hydrodynamic models of red giant mass loss, each mass ejection event likely expels of order $$\sim \! 10^{-4} \, M_\odot$$, imparting a small and randomly oriented kick to the star, with the net kick to the WD arising from a combination of many $$(N \sim 10^4)$$ events. Each kick is of order $$v_k \sim 5 \, {\rm m/s}$$, with a total kick of order $$v_{\rm k,tot} \approx \sqrt{N} v_k \sim 0.5 \, {\rm km/s}$$. We predict substantially larger net kicks for higher mass WDs. We also model the orbital evolution of binary stars experiencing a stochastic series of kicks, developing analytic models to explain numerical integrations. For nominal mass ejection parameters, widely separated binaries $$(a \gtrsim 10^3 \, {\rm AU})$$ can become unbound, with larger fractions of unbound systems at larger separations and for higher WD masses. The orbital eccentricities can also approach unity, causing stellar collisions that may result in luminous transients and eccentric common envelope events.
A white dwarf is a star the size of Earth but with a mass nearly that of the Sun. A teaspoon of its material would weigh several tons.
In Stanisław Lem's novel Solaris, the planet's ocean moves its orbit — here, the star itself gets a kick from its own "breathing."