Astronomers believe they have worked out what’s going on with a peculiar class of variable binary stars known as "heartbeat stars". These objects' change in luminosity is due to the intense force between them that literally squishes them.

In a paper, published in the Astrophysical Journal, the authors explain how the stars orbit each other in an elongated elliptical orbit, which takes them as close as just a few stellar radii and up to 10 times as far away in one orbit. This peculiar stellar ballet makes the brightness of the system vary like an electrocardiogram, with each orbit representing the beat of a heart.

When the stars approach each other, the mutual gravitational attraction tugs on the stars, causing them to appear more and more egg-shaped. Their close encounters make the stars vibrate, or "ring", and their diameter rapidly changes with every orbit.   

"You can think about the stars as bells, and once every orbital revolution, when the stars reach their closest approach, it's as if they hit each other with a hammer," said lead author Avi Shporer, from NASA's Jet Propulsion Laboratory, in a statement.  

"One or both stars vibrate throughout their orbits, and when they get nearer to each other, it's as though they are ringing very loudly." 

The stars were discovered by planet-hunter telescope Kepler, which carefully and regularly analyzes the light from distant stars to discover transiting planets. These objects were flagged for their unusual light curve.

For the study, the authors measured the orbits of 19 heartbeat stars and the properties of the individual objects. They appear to all be hotter and bigger than our Sun, but the authors don’t deny that smaller heartbeat stars are yet to be discovered.

^{This artist's impression shows two "heartbeat stars" and the light curve that they produce. NASA/JPL-Caltech}

While the research is the largest study yet performed on these stars, there’s still a lot more that we need to understand about these objects. Their peculiar orbit is not stable so some other forces might be at play.

"The mere existence of heartbeat stars is a bit of a puzzle," added co-author Susan Mullally from NASA's Ames Research Center in Moffett Field.

"All the tidal stretching of these heartbeat stars should have quickly caused the system to evolve into a circular orbit. A third star in the system is one way to create the highly stretched-out, elliptical orbits we observe."

The team is now planning to look for evidence of third stars in these systems and, hopefully, will lead to an even deeper understanding of heartbeat stars.