Astronomers track bubbles on star’s surface in most detailed video yet

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 images of the star, R Doradus, were obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), a telescope co-owned by the European Southern Observatory (ESO), in July and August 2023. They show giant, hot bubbles of gas, 75 times the size of the Sun, appearing on the surface and sinking back into the star’s interior faster than expected.

“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 on September 11 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.

Could be the future for our Sun

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 relatively 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 their velocities and timescales for the first time.”

A cool, big and bubbly star

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 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.

Even better observations lie ahead

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.

For all video and images, please go to: Astronomers track bubbles on star’s surface in most detailed video yet | ESO United Kingdom

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 are sharper and more detailed than previous ones and track the motion of the bubbles in a way that was not possible before.

More about the study

This research was presented in a paper entitled “One month convection timescale on the surface of a giant evolved star” in Nature.

The team is composed of Wouter Vlemmings, Theo Khouri, Behzad Bojnordi, Elivre De Beck, and Matthias Maercker; all at Chalmers University of Technology, Sweden.

The Atacama Large Millimeter/submillimeter Array (ALMA) is an international astronomy facility operated in partnership between the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. Read more. Chalmers University of Technology and Onsala Space Observatory have been involved in ALMA since its inception; receivers for the telescope are one of many contributions. Onsala Space Observatory is host to the Nordic ALMA Regional Centre, which provides technical expertise to the ALMA project and supports astronomers in the Nordic countries in using ALMA.

The European Southern Observatory (ESO) is supported by 16 member states and operates three observing sites in the Atacama desert in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. Read more about ESO

For more information, please contact:

Wouter Vlemmings
Chalmers University of Technology
Gothenburg, Sweden
Tel: +46 31 772 63 54
E-post: wouter.vlemmings@chalmers.se

Theo Kouri
Chalmers University of Technology
Gothenburg, Sweden
Tel: +46 31 772 6022
E-post: theo.khouri@chalmers.se

Behzad Bojnodi Arbab
Chalmers University of Technology
Gothenburg, Sweden
E-post: bojnordi@chalmers.se

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Mobil: +49 151 241 664 00
E-post: press@eso.org

Captions

Eso2412b: Astronomers have captured a sequence of images of a star other than the Sun in enough detail to track the motion of bubbling gas on its surface. The images of the star, R Doradus, were obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), a telescope co-owned by ESO, in July and August 2023. This panel shows three of these real images, taken with ALMA on 18 July, 27 July and 2 August 2023. The giant bubbles — 75 times the size of the Sun — seen on the star’s surface are the result of convection motions inside the star.
Credit: ALMA (ESO/NAOJ/NRAO)/W. Vlemmings et al.

Eso2412c:This chart shows the location of the nebula R Doradus in the southern-hemisphere constellation of Dorado (the Swordfish). This map shows most of the stars visible to the unaided eye under good conditions. The location of the star itself is marked with a red circle.
Credit: ESO, IAU and Sky & Telescope

Eso2412d: This wide-field view, created from Digitized Sky Survey 2 images, shows the region around R Doradus, the bright, orange star in the centre. The star’s surface was recently imaged in detail using the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner.
Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin