Astronomers have discovered a truly unique star. The object, known as HD 74423, is a pulsating star. That taken by itself is nothing too unusual – plenty of stars have big and small pulsations. What's peculiar about this one is that it only pulsates on one side.

As reported in Nature Astronomy, the star has tidally trapped pulsations, throbbing observed largely only on one hemisphere due to the presence of a close stellar companion of HD 74423. A red dwarf orbits the star every 39 hours and is so close to the star that its gravity has changed the shape of the star from a sphere to a teardrop, trapping the pulsations to a single half of the star.

“We’ve known theoretically that stars like this should exist since the 1980s,” co-author Professor Don Kurtz, from the University of Central Lancashire in Britain, said in a statement. “I’ve been looking for a star like this for nearly 40 years and now we have finally found one.”

The discovery was possible thanks to citizen scientists who scoured data from NASA’s TESS satellites for stars with tiny variations in their light, which is often (although not always) an indication of planets.

“What first caught my attention was the fact it was a chemically strange star,” said co-author Dr Simon Murphy, from the Sydney Institute for Astronomy at the University of Sydney. “Stars like this are usually fairly rich with metals – but this is metal poor, making it a rare type of hot star.”

The international team believes that many more stars like this one exist out there.

“The pulsation mode in HD 74423 is currently unique, but there must be a class of such stars that have their pulsation axes aligned with their tidal axes, and this discovery is an impetus to search for more,” wrote the team wrote in their paper.

HD 74423 is about 1.7 times heavier than the Sun and is located 1,500 light-years away. More observations of this object will hopefully clarify whether or not the hemisphere where the pulsations are trapped is facing its companion or pointing away from it.

“We expect to find many more hidden in the TESS data,” added co-author Professor Saul Rappaport from the Massachusetts Institute of Technology.