Whirling dance of vibrating ‘heartbeat stars’

Stars that spin round each other in far-flung elliptical orbits – called “heartbeat stars” for the electrocardiogram-like brightness graph they produce – have been presented in a new paper by US astronomers. 

Using the High Resolution Echelle Spectrometer (HIRES) on the W.M. Keck Observatory telescope in Hawaii, Avi Shporer from NASA’s Jet Propulsion Laboratory, California Institute of Technology, and colleagues examined the wavelengths of 19 of these heartbeat stars and calculated the speed and shape of their orbit over 43 nights in 2015.

In The Astrophysical Journal, they report that tidal forces also cause heartbeat stars to vibrate, with the stars’ diameter rapidly fluctuating as they orbit each other. This effect is most noticeable at the point of closest approach.

The heartbeat stars had orbited each other from seven to 90 days. They also seem to be hotter and bigger than the sun. “But it is possible that there are others with different temperature ranges that we did not yet measure,” Shporer says.

It’s their elliptical orbits that astronomers are interested in. Not only are they natural laboratories for studying the gravitational effects of stars on each other, but they’re also a bit strange.

“All the tidal stretching of these heartbeat stars should have quickly caused the system to evolve into a circular orbit,” Susan Mullally, a SETI Institute scientist and co-author of the study, says. 

“A third star in the system is one way to create the highly stretched-out, elliptical orbits we observe.”

More Kepler observations could unveil these extraneous bodies, the researchers write. And the upcoming Transiting Exoplanet Survey Satellite (TESS) from NASA and the European Space Agency’s Planetary Transits and Oscillations of stars (PLATO) missions will, no doubt, uncover more stellar heartbeats.