Supersonic tsunamis 3 times as tall as our sun are breaking on a distant 'heartbreak star'

A massive stellar behemoth is experiencing violent waves three times as tall as our sun crashing down on its surface. Known as a "heartbeat star," the extreme object also periodically pulses in brightness as the gravity of a close companion stretches it into an oblate shape. 

In fact, this particular heartbeat star's immense waves are being raised by that unseen companion as it swoops by on a highly elliptical orbit every 32.8 days. Just like how the moon's gravity serves as the primary source of Earth’s tides by pulling our planet's oceans around with it, the gravity of this heartbeat star's companion whips up material from the stellar body, then drags it around at supersonic speeds to form titanic waves.

The binary star system, known as MACHO 80.7443.1718, resides 169,000 light-years from Earth in the Large Magellanic Cloud. It incorporates a giant, 35-solar-mass primary star and a smaller secondary companion. Though first recognized as having variable brightness in 1990, no other stars that pulsed in this fashion were detected until, one day, NASA's Kepler Space Telescope mission spotted a bunch. 

Because the shapes of the larger stars in these systems are distorted, they alternately show their wider and narrower sides to us, leading to brightness pulses mimicking a beating heart. That's why scientists aptly named the stellar breed "heartbeat stars" to begin with.

Related: Mysterious cosmic 'heartbeat' detected billions of light-years from Earth

Normally, heartbeat stars are known to fluctuate in brightness by 0.1% – but MACHO 80.7443.1718 has always been different. It experiences regular episodes every 32.8 days that see its brightness increase by 20%, or 200 times more than the fluctuations of typical heartbeat stars.

Now, thanks to computer modeling of gas dynamics on the surface of the massive primary star in this system, astrophysicists Morgan MacLeod and Avi Loeb of the Harvard–Smithsonian Center for Astrophysics have determined that MACHO 80.7443.1718 contains more of a "heartbreak" star as great waves of plasma whipped up by the companion's passage violently break over its gaseous surface, unleashing a torrent of energy.

"Each crash of the star’s towering tidal waves releases enough energy to disintegrate our entire planet several hundred times over," MacLeod said in a statement.

The waves are absolutely huge, rising about 4 million kilometers (2.5 million miles) above the primary star's surface. They form when the companion star reaches what's known as periastron, which is the closest point in its 32.8-day orbit around the primary star. That primary star is also huge, with a gargantuan radius of  16.7 million kilometers (10.4 million miles) or 24 times the radius of our sun. The outer layers of this bloated star are diffuse and more weakly held by gravity, making it easier for the gravitational tides to distort them.

Rather than lead with their crest, like ocean waves that surfers ride, these gigantic stellar waves lead with their trough, the stellar material riding high behind it, in order to conserve angular momentum (the momentum of something moving in a circle or loop). Once the waves reach a peak, they start losing cohesion and begin breaking, dumping their energy and leaving "a big foamy mess," MacLeod said.

Much of this "foamy mess" is found in an envelope of hot stellar matter wrapped around the primary star. Every time the companion reaches periastron, it plunges through the envelope and destroys it, only to see it rebuilt by the resulting tsunami. The envelope itself is spun up by the energy of the wave, and rapidly rotates once every 4.4 days. This rate is inferred by the periodicity of the star’s regular "heartbeats." Considering the sheer size of the star, this rotation rate is incredibly fast. For context, our sun rotates once every 27 days at its equator. 

MacLeod and Loeb see the heartbreak star as a natural evolution of close binaries, but the primary star's high mass appears to be exacerbating the situation. 

Over the long lives of a pair of solar-mass stars, the orbits of the two stars around each other gradually become circular, and calmer, ultimately ending the sequence of close approaches and tidal distortions. However, massive stars, akin to the primary in MACHO 80.7443.1718, have much shorter lives. 

For instance, MACHO 80.7443.1718  is just six million years old, and will explode as a supernova within the next few million years. In fact, it has already ceased hydrogen burning in its core and has progressed to helium fusion, with hydrogen burning continuing in its outer layers. 

That marks a tell-tale sign of impending stellar death as the star rapidly burns through its fuel, moving from hydrogen to helium, then to carbon, oxygen, neon and silicon down to a core of iron – layer by layer, like peeling an onion. At the iron core is where reactions stop, and beyond that is when the star explodes. 

In this case, the transition from hydrogen to helium burning serves to expand the star’s outer layers by a factor of two or three, bloating the star and making it easier for the companion to disrupt it. The primary star's huge expanse further amplifies those tidal interactions, resulting in increasingly large waves.

A recent survey of data taken by the Optical Gravitational Lensing Experiment (OGLE), which ordinarily watches for fluctuations in the brightness of stars as evidence for small gravitational lenses, has found 991 more heartbeat stars, including 512 in our Milky Way Galaxy's central bulge, 439 in the Large Magellanic Cloud and 40 in the Small Magellanic Cloud. 

Strikingly, about 20 also feature large fluctuations in brightness, though none quite as severe as MACHO 80.7443.1718. Nonetheless, it seems heartbeat stars are more common than astronomers realized, and MACHO 80.7443.1718 may just be the tip of the iceberg. 

"This heartbreak star could just be the first of a growing class of astronomical objects," MacLeod says. "We’re already planning a search for more heartbreak stars, looking for the glowing atmospheres flung off by their breaking waves."

A paper about this work was published on Aug. 10 in the journal Nature Astronomy