For the first time, astronomers have obtained sufficiently detailed images of a star other than the Sun to track the movement of boiling gas on its surface.
The images of the star R Doradus were captured with the Atacama Large Millimeter/submillimeter Array (ALMA), a telescope owned by the European Southern Observatory (ESO), in July and August 2023. They show giant, hot gas bubbles, 75 times larger than the Sun, appearing on the surface and sinking back into the star faster than expected.
"This is the first time that the bubbling surface of a real star can be shown in this way"
(1), says Wouter Vlemmings, professor at Chalmers University of Technology, Sweden, and lead author of the study published today in the journal Nature. "We didn't expect the data to be of such quality that we could see so many details of the convection on the star's surface."
Stars produce energy in their core through nuclear fusion. This energy can be transported to the surface of the star in the form of enormous hot gas bubbles, which then cool and sink—just like a lava lamp. This mixing motion, called convection, distributes the heavy elements formed in the core, such as carbon and nitrogen, throughout the star. It is also thought to drive stellar winds that carry these elements into the cosmos to build new stars and planets.
Until now, convection movements had never been observed in detail in stars other than the Sun. Using ALMA, the team was able to capture high-resolution images of R Doradus's surface over the course of a month.
R Doradus is a red giant star, about 350 times the diameter of the Sun, located about 180 light-years from Earth in the constellation Dorado. Its large size and proximity to Earth make it an ideal target for detailed observations. Additionally, its mass is similar to that of the Sun, which means R Doradus is likely close to how our Sun will appear in five billion years, after it becomes a red giant.
"Convection is what produces the beautiful granular structure seen on the surface of our Sun, but it's difficult to observe in other stars," adds Theo Khouri, researcher at Chalmers and co-author of the study. "Thanks to ALMA, we were not only able to directly observe convective granules—75 times the size of those on our Sun!—but also measure, for the first time, how fast they are moving."
The granules on R Doradus appear to move in a cycle of about a month, which is faster than scientists expected based on how convection works in the Sun. "We don't yet know what accounts for this difference. It seems that convection changes as the star ages, in ways that we do not fully understand yet," explains Wouter Vlemmings. Observations like those of R Doradus help us understand the behavior of stars like the Sun, even when they become as cold, massive, and bubbling as R Doradus.
"It's spectacular that we can now capture a direct image of details from the surface of such distant stars and observe physical phenomena that, until now, could only be observed in our Sun," concludes Behzad Bojnodi Arbab, a PhD student at Chalmers who also contributed to the study.
Notes
(1) Convective bubbles have already been observed in detail on the surfaces of stars, notably with the PIONIER instrument on the ESO Very Large Telescope Interferometer. However, the new ALMA observations enable tracking the motion of the bubbles in a way that wasn't possible before.