A Star's Death Throes Involves a Lot of Kicking

When stars like our Sun age, they puff up into red giants. Their bubbling outer mass gradually escapes into space, and their remaining cores contract into white dwarfs. Since most stars end their lives this way, the universe is teeming with white dwarfs.

A new study from Caltech's Jim Fuller, professor of theoretical astrophysics, proposes a new model of the final death throes of Sun-like stars that shows how escaping mass from the stars' surfaces leads to a series of little kicks.

"In this model, blobs of matter are chaotically being ejected from the surface of the bloated stars in an asymmetric fashion," Fuller says. "And every time that happens, the star gets a little kick in the opposite direction. Like Newton said, for every action there is an equal and opposite reaction."

Fuller presented the results at the 248th meeting of the American Astronomical Society meeting in Pasadena. The study has been submitted to the Publications of the Astronomical Society of the Pacific.

According to Fuller's calculations, these stars—which are in their final stages of life before they become white dwarfs—will experience on the order of about 10,000 little kicks over hundreds of thousands of years, with each kick sending them traveling at a pace of about a few meters per second. "That's a slow jogging pace for humans," he says.

Even though the ejected mass is coming off in random directions, the process will result in a total net push in one direction. In math, this is known as a random walk. For instance, if you kept flipping a coin to decide which direction to move, you would eventually be somewhere offset from your starting point. In the new study, Fuller shows that the star's final net movement would be in a random direction at roughly a kilometer a second.

Other objects in the cosmos are known to experience even bigger kicks; for example, when a massive star explodes in a supernova, the explosion will propel the stellar remnant in one direction. Researchers had previously suspected that white dwarfs also receive kicks, but their case is not as dramatic because the stars do not explode.

Compelling evidence that white dwarfs receive little kicks comes from Kareem El-Badry, assistant professor of astronomy at Caltech, who found that widely spaced pairs of stars, or binaries, become less common once one of the stars has aged into a white dwarf. The reason for this, according to the study, is that the white dwarfs experience a net kick on the order of about 1 kilometer per second, which breaks apart the orbiting pair of stars, decoupling them.

"If the orbital speed of the binaries is less than the kick speed, the wide binaries will become gravitationally unbound," Fuller explains.

Fuller based his models on El-Badry's work as well as other computer simulations showing how the gurgling convective material on a dying red giant star will be ejected in an asymmetrical fashion. His model is the first to connect randomly oriented mass ejection events to the suspected kicks experienced by white dwarfs.

"I am pleased to see a physical model that can explain this observation, which has puzzled me for several years," El-Badry says.

Fuller's model also predicts something new: In some cases, the kicks to a dying red giant star orbiting with a companion could cause the two stars to collide, triggering an explosion. In the future, other astronomers may be able to search for evidence for such violent stellar unions and help test Fuller's model.

The submitted study titled "White Dwarf Kicks via Episodic Mass Ejection from Red Giant Stars" was funded by Caltech.