Boring Star Is Really a Stripped, Pulsating Core In the Sky, Scientists Say

Scientists think that a well-known bright star in the southern skies is secretly a “unique object” that has remained “hidden in plain sight” until now, a discovery that may offer an unprecedented glimpse into the mysterious innards of stars, reports a new study. 

Gamma (γ) Columbae, a star located about 870 light years from Earth, has long been classified as a typical massive star. Now, scientists have upended this view by suggesting that γ Columbae is the “stripped pulsating core of a massive star”—meaning that its atmosphere has been torn away to expose the nuclear fusion factory at the star’s center—according to a study published on Monday in Nature Astronomy.

The discovery of γ Columbae’s secret identity was “serendipitous,” said Norbert Przybilla, head of the Institute for Astro- and Particle Physics at the University of Innsbruck and a co-author of the study, in a call with Motherboard. “We were simply analyzing a large bunch of stars. From first sight, you wouldn't expect the star to be something special, but then, from more detailed analysis, it immediately became clear that it's something that we haven't seen so far.” 

“It's always surprising what you'll find if you look closer and closer,” he added.

Przybilla and his colleagues pointed to telltale signatures in γ Columbae’s light spectrum that indicate chemical abundances consistent with a star that has lost its outer atmospheric envelope. The team speculated that this gassy veil was likely tugged off by an unseen companion star that is close to γ Columbae, or that has possibly merged with it in recent years. 

As a result, γ Columbae has shrunk from a “normal” star that was about 12 times the mass of the Sun to an buck-naked core that is about five times as massive as the Sun. Astronomers can occasionally glimpse details about the cores of extremely massive stars in their late stages, called Wolf-Rayet stars, or the cores of “subdwarf” stars similar in scale to the Sun, but γ Columbae is the first exposed core in this mass range that has been spotted before. That makes the star “a unique testbed for stellar (binary) evolution, so far hidden in plain sight,” according to the study. 

“Having a naked stellar core of such a mass is unique so far,” Przybilla said, who called it an “oddball.” 

“We have through the Wolf-Rayet stars ideas about how the cores of very massive stars look, and through the subdwarf stars we know the the cores of low-mass stars look, but in the middle, in between, so far, we don't have much evidence,” he said. “This is the first step.”

Przybilla and his colleagues suggest that γ Columbae is currently going through a transitory phase of disequilibrium that will be incredibly brief, probably lasting just 10,000 years. Prior to this stage, γ Columbae was a regular massive star that had run out of hydrogen fusion in its core, prompting its outer gassy layers to expand and encompass a companion star in a common atmospheric envelope. Instabilities from this interaction then triggered the ejection of the envelope, and possibly a merger between the two stars. 

What remains is the incredibly hot center of the star, which is likely burning through helium at this point. The star will eventually regain its equilibrium by becoming an extremely hot core that will fuse heavier elements together for another million years or two, before ending its life in a dramatic type of stellar explosion called a stripped core-collapse supernovae. After that, the star will enter a long afterlife as a type of extremely dense remnant called a neutron star.

It is “very unique” to “find an object in this phase, because it only lasts for a few thousand years, probably—long for us humans but in astronomical timescales, very very short,” Przybilla said. “It will always stay as a peculiar object.” 

For this reason, γ Columbae offers an unparalleled window into the core forces that power stars, with the potential to unlock answers to a host of questions in astrophysics. In particular, the object can provide insights about the evolution of binary star systems, which can have more complicated lives than single-star systems, such as our solar system. Przybilla and his colleagues suggest that asteroseismology, which is the study of oscillations inside stars, would be especially helpful in probing the structure of the star.

“If you analyze earthquakes, you can look, really, into Earth, and how it is built on the inside, and this has to be done with γ Columbae,” Przybilla said. “I think we will get a very, very good idea how such a core looks on the inside.” 

“This is probably the most interesting factor of all, in terms of scientific outcome, because all the cores are hidden in the other stars and here we have a naked one, a stripped one, and that will leave a very particular signal in its pulsations,” he concluded. “We have to follow up on that.”