Although stars are huge, they are extremely distant and appear as point sources in telescopes. You never normally see more than a pixel. Now astronomers have used the Atacama Large Millimeter/submillimeter Array (ALMA) to resolve details on the surface of the star R Doradus and tracked its activity for 30 days. The images revealed huge, hot bubbles of gas that are 75 times larger than the entire sun. R Doradus is 350 times larger than our sun, but only 180 light-years away.
“This is the first time that the bubbling surface of a real star has been imaged in this way,” said Wouter Vlemmings, a professor at Chalmers University of Technology in Sweden and lead author of the study, in a press release from the European Southern Observatory (ESO). “We never expected the data to be of such high quality that we could see so much detail of the convection on the stellar surface.”
In the study published in Nature, the astronomers detail how they observed R Doradus, a massive red supergiant star, over four weeks between July 2 and August 2, 2023. The observations at about 338 GHz were carried out using the longest ALMA baselines available. The images revealed a stellar disk with distinctive small-scale features that provide the structure and motions of convection on the star's surface.
Convection is the mixing of gases within a star, with heated gas from the star's interior, created by nuclear fusion in the core, rising to the surface and the cooler, denser gas in the star's photosphere sinking. This continuous movement also distributes the heavy elements formed in the core, such as carbon and nitrogen, throughout the star. Convection is also thought to be responsible for the stellar winds that carry these elements out into the cosmos to form new stars and planets.
“Convection creates the beautiful granular structure that you see on the surface of our Sun, but that is difficult to see on other stars,” said Theo Khouri, a researcher at Chalmers and co-author of the study. “With ALMA, we have now not only been able to directly see convective granules – 75 times the size of our Sun! – but also, for the first time, measure how fast they are moving.”
Although convection bubbles have been observed in detail on the surface of other stars before, including in another observation of a red giant star using the PIONIER instrument on ESO's Very Large Telescope Interferometer, ALMA's higher resolution allowed astronomers to follow the bubbles' motion in a way not possible with other telescopes.
The researchers found that the granules of R. doradus appear to move in a one-month cycle, which is faster than scientists expected based on convection in the sun.
“We don't yet know what the reason for this difference is. It seems that convection changes as a star ages in ways we don't yet understand,” Vlemmings said. In their paper, the team wrote: “This suggests a possible difference between the convection properties of evolved low-mass and high-mass stars.”
This wide-field view, created from images from the Digitized Sky Survey 2, 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. Image credit: ESO/Digitized Sky Survey 2. Credit: Davide De Martin
Red giant stars are what main sequence stars like our Sun become when they use up their hydrogen fuel and expand to hundreds of times their normal diameter. Since R Doradus has a similar mass to our Sun, this red giant star is probably a good example of what our Sun will look like in about five billion years.
“It is spectacular that we can now directly image the details on the surface of such distant stars and observe physical phenomena that were previously mostly only observable in our Sun,” said Behzad Bojnodi Arbab, a doctoral student at Chalmers University who was also involved in the study.
Like this:
Load…
Comments are closed.