Breaking waves on a stellar shore

For the first time a team identifies breaking, gaseous waves on a stellar surface.
Published in Astronomy
Breaking waves on a stellar shore
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This is a story about tides and waves, but not in the context we usually think about them. 

I grew up on an island on the Maine coast, watching the ocean wash in and out. Now, I trace these familiar phenomena out to distant stellar systems, which has been immensely satisfying — and immensely complicated. 

Together with my collaborator Abraham Loeb, I recently published a paper in Nature Astronomy that describes stellar activity that’s never been seen before: waves of gas breaking on the surface of a star each time it passes close to an orbiting companion. But to understand this feature of outer space — on a star conveniently named MACHO 80.7443.1718 — it helps to start with how tides and waves affect Earth.

Ocean tides on Earth occur because of the gravitational pull of the moon. The side of the Earth facing toward the moon bulges out a bit, because the moon’s gravitational pull there is stronger than it is at Earth’s center. 

The side away from the moon bulges the opposite direction because the gravitational pull of the moon is weaker than at Earth’s center. But all things considered, the amplitude of our tides is actually quite small. They’re typically a meter or so in range, compared to Earth’s 6.4 million-meter radius. Over the course of a day, these tides cause a variation of just  0.000016% in the height of Earth’s oceans. 

The gravity of Earth's moon causes tidal bulges in our liquid oceans -- one facing toward the moon and one away.

Tides can also be raised in gas, just as they are in the liquid of the oceans And in fact, Earth’s gaseous atmosphere oscillates much like the oceans in response to the moon’s gravity. 

Stars also experience tides when they have close-orbiting companions, like planets or other stars. This is more common than we might imagine: Although our sun is a solitary star, over half of stars actually lie in close pairs, known as binary stars, in which the two orbit around each other. If these stars pass close enough together, each one’s gravity can distort the other and create tides.

Certain pairs of these stars have become known as “heartbeat” stars. These systems contain pairs of stars that orbit past each other quite closely. And during each pass, each star’s huge gravitational forces create extreme tides on the other — so big, in fact, that they fly away “ringing,” as their shapes wobble and shift around.  

Those wobbles affect the amount of light each star shines toward us here on Earth at a given time, and graphs of their brightness over time look a lot like a heart monitor’s output  — earning them their name.  

Astronomers using data from NASA’s Kepler satellite discovered sun-like stars in this binary arrangement with close companions. And the amplitudes of their tides are typically on the order of 0.1% — 6,000 times larger than Earth’s oceanic tides but still a small change in the stars’ overall shape. 

More recently, in 2019, scientists announced that they’d found a “massive” heartbeat system in a nearby galaxy, with one star that’s 35 times the size of the sun and extreme tides in the range of 20% of the star’s normal shape. That star is MACHO 80.7443.1718, the subject of my new paper. 

As scientists studied MACHO 80.7443.1718, an enigmatic picture emerged. The star is spinning so fast that it flattens itself, such that its equator extends 50% wider than its poles. It is surrounded by an extended atmosphere of glowing and fast rotating gas — think spinning pizza crust flinging off chunks of cheese and sauce —  and its month-long orbit around its companion star period is shrinking by nearly 11 seconds every year. 

I’ve been interested in this star since its initial discovery in 2019. I was particularly curious how the star could support such extreme tides without breaking apart — and where its rapid spin and rotating gas disk might come from. 

Much of my own work uses computer simulations to understand astronomical objects like stars, and the way that gasses behave in them. 

This work is a much-needed supplement to observations in astronomy, like looking at stars with powerful telescopes. After all, we can’t  send two stars flying by each other in a laboratory as we stand by and watch from close range. 

The next best thing is to try to recreate these dynamics in a computer model, and use what we learn to better understand what we can observe in the sky. Given that background, I set out to build a model of the tides in the MACHO 80.7443.1718. 

After much trial and error, my model showed that the rise and fall of this star’s huge tides happens fast — over just a few days of each month-long orbit around its companion star. This is so fast that the tides form enormous waves that actually break as they travel around MACHO 80.7443.1718. In turn, the smooth motion of each wave crashes back into itself and devolves into the disordered motion of turbulence, much like the foamy mess that forms as Earth’s ocean waters crash back into the shore.

Back on MACHO 80.7443.1718, our simulations show that the star’s huge tides are marked by supersonic flow of gas that gets in each tidal wave. Shock waves also occur where the supersonically-moving wave buckles on itself and collapses. These shock waves transform some of the energy contained in the wave into heat, much like the foamy crest of a breaking wave at the beach. 

MACHO 80.7443.1718’s rotations fling these heated debris from the breaking waves into an extended, glowing atmosphere around the star. And as each wave crashes down on the star, it spins the surface faster and faster until its bulging middle is flattened and expanded to the point where the star’s own gravity can barely hold it. 

Each orbit repeats this process of raising a new tidal wave, its collapse, and shock-heating that supplies a new atmosphere around the star. With each breaking wave, energy dissipates and then radiates away from the star’s extended atmosphere. The brightness of this atmosphere is nearly 100 times the Sun’s own power output, implying that each breaking tidal wave releases nearly 1,000 times all the gravitational energy that holds Earth together. And, the energy released by this process matches the stars’ decaying orbit — showing that the system’s orbit is decaying because of the energy released by the breaking tidal waves. 

As a heartbeat star with breaking waves, we call MACHO 80.7443.1718 the first of a new category of “heartbreak” stars. There should be many more, especially among massive heartbeat stars, and studying them in detail will open a new window to the physics of how tides work,  and how they transform stellar and planetary systems.

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Astronomy, Cosmology and Space Sciences
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences

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