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The Uncanny Sight of Waves Breaking on a Star

It starts with a slow and steady tick. Hans Zimmer’s “Mountains” begins playing, in the film Interstellar, when the astronauts land on a watery planet orbiting a black hole, dilating time. An hour there is seven years back on Earth. The ticks begin to ramp up, marking off not moments in time anymore but something approaching their spaceship. What one of the astronauts thinks are “mountains” is actually a wave. The pilot orders everyone back to the ship. “The second wave is coming!” he says, awestruck by the colossal otherworldly crest, a tidal spectacle courtesy of the black hole. “And we’re in the middle of a swell.”

Enormous waves in space aren’t only the stuff of science fiction anymore. A new paper in Nature Astronomy reports about a star that experiences tides resulting in waves of gas and plasma. A small star in a binary star system located in the Large Magellanic Cloud—a galaxy near the Milky Way—creates the waves when it passes by its larger companion star at the closest point in their orbital dance, called the periapse. The authors, Harvard astrophysicists Morgan MacLeod and Avi Loeb, write that, according to their simulations, the small star, with each periapse passage, “raises tidal waves so large” on its larger companion star, 35 times the mass of our sun, that they “break” on the larger star’s surface. This is, they say, the first time anyone has ever witnessed (if you count creating a simulation as a kind of “seeing”) a star rocking waves of such magnitude. “Each crash of the star’s towering tidal waves,” MacLeod said, “releases enough energy to disintegrate our entire planet several hundred times over.” 

When the waves break, they can propel massive shockwaves through the larger star. 

What piqued MacLeod’s curiosity about the binary star system MACHO 80.7443.1718 was how much it dimmed and brightened over time—far outside the norm for “heartbeat stars.” (If you plot the changing brightness of heartbeat stars on a graph, this “light curve” will resemble what a heartbeat looks like on an electrocardiogram: a spiky but regular line, basically.) The researchers surmise that what causes the dramatic shifts in MACHO 80.7443.1718’s brightness is “extreme, nonlinear distortion.” When the smaller star mangles the shape of the big star—by stretching and compressing it, and churning waves on it—it affects how much light heads toward us. 

The waves aren’t quite like the ones that break on our familiar shores. “Unlike a breaking ocean wave, where the crest leads the trough,” the researchers write, “the oscillating stellar fluid conserves angular momentum, which implies that the wave crests lag behind the troughs.” The high points don’t catch up to the low points because the fluid within the star keeps up its spinning motion. It’s just like when you spin around and then stop suddenly—you feel slightly dizzy. Similarly, in the larger star, the fluid keeps its spinning or rotational energy even as it moves around and creates those waves. It’s as if the stellar fluid remembers its spin, causing the wave crests to lag behind the troughs.

When the waves break, they can propel massive shockwaves through the larger star. This happens when waves fall faster than the speed of sound traveling on the star’s surface. “Where the velocity of this fallback exceeds the surface sound speed,” MacLeod and Loeb write, “shocks form.” It’s like what happens to the air when supersonic planes travel faster than the speed of sound—pressure waves combine and explode. Similarly, the shockwaves in the star come from the stellar fluid suddenly zooming faster than the speed at which disturbances can move through the star’s outer layers.

While MACHO 80.7443.1718 is “remarkable”—the researchers cheekily call it a “heartbreak” star—it’s apparently not entirely alone in the universe. A 2022 paper found that there’s “one thousand heartbeat stars in the galactic bulge and Magellanic Clouds,” and around 20 of them dim and brighten as much as MACHO 80.7443.1718. So, MacLeod and Loeb write, it’s “likely just the first of a growing class of objects.” 

Since MacLeod’s visualization of the star’s waves has no sound, I recommend watching it on the slowest playback speed while listening to Zimmer’s “Mountains.” It may help you appreciate the uncanny sight of star waves crashing in space. 

Credit: Morgan MacLeod / YouTube

Credit: Jennyni20 (Epic Music) / YouTube

Lead image: Nazarii_Neshcherenskyi / Shutterstock

  • Brian Gallagher

    Posted on August 17, 2023

    Brian Gallagher is an associate editor at Nautilus. Follow him on Twitter @bsgallagher.

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