For the first time, astronomers have captured detailed images of the surface of a star other than the Sun, revealing massive convection bubbles of gas moving across its surface.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the research team focused on R Doradus, a red giant star approximately 180 light-years from Earth. The observations, taken between July and August 2023, offer new insights into how stars like our Sun behave as they evolve into red giants.
A Closer Look at R Doradus and its Turbulent Surface
The star R Doradus, located in the southern constellation Dorado, is roughly 350 times larger than the Sun, making it an ideal candidate for high-resolution imaging. Astronomers used ALMA to capture a time-lapse of the star’s surface, showing massive bubbles of gas, each about 75 times the size of the Sun, rising and sinking back into the star. These bubbles are a result of convection, a process where hot gas rises from the star’s core to its surface before cooling and sinking back down. This convection helps to transport heavy elements, like carbon and nitrogen, throughout the star and into space, where they can eventually contribute to the formation of new stars and planets.
Lead researcher Wouter Vlemmings, from Chalmers University of Technology in Sweden, described the breakthrough: “This is the first time the bubbling surface of a real star can be shown in such a way.” He emphasized that the team had not expected the data to reveal such detailed convection patterns on the stellar surface. The study was published in the journal Nature, and the findings are expected to help scientists better understand the behavior of evolved stars.
Surprising Speed of Convection in Red Giants
While convection on the Sun occurs on a relatively slow timescale, with granules on the Sun's surface appearing and disappearing over several minutes, the convection on R Doradus operates on a much faster scale. The team observed that the bubbles on the surface of the red giant move on a one-month cycle, faster than what scientists had previously expected. This discovery suggests that convection behaves differently in stars as they age and expand into red giants. “We don’t yet know the reason for this difference,” Vlemmings said, adding that it highlights the need for further study into how stellar physics evolve in older stars.
Despite its enormous size, R Doradus has a mass similar to the Sun's. This makes it an important object of study because it provides clues to what might happen to our Sun when it enters its red giant phase in about five billion years. As stars like the Sun age, they expand significantly and go through a phase of intense mass loss, shedding much of their outer material into space. Observing this process in detail on R Doradus gives astronomers a rare opportunity to see the future of our own solar system.
Impact of ALMA's Observations on Stellar Physics
The ALMA images of R Doradus represent a significant leap forward in our ability to study distant stars. Prior to this, such detailed observations of convection were only possible on the Sun due to its proximity to Earth. Now, with ALMA's unprecedented resolution, astronomers can directly image the surfaces of distant stars, revealing the physical processes that drive their evolution. “It is spectacular that we can now directly image the details on the surface of stars so far away,” said Behzad Bojnodi Arbab, a doctoral student at Chalmers and co-author of the study.
The study also sheds light on a mystery regarding the star’s rotation. Previous ALMA data suggested that R Doradus was spinning much faster than expected for a red giant star. However, the new findings show that the high-speed rotation is not an illusion caused by the star’s bubbling surface, as had been proposed in a similar case involving Betelgeuse, another well-known red giant. Instead, the research team has confirmed that R Doradus has a slower rotation than initially suspected, offering new insights into the behavior of red giants.
Future Studies and Implications for Understanding Red Giants
The ability to track convection and other surface phenomena on distant stars like R Doradus is a major advancement in astrophysics. As red giants play a key role in the chemical enrichment of the universe, understanding their behavior is critical for grasping the lifecycle of stars and the formation of planetary systems. The study’s findings could have broader implications for how scientists model the late stages of stellar evolution, particularly for stars similar to the Sun.
As our Sun ages, it too will expand into a red giant and lose much of its mass, ultimately influencing the orbits of planets like Mercury and Venus. By studying stars like R Doradus, scientists can better predict how these processes will unfold in our own solar system and beyond.
The team plans to continue observing R Doradus with ALMA to gain further insights into the star’s convection and surface dynamics, contributing to a deeper understanding of stellar physics and the future of our Sun.