New observations show the bubbling surface of the red giant star R Doradus, a feat previously only achievable with the Sun.
This series of ALMA images of the star R Doradus shows in incredible detail the bubbles forming on the star's surface. Credit: ALMA (ESO/NAOJ/NRAO)/W. Vlemmings et al.
The stars in the sky are so distant that they often appear as tiny pinpoints to even our most powerful telescopes. Aside from the Sun, only a few nearby, puffy stars appear large enough to pick out any surface features, however rudimentary. But new images released Sept. 11 capture one of the most detailed images ever obtained of a star other than the Sun — and it's one that foreshadows the fate of our own solar system.
In a research published in NatureAstronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile have captured images of the star R Doradus in such detail that bubbles of gas can be seen on its surface. Previously, this was only possible for the Sun.
Approaching R Doradus
Several factors have contributed to ALMA being able to make out such fine details. The star is only 180 light-years away. As a red giant, it is huge (350 times the diameter of the Sun) and relatively cool. And each of the bubbles on the surface is about 75 times the size of the Sun.
But don't let its massive size fool you. R Doradus' mass is comparable to that of our Sun. When stars like the Sun exhaust their supply of hydrogen, the outer layers expand, turning them into a red giant star. This expansion greatly reduces the density of the star's outer atmosphere, meaning that despite the size of R Doradus' bubbles, they are quite diffuse – several-fold. In fact, our own Earth's atmosphere is denser than R Doradus' outer layers.
R Doradus is classified as an asymptotic giant branch, or AGB star, whose core is no longer undergoing fusion, but where helium and hydrogen are still fusing in “shells” around the inert carbon-oxygen core.
“We chose R Doradus because it is one of the closest AGB stars, possibly the closest,” says study author Wouter Vlemmings of Chalmers University of Technology in Sweden. “It is also the largest star (after the Sun) in angular size in the sky.” The researchers therefore hoped it would be able to provide the level of detail they wanted.
A glimpse into the future of the Sun
The images are stunning, and Vlemmings says they closely match predictions of what such a red giant star should look like. That means our predictions of the path our Sun might take in its death spiral in a few billion years are probably correct – and R Doradus offers a little insight.
So how much time does R Doradus have left? Vlemmings says it’s hard to say, but “the typical duration of this stellar phase is on the order of a million years. So it still has a few hundred thousand years left.”
The team is planning further observations of the star, hoping to learn more about how gas behaves in the atmosphere. They also want to study similar stars for flares or hot spots to better understand how convection (the transfer of heat outward) might work on the surface.
For most future observations, they will still need ALMA. The array of radio telescopes does not form traditional images, like those you might see when looking through a telescope. Instead, the 66 receivers work in tandem to gradually build up a huge observation of an object in wavelengths longer than visible light.
This means that ALMA can be “tuned” to gather details about objects too bright to be analysed in visible or infrared light. “[These]stars are actually too bright for the JWST, meaning they would be very difficult to study without saturating the telescope,” says Vlemming, although he adds that the space telescope could be used to survey the region around the star for gas eruptions.
So while JWST is typically used to peer deep into the universe's past, researchers are using ALMA to gain insight into its future.
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