As if the waves of ocean surf crashing on a sandy shore, huge plasma waves may crash against the surface of a massive star.
The star is part of a pair, stretched and pulled by its companion’s gravity. That gravitational tug-of-war causes the star’s brightness to change drastically and rhythmically. A computer simulation has shown that the steady pulse of starlight is caused by gravitational tug-of-war. is caused by giant tidal waves undulating and breaking on the star’s surfaceResearchers report August 10, Nature Astronomy. The waves can reach up to three-times the diameter of a sun.
“It’s quite rare to see these really kind of dramatic but transformative moments in action,” says astrophysicist Morgan MacLeod of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
This star system is located in the Large Magellanic Cloud, about 160,000 light years away from Earth. One visible star is 35 times as massive as the sun, and there’s another star at least 10 solar mass away. About once a month, as they orbit each other, they pass near enough that gravitational forces raise tides on both stars’ surfaces, scientists suspect, much the way the moon tugs on Earth’s oceans (SN: 4/5/11).
The tug on the stars would be much more intense. “Instead of being a few meters tall, [the tide] can be 10 percent of the diameter of the star” that’s visible, says astrophysicist Jim Fuller of Caltech, who was not part of the study. On a star as big as that visible star — about 24 times as wide as the sun — that corresponds to a tidal wave roughly 3.3 million kilometers tall.
The new study, Fuller says, “shows how complicated and interesting the dynamics get when you have an extreme system like this.”
Astronomers can’t see the shapes of these stars through a telescope. but they can track how the brighter star’s light changes over time. While the brightness of most known “heartbeat stars” changes by about a tenth of a percent, the brightness of this system changes by 20 percent.
MacLeod wanted a better understanding of the changes that are visible in this star system. He and Harvard astronomer Avi Löeb then simulated the movement of plasma on and between stars as they orbited one another.
The waves can get big enough that they actually break and crash across the brighter star’s surface, the study suggests. When an ocean wave is far from shore, it’s a rolling, undulating wave. It rises, then collapses as it approaches the shore. “Something kind of parallel is happening here,” MacLeod says. The top of the wave steepens, “gets out of phase with the bottom, and it folds over on itself, and it crashes.”
After it crashes on the stellar surface, he says, “debris that’s thrown off is fed into this atmosphere around the star,” like the foamy surf left behind on a beach. Energy is lost as the waves crash. That crashing, the study suggests, causes the stars’ orbits to shrink, meaning eventually these stars could collide and possibly merge.
