First-of-Its-Kind Tear-Shaped Star That Pulsates on Just One Side Discovered

A never-before-seen star that is tear-shaped and pulsates on just one side has been discovered by astronomers. The star, named HD74423, sits 1,500 light-years away and is part of a binary system with a red dwarf, which researchers say is the cause of the warped shape and unusual pulsations.

Pulsating stars are a type of variable star that increase and decrease in size periodically, causing a change to their brightness due to physical changes within the star. Researchers believe some stars should pulsate in certain parts, along the strong magnetic fields, but until now, evidence of them has been lacking.

"We've known theoretically that stars like this should exist since the 1980s," Donald Kurtz, from the U.K.'s University of Central Lancashire, said in a statement. "I've been looking for a star like this for nearly 40 years and now we have finally found one."

Kurtz is co-author of a study published in Nature Astronomy announcing the discovery of the star. It was first identified by citizen scientists using publicly available data from NASA's Transiting Exoplanet Survey Satellite (TESS), which looks for planets beyond our solar system via the transit method. This is where a planet passing in front of its star blocks out some of the light at regular intervals. But to do this, you need to know the size and mass of the star.

The team behind the latest study does just that. They observe light variations in stars caused by sound waves in the stars. "This makes the stars very similar to musical instruments, so this is the real Music of the Spheres," Kurtz told Newsweek. "We can then 'see with sound'—just as a bat echo-locates to see its environment, or as technicians in a hospital can image the face of a fetus using ultrasound."

He said the amateur astronomers had noticed a light curve for HD74423 that was "out of the ordinary." They passed this information on to Saul Rappaport, from the Massachusetts Institute of Technology, who contacted another colleague for more insight. Eventually, the star landed in the lap of Kurtz, who had first predicted the existence of stars that pulsate along their strong magnetic fields four decades ago. He said that he knew immediately this was what HD74423 was doing.

"But it was odd beyond that," he said. "It seemed to be pulsating only on the side facing its companion." At this time, Jim Fuller, from California Institute of Technology, had been working on an idea to explain how pulsations would behave on tidally distorted stars, so joined the team in analyzing HD74423.

Artist impression of HD74423. The star is being distorted by the red dwarf it orbits. Gabriel Pérez Díaz IAC

While working on the star, they found another scientist, Simon Murphy, from the University of Sydney, had also noticed the star had some unusual chemical properties. They eventually teamed up to study it.

Using computer programs he had written, Kurtz was able to almost immediately confirm the star was pulsating along its tidal axis. "However, there were a few decades of preparation that made this go so fast," he added.

HD74423, which is about 1.7 times the mass of our sun, is part of a binary system with a red dwarf. The two stars orbit one another every 1.6 days, meaning they are close together, astronomically speaking. The gravitational pull exerted on each distorts them, making the larger, hotter, less compact HD74423 stretch out in a tear shape.

The team found that while the star was pulsating globally, on both sides, because it was being stretched out from the gravitational pull of the red dwarf, the density, pressure and gravity was far lower on one side. This meant the pulsations could grow to far larger amplitudes, Kurtz explained. "On the other side, where the gravity is stronger, and the star is not stretched out, the amplitude is so small that it is much harder to detect. So it does mostly look like the star is only pulsating on one side."

Kurtz said they have already identified another star that is pulsating on one side and are in the process of publishing a study on it: "It is known as a metallic-lined star and we understand in its case the radiation pressure—the pressure of light as it travels out of the star—levitates metals into the star's atmosphere making it peculiar." They have also found a third star, which they are still studying.

"We expect to find many more hidden in the TESS data," Rappaport said in a statement.

Giovanni Mirouh, a research fellow in stellar astrophysics at the U.K.'s University of Surrey, who was not involved in the research, told Newsweek the system identified by the team "is, at the moment, a puzzle."

He said: "Indeed, as binary stars are common—even close pulsating binaries—t is very likely we find more of these stars. Asteroseismology missions such as TESS... cover most of the sky and will let us know whether this star is an exception to our theory of stellar evolution. It will thus drive the effort in stellar population studies: to predict the number of HD74423-like systems, motivate theoretical studies of stellar wave propagation and to describe the tidal mechanism that tilts the propagation axis. "

This story has been updated to remove duplicated information and to include comments from Giovanni Mirouh.