Video: A time-lapse image of R Doradus shows hot bubbles – 75 times the size of the Sun – emerging and sinking on the surface of the red giant star. © ALMA (ESO/NAOJ/NRAO)/W. Vlemmings et al.
Similar to our bubbling sun, a distant giant is also throwing bubbles: researchers have “filmed” the convection movements on the surface of a distant star for the first time. The images from the ALMA telescope documented the changes in the stellar surface of the red giant R Doradus over the course of a month. The development of hot bubbles that are 75 times the size of the sun can be seen. They appear on the surface and then sink back into the interior of the bloated star faster than expected, the researchers report.
The basic principle is like that of a lava lamp: material is heated at the bottom, expands and rises to the top. There it then cools down, condenses and sinks back down. In the case of stars, this process, known as convection, is driven by the heat generated by nuclear fusion in the interior. The dynamics ensure that the star material is mixed. Convection is therefore very important in the various stages of star development. The effects of this thermal process can be clearly observed in our sun: its surface bubbles – the rising material causes so-called granules on its surface. These bubble-like structures have an average diameter of 1500 kilometers and only exist for a few minutes at a time.
A seething giant
As far as signs of convection in distant stars are concerned, they have so far only been detected statically: it has already been possible to image bubble structures in a red giant, which have been interpreted as giant counterparts to the granules of our sun. Evidence of convection movements, however, has so far been lacking. But the new detailed look at the phenomenon has now also provided clues to its dynamics. The results of the researchers led by Wouter Vlemmings from Chalmers University of Technology in Gothenburg are based on data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
The focus of this telescope system was the star R Doradus, which is located 180 light years from Earth in the constellation of Swordfish. It is a so-called red giant. Stars like our sun reach this stage of development when helium nuclei fuse to form heavier elements in the second stage of the nuclear fusion process. The heat released in this process causes the star to expand significantly – as in the case of R Doradus: It has about the mass of the sun, but has swelled to 350 times its diameter. Its size and proximity to Earth make it an ideal target for detailed observations, the researchers explain.
Dynamics captured for the first time
As the astronomers report, they were able to reconstruct high-resolution images of the surface of R Doradus from the observation data over a period of 30 days in the summer of 2023. The structures were most clearly visible on July 18 and 27 and August 2. “We never expected the data to be of such high quality that we could see details of the convection on the star’s surface,” says Vlemmings. In the case of the bloated red giant, the counterparts to the granules of our sun are gigantic in size: the hot bubbles of matter are 75 times the size of the sun, the researchers report.
The images taken at different times then made it possible to show the convection process like in a video. “With ALMA, we were now not only able to see the star’s granules directly, but also for the first time to see how fast they are moving,” says co-author Theo Khouri from Chalmers University. The team recorded speeds of up to 20 kilometers per second. Overall, the granules of R Doradus appear to develop in a one-month cycle, the researchers report. “It is fascinating that we can now directly image details on the surface of distant stars and observe physical processes,” says co-author Behzad Bojnodi Arbab from Chalmers University.
According to the team, however, the results were surprising: the convection process in the red giant is faster than would have been expected based on projections of the known dynamics in the sun. “We don’t yet know where the difference comes from. It seems that convection changes as a star ages in a way that we don’t yet understand,” says Vlemmings.
Source: European Southern Observatory (ESO), scientific article: Nature, doi: 10.1038/s41586-024-07836-9