Astronomers get a rare glimpse of the exposed core of a star

At first glance, the star Gamma Columbae — a bright blue point of light about 870 light-years away in the southern hemisphere constellation Columba — looks seems like your average celestial body. But according to a team of astrophysicists, it’s “anything else but normal.”

A recent study of the star’s surface, published in the journal Nature Astronomy, says that we’re seeing Gamma Columbae in a short, deeply weird phase of a very eventful stellar life, one that lets astronomers look directly into the star’s exposed heart.

What’s New – The mix of chemical elements on the surface of Gamma Columbae look like the byproducts of nuclear reactions that should be buried in the depths of a massive star, not bubbling on its surface.

University of Geneva astrophysicist Georges Meynet and his colleagues observed light from the star, which had been split into the individual wavelengths that make it up — exactly like when light shines through a prism, and we see a rainbow. Each molecule absorbs and emits light at different wavelengths, so looking at the spectrum of light from an object can reveal what it’s made of. Astronomers had never studied Gamma Columbae’s surface composition in detail before, and what Meynet and his colleagues saw surprised them.

In particular, Gamma Columbae’s surface boasts much more helium and nitrogen —compared to hydrogen, carbon, and oxygen — than should be present on the surface of a star. These ratios look like the mix of elements left over from nuclear reactions in the heart of a massive star, in which certain isotopes of carbon, nitrogen, and oxygen play a role in the reactions that fuse hydrogen atoms into helium.

Meynet and his colleagues describe that material as “nuclear ashes,” and usually only a little bit of it gets mixed into the star’s outer layers, thanks to the churning currents of convection. But the spectrum of light from Gamma Columbae’s surface reveals too strong a signature to be from just a handful of nuclear ashes stirred into (what should be) the star’s hydrogen-rich outer layers.

“In order to observe this at the surface of a star, you need to remove a lot of mass above these deep layers, to uncover the core of the star,” Meynet tells Inverse.

In other words, although Gamma Columbae looks like a typical bright main-sequence star (about as normal as it gets), it’s actually “the stripped, pulsating core of a previously much more massive star,” write Meynet and his colleagues.

Digging Into The Details – At the moment, Gamma Columbae is about four or five times the mass of our Sun, so it’s still not exactly small. But in its younger years, Meynet and his colleagues estimate that it probably weighed in at around twelve times the mass of our Sun. That’s based on the ratios of the chemical elements nitrogen, carbon, and oxygen visible in the light from its surface, which “nicely match” the expected makeup of the core of a twelve solar-mass star, specifically, one that’s burned up all the hydrogen in its core and is ready to transition to burning helium.

So what happened?

The explanation that best fits the observations, according to Meynet and his colleagues, is that Gamma Columbae is, or was, part of a binary star system: Two stars orbiting a common center of gravity, like Alpha Centauri A and B, or the twin suns of Tatooine if you’re a sci-fi fan. When Gamma Columbae finished its hydrogen-burning phase, its outer layers would have expanded outward (just like our Sun will do one day). That swollen envelope of gas and plasma fell prey to the gravitational pull of a smaller companion star, perhaps around three times the mass of our Sun.

Meynet says that process probably took about 10,000 years, with the companion star pulling away about 0.01 percent of the mass of our Sun from Gamma Columbae every year, until all that was left was the core of the star, stripped bare.

Why It Matters – All of that adds up to make Gamma Columbae extremely unusual. What happened to gamma Columbae doesn’t happen often, and the handful of examples that astronomers know of are all much smaller stars, about the size of our Sun. But Gamma Columbae is unusually large and bright; it’s bright enough to see with the unaided eye, in fact.

Astronomers also know of another weird group of stars called Wolf-Rayet stars. These stars were once much, much larger than Gamma Columbae, about 60 times the mass of our Sun. They blasted away their own outer layers with powerful stellar winds. But there’s no sign of that kind of stellar wind coming from Gamma Columbae. Apparently, it’s in a class by itself.

And it’s a blink-and-you-miss-it phenomenon, at least in astronomical terms. Right now, we see Gamma Columbae as the exposed core of a hydrogen-burning star, but it’s only going to be that way for another few thousand years.

“The phase in which Gamma Columbae has been observed is a short phase of its life,” says Meynet. “So that is why it’s very unique because it’s a short timescale. It is rapidly evolving now.”

First, the core will contract, falling inward under its own weight, until the pressure at its center is enough to kickstart the process of fusing helium atoms together. At that point, Gamma Columbae will become an even brighter, hotter blue star, with probably another 2 million years left to live before it dies in a spectacular supernova.

For now, though, it gives astronomers a rare chance to look directly at the heart of a star.

What’s Next – To learn more about what’s going on inside Gamma Columbae, Meynet and his colleagues point to a technique called asteroseismology: measuring small changes in the light on a star’s surface and using that to infer things about its internal structure.

“Asteroseismology is an extraordinary technique to probe the physics of the interior of stars,” says Meynet.

The researchers also hope to learn more about the fate of Gamma Columbae’s small, hungry companion. It may be that the smaller star’s light is just lost in the bright glow of gamma Columbae, but it’s also possible that the two stars merged at some point in their history.

Depending on how much Gamma Columbae expanded, and how closely the two stars orbited their shared center point, they could have gone through what astrophysicists call a “common-envelope phase.” That means the two stars orbited each other so closely, and Gamma Columbae swelled outward so far, that the little companion star was actually inside Gamma Columbae’s outermost layers — feasting on the larger star from the inside.

If that’s what happened, then the mechanics of the whole system mean that the two stars would gradually have spiraled closer to each other — so close that it’s possible that Gamma Columbae may actually have absorbed its smaller, envelope-thieving partner. In the process, whatever material the smaller star didn’t “eat” would have been tossed out of the star system by gravity or a brief gust of stellar wind.

We told you this star was weird.

To uncover whether Gamma Columbae still has a companion star, astronomers could turn to a method that’s often used to find exoplanets. By very precisely measuring how the star’s light changes over time, they could see the star wobbling slightly back and forth on its axis. This would mean it’s being tugged ever so slightly by the gravity of something in its orbit, like an exoplanet or a small companion star.