Watch the motion of star bubbles on the surface of the star R Doradus in the video above.

• Stars – including our own sun – have giant bubbles of hot gas on their surfaces.
• These convective bubbles, called granules, had never been tracked in detail in stars besides the sun, until now.
• A new video of the star R Doradus captures these granules in the best detail yet for stars outside our own.

ESO published this original story on September 11, 2024. Edits by EarthSky.

Detail on a star far beyond the sun

For the first time, astronomers have captured images of a star other than the sun in enough detail to track the motion of bubbling gas on its surface. The Atacama Large Millimeter/submillimeter Array (ALMA) obtained the images of the star R Doradus in July and August 2023. They show giant, hot bubbles of gas, 75 times the size of the sun. The video shows the bubbles appearing on the surface and sinking back into the star’s interior faster than expected.

Wouter Vlemmings, a professor at Chalmers University of Technology, Sweden, was the lead author of the study published today in Nature:

This is the first time the bubbling surface of a real star can be shown in such a way. 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.

Star bubbles are granules due to convection

Stars produce energy in their cores through nuclear fusion. This energy can be carried out toward the star’s surface in huge, hot bubbles of gas. These bubbles 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. Scientists also think convection is responsible for the stellar winds that carry these elements out into the cosmos to build new stars and planets.

This is the first time convection motions have been tracked in detail in stars other than the sun. By using ALMA, the team was 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. It’s 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. That means R Doradus is likely fairly similar to how our sun will look like in 5 billion years, when it becomes a red giant.

Theo Khouri, a researcher at Chalmers who is a co-author of the study, said:

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

R Doradus is our sun’s future

The granules of R Doradus appear to move on a one-month cycle. That’s faster than scientists expected, based on how convection works in the sun. Vlemmings said:

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.

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.

Behzad Bojnodi Arbab, a PhD student at Chalmers who was also involved in the study, said:

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.

Bottom line: For the first time, astronomers have gotten a good look at the details of a star beyond the sun, spotting the motion of star bubbles, or the convection of hot gas.

Source: One month convection timescale on the surface of a giant evolved star

Via ESO