The outer layers of most stars are like a pot of boiling water, with giant bubbles known as convection cells transporting energy and hot gases from deep inside the star to the surface. Bubbles on the Sun are as big as Texas, and they percolate to the surface and fall again over a period of a few minutes.
Astronomers have now measured both the size and cadence of bubbles on a star other than the Sun for the first time. The star, R Doradus, is a red giant roughly 350 times the Sun’s diameter. Its convection cells are dozens of times the size of the Sun itself, and they rise and fall over a period of a few weeks. These data should help theorists revise models of the final stages of life for the Sun and similar stars.
“This result isn’t a big shock, but it’s very pleasing,” said Christopher Sneden, an astronomer at the University of Texas at Austin who wasn’t involved in the study. “It’s especially interesting because they caught an ordinary, garden-variety star,” which began life with a mass similar to the Sun. “It’s not some exotic, ‘Betelgeuse is going to explode any minute folks, run for your lives!’ kind of star. And that’s what makes this study especially welcome.”
A Mighty Big Target
Researchers were studying how dying stars eject elements created through nuclear reactions in their cores into the interstellar medium. “We are all made of ‘stardust,’ and much of the material around us is made in stars,” said Wouter Vlemmings, an astronomer at Chalmers University of Technology in Sweden and the study’s lead author. “How this material is ejected from old stars to be incorporated into new stars and planets is still not completely clear.”
R Doradus was a primary target because it is relatively close—roughly 180 light-years away—and quite large. It spans about 3.3 astronomical units (1 astronomical unit is the average distance from Earth to the Sun), so if it replaced the Sun in our own solar system, it would engulf the four innermost planets, including Earth. The combination makes R Doradus one of the largest stars in the sky.
R Doradus is so big because it has evolved into an asymptotic giant branch star—the final and most impressive red giant phase—as nuclear fusion shut down in its core. Today, R Doradus is fusing hydrogen and helium in thin shells around the core. Radiation from these reactions pushes on the surrounding layers of gas, inflating the star.
Despite its great expanse, R Doradus is only slightly less massive than the Sun. It probably began life no more than 1.25 times the Sun’s mass, so it gives us a preview of what the Sun will look like when it enters its own red giant phase in about 5 billion years.
The astronomers observed R Doradus with ALMA (Atacama Large Millimeter/submillimeter Array), a telescope high in the Andes Mountains of Chile, during five sessions spaced roughly 1 week apart in July and August 2023. ALMA’s high resolution allowed the team to map motions at the star’s millimeter “surface.” (Optical and shorter wavelengths can see slightly deeper into a star’s atmosphere, so “there is not a single surface that can be defined,” Vlemmings said.)
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The observations revealed convection features up to 0.72 astronomical unit in diameter—more than 75 times the diameter of the Sun. “The size corresponds well to the expectation of convection cells on these types of stars, so that’s what we think we’re seeing,” Vlemmings said.
Bubbling Faster Than Expected
Though previous studies have imaged convection cells on a few large stars—mainly red supergiants such as Antares, which are much more massive—the R Doradus study was the first to measure their velocity. It showed that the cells were moving faster than expected, bubbling up and falling away on timescales of 3 weeks to a month.
“This is the first time the timescales of convection can be measured [anywhere] except for the Sun,” Vlemmings said. “But based on theoretical extrapolations from the Sun, we were expecting the timescales to be 3 to 4 times longer.”
So far, the astronomers have no explanation for the discrepancy. They hope to learn more, however, through continued analysis of the R Doradus observations and those already made of two other stars, along with future observations of a few other target stars.
“I’d love to see this kind of observation for another star,” Sneden said. “If you see this behavior in a second star, then you know you’re on track, you know what you’re doing, and you can force the modelers to address this issue.”
Additional modeling could add insights into the next stage for R Doradus and similar stars—a planetary nebula. In this phase, the star will expel its outer layers, briefly enveloping itself in a colorful shell of expanding gas and dust. That will leave behind only a now-dead core, a white dwarf—the fate that awaits the Sun after perhaps a billion years as a red giant.
“One part of me wants to ask if this is the start of a planetary nebula,” Sneden said. “One of the standard ways people wave their hands about planetary nebulae is that there are certain big ‘burps’ from the interior, which eventually puff out even more.” “Eventually” could be millions of years in the future, Sneden said, “but millions of years is short for a star.”
This article originally appeared in Eos Magazine.
