Astronomers get a rare glimpse of a star's exposed core

Astronomers get a rare glimpse of a star’s exposed core

At first glance, the star Gamma Columbae – a bright blue point of light about 870 light-years away in the southern hemisphere constellation Columba – resembles your average celestial body. But according to a team of astrophysicists, this is “anything but normal”.

A recent study of the star’s surface, published in the journal natural astronomysays we see Gamma Columbae in a short and profoundly strange phase of very turbulent stellar life, which allows astronomers to look directly into the star’s exposed core.

What’s new – The mixture of chemical elements on the surface of Gamma Columbae resembles the by-products of nuclear reactions that should be buried in the depths of a massive star, without bubbling up 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 – just like when light shines through a prism and we see a rainbow. Each molecule absorbs and emits light at different wavelengths, so looking at an object’s light spectrum 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, the surface of Gamma Columbae contains far more helium and nitrogen – relative to hydrogen, carbon and oxygen – than there should be on the surface of a star. These ratios resemble the mixture of elements left behind by nuclear reactions in the core of a massive star, in which certain isotopes of carbon, nitrogen and oxygen play a role in reactions that fuse hydrogen atoms together. in helium.

Meynet and his colleagues describe this material as “nuclear ash”, and usually only a small part mixes with the outer layers of the star, thanks to convection currents. But the spectrum of light from Gamma Columbae’s surface reveals too strong a signature to come from a handful of nuclear ash stirred up in (what should be) the star’s hydrogen-rich outer layers.

“In order to observe this on the surface of a star, you have to remove a lot of mass above those deep layers, to uncover the core of the star,” Meynet explains. Reverse.

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-down, pulsating core of a previously much more massive star.” , write Meynet and his colleagues.

Dig into the details – Right now, Gamma Columbae is about four or five times the mass of our Sun, so it’s still not exactly small. But in its youth, Meynet and his colleagues estimate that it probably weighed about twelve times the mass of our Sun. This is based on the ratios of the chemical elements nitrogen, carbon, and oxygen visible in light from its surface, which “perfectly match” the expected composition of the core of a twelve-solar-mass star, especially one that has burned up all the way. hydrogen in its core and is ready to switch to burning helium.

So what happened?

This illustration shows a star stealing mass from its companion, in a process similar to that which left Gamma Columbae’s core exposed.MARK GARLICK/SCIENTIFIC PHOTO LIBRARY/Science Photo Library/Getty Images

The explanation that best fits the observations, according to Meynet and 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 completed its hydrogen burning phase, its outer layers expanded outward (just as our Sun will one day). This bloated envelope of gas and plasma fell prey to the gravitational pull of a smaller companion star, perhaps about three times the mass of our Sun.

Meynet says this process probably took about 10,000 years, with the companion star removing about 0.01% of the mass from our Gamma Columbae Sun each year, until only the core of the Gamma Columbae was left. star, laid bare.

Why is it important – It all adds up to make Gamma Columbae extremely unusual. What happened to gamma Columbae doesn’t happen often, and the few examples astronomers know of are all much smaller stars, the size of our Sun. But Gamma Columbae is exceptionally large and bright; it’s bright enough to be seen with the naked eye, in fact.

Astronomers are also familiar with another strange 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 destroyed their own outer layers with powerful stellar winds. But there is no sign of this kind of stellar wind coming from Gamma Columbae. Apparently it’s in a class of its own.

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

“The phase in which Gamma Columbae was observed is a short phase of its life,” explains Meynet. “That’s why it’s very unique because it’s a short time scale. It’s moving fast now.

First, the nucleus will contract, falling inward under its own weight, until the pressure at its center is sufficient to start the process of fusing the helium atoms. At this point, Gamma Columbae will become an even brighter and hotter blue star, with likely another 2 million years to live before it dies in a spectacular supernova.

For now, however, it gives astronomers a rare chance to look directly at a star’s core.

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

“Asteroseismology is an extraordinary technique for probing the physics of star interiors,” Meynet explains.

Researchers also hope to learn more about the fate of Gamma Columbae’s hungry little companion. It may be that the light from the smaller star is simply 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 the expansion of Gamma Columbae and how close the two stars are around their common center point, they could have gone through what astrophysicists call a “common envelope phase.” This means that the two stars were orbiting so close to each other and Gamma Columbae was bulging outward so much that the smaller companion star was actually inside the outermost layers of Gamma Columbae – feasting on the biggest star in the interior.

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

We told you that star was weird.

To find out if Gamma Columbae still has a companion star, astronomers could turn to a method often used to find exoplanets. By very precisely measuring the evolution of the star’s light over time, they were able to see the star oscillate slightly back and forth on its axis. That would mean it’s being pulled slightly by the gravity of something in its orbit, like an exoplanet or a small companion star.

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