© 2024 Nash Cruz

For as long as humans have been on Earth, we have looked up at (almost) the same sky and wondered whether the goings-on up above might mirror the events happening down below. Scientists on the OGLE project - an astronomical endeavour started by the University of Warsaw in 1992 – noticed that the brightness of certain stars fluctuated rhythmically over time. When they plotted the light output of these stars on a graph, they noticed how much they resembled heartbeats on an electrocardiogram.

Electrocardiogram: A test used to record the electrical activity of a beating heart, like its rate and rhythm.

We now know that this starlight ‘heartbeat’ comes from pairs of stars orbiting one another, with the distance between them changing drastically as they complete an orbit. When the stars are far apart, they possess a spherical shape.  When the partner stars are close together, gravity causes them to stretch and adopt ellipsoidal shapes, changing their brightness as seen from lightyears away. Since the OGLE project was first started, dozens more heartbeat stars have been discovered by NASA’s Kepler Space Telescope which was launched in 2009.

Massive astrophysical compact halo object 80.7443.1718 - or MACHO 80.7443.1718 for short – is the biggest heartbeat star discovered to date, as published in Nature Astronomy in August of 2023. MACHO 80.7443.1718 sits in the Large Magellanic Cloud 160,000 lightyears away from Earth and is 35 times heavier than the sun we know and love. Its partner star, while much smaller, is still at least 10 times the mass of our sun.

Scientists Morgan MacLeod and Avi Loeb from the Harvard-Smithsonian Center for Astrophysics used a computer simulation to model these stars orbiting one another. Once again, astronomers observed a phenomenon that looked a little familiar. Every month, as the partner stars close their gap, their interacting gravitational fields raise enormous tides of plasma on their surfaces – much like the pull of the moon controls the tides on Earth. These waves are so big that they fold in on themselves and collapse towards the center of the star, just like tall ocean waves prematurely break before reaching the shore. However, the plasma waves undulating off MACHO 80.7443.1718 are 100 million times taller than the biggest ocean waves ever recorded on our home planet. The huge tidal waves of plasma MacLeod and Loeb modelled explained some of the strange behaviour they observed from MACHO 80.7443.1718 and its partner. Normally, the brightness of heartbeat stars changes by 0.1% as they complete their paired dance, but MACHO 80.7443.1718’s glow changes by a whopping 20%. Such a high-energy system cannot last forever. Every successive orbit and crashing of these plasma waves causes energy to be lost. As the stars’ orbits are shrinking, it is very possible that they will eventually collide and merge into a single celestial body.

Journal references

M. MacLeod and A. Loeb. Breaking waves on the surface of the heartbeat star MACHO 80.7443.1718. Nature Astronomy. Published online August 10, 2023. doi: 10.1038/s41550-023-02036-3.