The University

Super-sharp images show gigantic bubbles on surface of cold giant star

30 January 2018

Star model. Complex computer models are being developed at Uppsala University to study the processes that control the final stages in the lives of sun-like stars.

An international research team has taken extremely sharp images of the surface of a nearby giant star using the VLT telescope in Chile. The images show how huge bubbles, two hundred times as large as our own sun, transport energy from inside the giant star up to its surface. The images can be used to understand how the intense stellar winds of giant stars gradually lead to the star’s death.

Until recently, the sun was the only star whose surface could be studied in detail. All other stars are so far away that they only look like small dots, even with the world’s largest telescope. But by connecting together several telescopes at the VLT facility in Chile, a research team was able to take pictures of a nearby giant star with such a high resolution level that it is possible to distinguish structures on the surface.

If you look at the surface of the sun, you can see lots of small bubbles covering the entire surface. Just as when you’re boiling spaghetti and the heat from the cooker makes the water bubble, energy from the inner layers of stars are transported up to the surface through large gas bubbles. When the sun or other stars of the same size grow old, they swell up and become giant stars equal in size to the Earth’s path through the solar system. The gas disperses and the layers furthest out are much thinner than what they are on our sun now. The energy formed furthest inside the star is still transported in the same way, with bubbles, but because the gas is much thinner, the bubbles become much larger. The bubbles on the images of π1 Gruis are the same size as two hundred suns. Just a handful of bubbles cover the entire star and each bubble is about 120 million kilometres across. For comparison, the sun is covered by a couple million bubbles at any given point in time.

To study the processes that control the final stages in the lives of sun-like stars, complex computer models are being developed at Uppsala University. The models are used to calculate how a giant star transports energy and how the star’s intense stellar winds arise. The star models can now be compared with the new images to see if they produce similar results.

“For hundreds of years, the sun has been the only star in which we have been able to study dynamic processes directly on the surface,” says Bernd Freytag, researcher at the Department of Physics and Astronomy, Uppsala University, who is leading the development of the models that are now being compared with the new images in the article in Nature. “The sun has therefore until now been our only reference point when we develop numeric models of the atmospheres of sun-like stars. With our new observations, we get a new reference point for another type of star, a red giant star, and can therefore gain much better insight into how different types of stars work.”

“There are really exciting advances happening in star research right now,” says Sofia Ramstedt, docent at the Department of Physics and Astronomy, Uppsala University, who also took part in the study. “Technological advancements have enabled amazing astronomical images and this coincides with the development of realistic 3D models of stars. By comparing observations and theory, we can make new discoveries and major advances.”