"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, Sweden, and lead author of the study published 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 generate energy in their cores via nuclear fusion, which results in hot gas bubbles moving towards the surface. As these bubbles cool, they sink back into the star's interior. This process, called convection, spreads heavy elements like carbon and nitrogen throughout the star and contributes to stellar winds that eventually carry these elements into space, fueling the creation of new stars and planets.
Until now, scientists had never been able to track the convection motions in stars other than the Sun in such detail. With ALMA's high-resolution imaging, the team observed the surface of R Doradus for a month, capturing unprecedented details. The star, a red giant with a diameter about 350 times that of the Sun, is similar in mass to the Sun and offers a glimpse of what our own star might look like in five billion years when it evolves into 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," added Theo Khouri, a researcher at Chalmers and co-author of the study. "With ALMA, we have now been able to not only directly see convective granules - 75 times the size of our Sun - but also measure their speed for the first time."
The observations revealed that the granules on R Doradus cycle on a monthly basis, which is faster than expected based on models of convection 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," explained Vlemmings. These findings are helping astronomers better understand the behavior of stars like the Sun as they age and evolve into red giants.
"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," concluded Behzad Bojnodi Arbab, a PhD student at Chalmers involved in the study.
Research Report:One month convection timescale on the surface of a giant evolved star
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Stellar Chemistry, The Universe And All Within It
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