Teardrop-Shaped Star Only Pulses on One Side, Making It the First of Its Kind

A close binary star distorts its pulsation patterns.
Fabienne Lang
Artist's illustration of the teardrop-shaped star next to the red dwarf starGabriel Pérez Díaz/IAC

When you look up at the night sky, you'll notice that stars twinkle. That twinkle is in fact pulsations, as stars tend to pulsate. This is usually the same for all stars, however, amateur astronomers have recently discovered a star that pulsates only on one side. 

Referred to by the rather un-twinkling name of HD74423, the one-sided pulsating star has taken on a teardrop shape because of a close companion pulling it out of its regular pattern. 

The findings were published in Nature Astronomy on Monday.

A one-sided twinkle

The star in question is around 1.7 times the mass of the Sun and is located about 1,500 light-years away from Earth. But the star isn't all on its own out there, it's accompanied by a red dwarf star. These two stars orbit each other in the short span of two Earth days.

It is precisely because of this gravitational pull as they orbit each other that HD74423 is pulled into a teardrop shape and pulses just on one side. This is what is making the star pulsate in this extraordinary manner. 

"Stars that pulsate have been known in astronomy for a long time," said Zhao Guo, an author of the study. "The rhythmic pulsations of the stellar surface occur in young and in old stars, can have long or short periods, a wide range of strengths, and different causes. There is however one thing that, until now, all of these stars had in common: The oscillations were always visible on all sides of the star."

However, with HD74423 that's not quite the case as it only pulsates on one side. The amateur astronomers discovered this by closely analyzing data from the planet-hunting TESS satellite. 


"What first caught my attention was the fact it was a chemically peculiar star," said Simon Murphy, study co-author and postdoctoral researcher 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."

An animation showing the pulsation patterns of the teardrop-shaped star HD74423
An animation showing the star pulsating just on one side, next to the red dwarf star. Source: Gabriel Pérez Díaz/IAC

Don Kurtz, study co-author and inaugural Hunstead Distinguished Visitor at the University of Sydney from the University of Central Lancashire in Britain explained "We've known theoretically that stars like this should exist since the 1980s. I've been looking for a star like this for nearly 40 years and now we have finally found one."

It's an exciting time for astronomers, as this is most likely not the last time that this phenomenon will occur.


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