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

At first sight, 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, it’s “everything different from normal.”

A recent study of the star’s surface, published in the magazine Natural Astronomysays that we see Gamma Columbae in a short, very strange phase of a very eventful stellar life, one in which astronomers can look directly into the exposed heart of the star.

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

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

The surface of Gamma Columbae in particular has much more helium and nitrogen – compared to hydrogen, carbon and oxygen – than should be present on the surface of a star. These ratios resemble 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 ash,” and usually only a small amount of it is mixed in the star’s outer layers, thanks to the swirling convection currents. But the light spectrum from Gamma Columbae’s surface reveals too strong a signature to come from just a handful of core axes that are in (what should be) the star’s hydrogen-rich outer layers.

“To observe this at the surface of a star, you have to remove a lot of mass above these deep layers to expose the core of the star,” Meynet said. Inverse.

In other words, while 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,” Meynet and colleagues write.

Dive 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 younger years, Meynet and his colleagues estimated that it probably weighed about 12 times the mass of our sun. That’s based on the proportions of the chemical elements nitrogen, carbon and oxygen visible in the light from the surface, which “match nicely” with the expected composition of the core of a star with 12 solar masses, specifically one that contains all the hydrogen in the 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, such as Alpha Centauri A and B, or Tatooine’s twin suns. if you are a sci fi fan. When Gamma Columbae finished its hydrogen combustion phase, the outer layers would have expanded outwards (just like our sun will ever do). That swollen shell 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 that process probably took about 10,000 years, with the companion star stripping about 0.01 percent of our sun’s mass from Gamma Columbae each year, until only the star’s core remained, stripped bare.

Why it matters – All this makes Gamma Columbae extremely unusual. What happened to gamma Columbae doesn’t happen often, and the handful of examples known to astronomers are all much smaller stars, about the size of our sun. But Gamma Columbae is unusually large and bright; it’s even bright enough to see with the naked eye.

Astronomers also know of 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 blew away their own outer layers with powerful stellar winds. But there is no sign of that kind of stellar wind from Gamma Columbae. Apparently it’s in a class of its own.

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 that will only be for a few thousand years.

“The stage in which Gamma Columbae has been observed is a short life stage,” says Meynet. “So that’s why it’s very unique because it’s a short span of time. It’s evolving quickly now.”

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

But for now, it gives astronomers a rare chance to look directly into the heart of a star.

What’s next – To learn more about what’s happening in Gamma Columbae, Meynet and his colleagues point to a technique called asteroseismology: measuring tiny changes in light on a star’s surface and using that to tell things about its internal structure. to distract.

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

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

Depending on how much Gamma Columbae expanded and how closely the two stars orbited their shared center, they would have gone through what astrophysicists call a “common-enveloping phase.” That means the two stars orbited so closely, and Gamma Columbae swelled so far out, that the small companion star was actually in the outer layers of Gamma Columbae — feasting on the larger star from the inside.

If that’s what happened, then the mechanics of the whole system means the two stars would have gradually spiraling closer together – so close that it’s possible that Gamma Columbae has absorbed its smaller, enveloping partner. In the process, any material that doesn’t “eat” the smaller star would have been flung out of the galaxy by gravity or a brief gust of wind.

We told you this star was weird.

To find out if Gamma Columbae still has a companion star, astronomers could turn to a method commonly used to find exoplanets. By very accurately measuring how the star’s light changes over time, they were able to see the star wiggle a bit on its axis. This would mean that something in its orbit, such as an exoplanet or a small companion star, is being pulled slightly by the gravitational pull of something in its orbit.