Astronomers have successfully taken detailed images of a star outside our own Sun, enabling them to observe the dynamic movement of gas bubbles on its surface. These images, focusing on the star R Doradus, were captured using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, which is a joint project of the European Southern Observatory (ESO), during July and August 2023. The observations reveal massive, heated gas bubbles that are 75 times larger than the Sun, appearing on the star’s surface and submerging back into its interior at a surprisingly quick pace.
For the first time, astronomers have taken images of a star besides our Sun that are detailed enough to observe gas bubbles rising and falling on its surface. These images of R Doradus were captured using the Atacama Large Millimeter/submillimeter Array (ALMA), a telescope shared by the European Southern Observatory (ESO), during July and August 2023. They reveal enormous hot gas bubbles, which are 75 times the size of the Sun, emerging on the surface and sinking back into the star’s interior more quickly than scientists had anticipated.
“This is the first time we can visually demonstrate the bubbling surface of a true star in such detail,” states Wouter Vlemmings, a professor at Chalmers University of Technology in Sweden, and lead author of the study recently published in Nature. “We never anticipated the data would be of such excellent quality that we could discern so many intricate details of the convection occurring on the star’s surface.”
Stars generate energy in their centers via nuclear fusion. This energy moves towards the outer layers in large, hot gas bubbles, which eventually cool and sink back down—similar to the motion seen in a lava lamp. This action, known as convection, plays a crucial role in spreading heavy elements, such as carbon and nitrogen, created in the star’s core, throughout the star. It’s also thought to influence stellar winds that carry these elements into space, contributing to the formation of new stars and planets.
Until this study, detailed tracking of convection movements had not been achieved in stars other than the Sun. Utilizing ALMA, the team collected high-resolution images of R Doradus over approximately one month. R Doradus is a red giant star with a diameter around 350 times that of the Sun and is located about 180 light-years away from Earth in the Dorado constellation. Its significant size and relative proximity make it an excellent candidate for close examination. Additionally, since its mass is comparable to that of the Sun, R Doradus offers insights into how our Sun might appear in about five billion years when it transforms into a red giant.
“Convection forms the stunning granular patterns observed on the Sun’s surface, but it has been challenging to witness this on other stars,” remarks Theo Khouri, a researcher at Chalmers and co-author of the study. “With ALMA, we’ve not only been able to visually capture convective granules—each 75 times larger than our Sun—but also measure their movement speeds for the first time.”
The granules of R Doradus seem to follow a one-month cycle of motion, which is quicker than prior expectations based on convection patterns in the Sun. “The reason for this difference is still unknown. It appears that convection undergoes changes as a star ages in ways we haven’t yet comprehended,” Vlemmings explains. Observations like those of R Doradus aid in unraveling the behaviors of sun-like stars as they expand into cool, oversized, and bubbly forms like R Doradus.
“It is extraordinary that we can directly image the details of distant star surfaces, and witness physical phenomena previously only observable in our Sun,” concludes Behzad Bojnodi Arbab, a PhD student at Chalmers also involved in the project.
Notes
*While convection bubbles have been previously studied in detail on other stars—such as with the PIONIER instrument on ESO’s Very Large Telescope Interferometer—the latest ALMA observations uniquely track the movement of these bubbles differently and more effectively than before.
