As stars similar to the Sun age, they expand and become red giants. During this stage, their outer layers are stirred up and gradually drift into space, while the dense cores left behind shrink into white dwarfs. This is the fate of most stars, so white dwarfs are one of the most common stellar remnants in the universe.
A new model by Jim Fuller, a theoretical astrophysicist at the California Institute of Technology, suggests that this transformation may be much less orderly than it appears. According to his calculations, the uneven bursts of escaping material could repeatedly push the dying star in different directions, giving the white dwarf thousands of small kicks before it fully forms.
Chaotic eruption pushes out dying star
“In this model, clumps of material are randomly ejected from the asymmetrically expanded surface of the star,” Fuller said. “And every time that happens, the star gets kicked a little bit in the opposite direction. As Newton said, for every action there is an equal and opposite reaction.”
Fuller presented his findings at the 248th meeting of the American Astronomical Society in Pasadena. This research was submitted to the Pacific Astronomical Society Publications.
According to Fuller’s calculations, a star approaching the white dwarf stage could experience around 10,000 small kicks over hundreds of thousands of years. Each push would move the star only a few meters per second. “This is slow for a human jogger,” he says.
Thousands of kicks pile up
The escaping material is shot in random directions, but the kicks do not completely cancel each other out. Over time, a general shift in one direction occurs through a mathematical process known as a random walk.
A simple comparison would be tossing a coin repeatedly to determine whether it moves in one direction or the other. Each step is random, but you’ll end up gaining some distance from where you started. According to Fuller’s model, the combination of kicks could cause the dying star to move about a kilometer per second in a random direction.
Some objects in space experience stronger pressures. For example, when a massive star explodes as a supernova, the explosion can send stellar debris flying in one direction at high speed. Astronomers have long suspected that white dwarfs also undergo shocks, but since these stars don’t explode, the process is much more gentle.
Why wide binary stars break up
Evidence for a white dwarf kick comes from research led by Kareem El-Badry, assistant professor of astronomy at the California Institute of Technology. He found that pairs of widely separated stars, known as binaries, become less common when one member of the pair becomes a white dwarf.
A new model offers a possible explanation. A net kick of about 1 kilometer per second could disrupt the orbits of a loosely bound pair of stars, potentially causing them to separate.
“If the binary star’s orbital velocity is less than its kick velocity, the wide binary star is freed from its gravitational constraints,” Fuller explains.
Fuller developed the model using El Badry’s observations and computer simulations of convection inside aging red giant stars. These simulations show that the stirred material near the surface can escape non-uniformly rather than flowing away in a smooth and balanced pattern.
This model is the first to directly link the many randomly directed ejection events to the motions that astronomers suspect white dwarfs undergo.
“I’m happy to see a physical model that can explain this observation that has puzzled me for several years,” El-Badri says.
Stella kicks can cause collisions
The model also makes new predictions. In some binary star systems, repeatedly kicking a dying red giant star can change its orbit and cause it to collide with its companion star. Such a collision can cause an explosion.
Astronomers may eventually be able to look for signs of these violent stellar mergers, providing a way to test whether Fuller’s model accurately describes the final stages of a Sun-like star.
The research, submitted titled “Kicking a white dwarf via transient mass ejection from a red giant star,” was funded by the California Institute of Technology.
