How a dying star is similar to a lava lamp

For the first time, astronomers have watched gas boil and bubble on the surface of a distant star.

Scientists observed the red giant star R Doradus with the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile over the course of four weeks in July and August 2023. The series of images shows large cells of gas rising to the star’s surface and sinking again, the team reports in Nature September 11.

Those bubbles are the hallmark of convection, the process that transports heat and energy around the insides of stars. “It’s kind of the principle of a lava lamp or boiling water,” says astronomer Wouter Vlemmings of the Chalmers University of Technology in Gothenburg, Sweden.

Similar bubbles have been seen on other giant stars. But this is the first time the bubbles’ speeds and motions have been tracked in a star other than the sun.

R Doradus is about 180 light-years from Earth and is nearing the end of its lifetime (SN: 7/23/21). As part of its death process, it has puffed up to about 350 times the width of the sun, though both stars have about the same mass.

The convective cells on the star’s surface are correspondingly enormous. A single cell spans 75 times the width of the sun. The cells rise and fall within the star at about 20 kilometers per second, about 60 times the speed of sound. That’s faster than astronomers expected based on how convection works on the sun, and fast enough that a small fraction of the gas could escape into space (SN: 12/5/13).

These observations and others like it could help illuminate the origins of the elements that make up stars, planets and people (SN: 11/29/20). The majority of the stardust that goes on to become new objects “comes from stars like the one we looked at,” Vlemmings says. “But the process of how this works is still not fully understood. We’d like to know the physics, the details of how this works.”