High Resolution Images Show Bubbling Gas on the Surface of Another Star

Although stars are enormous, they’re extremely far away, and appear as point sources in telescopes. Usually, you never get to see more than a pixel. Now astronomers have used the Atacama Large Millimeter/submillimeter Array (ALMA) to resolve details on the surface of the star R Doradus and track its activity for 30 days. The images revealed giant, hot bubbles of gas 75 times larger than the entire Sun. R Doradus is 350 times larger than our Sun, but only 180 light-years away.

“This is the first time the bubbling surface of a real star can be shown in such a way,“ said Wouter Vlemmings, a professor at Chalmers University of Technology in Sweden, and lead author of the study, in a press release from the European Southern Observatory (ESO). “We had never expected the data to be of such high quality that we could see so many details of the convection on the stellar surface.”

In the study, published in Nature, the astronomers detailed how they observed R Doradus, a massive red supergiant star, over four weeks between July 2 and August 2, 2023. The observations were made using the longest available ALMA baselines. The images revealed a stellar disk with prominent small-scale features that provide the structure and motions of convection on the stellar surface.

Convection is the mixing of gas within a star, where heated gas from the interior of the star created by nuclear fusion in the core rises to the surface and the cooler, denser gas on the star’s photosphere sinks. This continuous motion also distributes the heavy elements formed in the core, such as carbon and nitrogen, throughout the star, and convection is also thought to be responsible for the stellar winds that carry these elements out into the cosmos to build new stars and planets.

“Convection creates the beautiful granular structure seen on the surface of our Sun, but it is hard to see on other stars,” said Theo Khouri, a researcher at Chalmers who is a co-author of the study. “With ALMA, we have now been able to not only directly see convective granules — with a size 75 times the size of our Sun! — but also measure how fast they move for the first time.”

While convection bubbles have been previously observed in detail on the surface of other stars, including another observation of a red giant star using with the PIONIER instrument on ESO’s Very Large Telescope Interferometer, ALMA’s higher resolution allowed astronomers to track the motion of the bubbles in a way that was not possible with other telescopes.

The researchers found that the granules of R Doradus appear to move on a one-month cycle, which is faster than scientists expected based on how convection works in the Sun.

“We don’t yet know what is the reason for the difference. It seems that convection changes as a star gets older in ways that we don’t yet understand,” said Vlemmings. In their paper, the team wrote, “This indicates a possible difference between the convection properties of low-mass and high-mass evolved stars.”

Red giant stars are what become of main sequence stars like our Sun once they have exhausted their hydrogen fuel, and they expand to becomes hundreds the size of times their normal diameter. Since R Doradus has a mass similar to that of our Sun, this red giant star is likely a good example of how our Sun will look like in approximately five billion years.

“It is spectacular that we can now directly image the details on the surface of stars so far away, and observe physics that until now was mostly only observable in our Sun,” said Behzad Bojnodi Arbab, a PhD student at Chalmers who was also involved in the study.