Unique images show gas bubbles on red giant star's surface

“This is the first time the bubbling surface of a real star can be shown in such a way,“ [1] says Wouter Vlemmings, a professor at Chalmers University of Technology, Sweden, and lead author of the study published today in Nature. “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.”

Stars produce energy in their cores through nuclear fusion. This energy can be carried out towards the star’s surface in huge, hot bubbles of gas, which then cool down and sink — like a lava lamp. This mixing motion, known as convection, distributes the heavy elements formed in the core, such as carbon and nitrogen, throughout the star. It is also thought to be responsible for the stellar winds that carry these elements out into the cosmos to build new stars and planets.

What our Sun will look like

Convection motions had never been tracked in detail in stars other than the Sun, until now. By using ALMA, the team were able to obtain high-resolution images of the surface of R Doradus over the course of a month. R Doradus is a red giant star, with a diameter roughly 350 times that of the Sun, located about 180 light-years away from Earth in the constellation Dorado. Its large size and proximity to Earth make it an ideal target for detailed observations. Furthermore, its mass is similar to that of the Sun, meaning R Doradus is likely fairly similar to how our Sun will look like in five billion years, once it becomes a red giant.

“Convection creates the beautiful granular structure seen on the surface of our Sun, but it is hard to see on other stars,” adds 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.”

Faster bubbles than expected

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,” says Vlemmings. Observations like those now made of R Doradus are helping us to understand how stars like the Sun behave, even when they grow as cool, big and bubbly as R Doradus is.

“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,” concludes Behzad Bojnodi Arbab, a PhD student at Chalmers who was also involved in the study.

New telescope, new observation opportunities

In the future, new observations will enable even more spectacular movies of this star and others like it. On 10 and 11 September 2024, scientists met to share the scientific potential for the world’s next boundary-breaking radio telescopes, those of the SKA Observatory in South Africa and Australia, at Sweden’s second National SKA Science Days held in Gothenburg, Sweden. Behzad Bojnordi Arbab presented his hopes for observing this star with the SKA.

 “With the SKA telescopes we will be able to get high-resolution observations of the higher atmosphere of R Doradus. We want to see something we’ve not yet been able to: how the star’s bubbles could help create the star’s dusty wind. That will help us understand how the cosmic ecosystem works”, he says.

More info:

Read the press release: Astronomers track bubbles on star’s surface in most detailed video yet, where all images are available in high resolution on the ESO website. 

Notes: 

[1] Convection bubbles have been previously observed in detail on the surface of stars, including with the PIONIER instrument on ESO's Very Large Telescope Interferometer. But the new ALMA observations track the motion of the bubbles in a way that was not possible before.