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Astronomers have observed for the first time the detailed movements of giant bubbles of gas on the surface of a nearby star, rising and falling like the inside of a lava lamp.
The massive bubbles of hot gas are 75 times the size of the sun and appear to be sinking into the star’s interior faster than expected, according to a team of astronomers at Chalmers University of Technology in Sweden.
The images show the surface of the star R. Doradus, a red giant star located 180 light-years away in the constellation Dorado. The star has a diameter about 350 times that of the Sun and serves as a preview of the future of our Sun.
In about five billion years, our sun will become a red giant, swelling and expanding as it releases layers of material and likely vaporizes the solar system’s inner planets; however, the fate of Earth remains unclear, according to NASA.
The observations, made by the Atacama Large Millimeter/submillimeter Array (ALMA) of telescopes in Chile, mark the first time researchers have been able to track such detailed movements on the surface of a star other than the Sun.
They published their results on Wednesday in the journal Nature.
“We wanted to observe the gas in the atmosphere around the star and hoped to find signs of the expected ‘convection’ bubbles,” said lead study author Wouter Vlemmings, professor of astronomy and plasma physics at Chalmers University, in an email. “However, we did not expect to see them in such detail and to actually be able to observe their movement.”
When stars age
Vlemmings and his colleagues are studying what happens when stars approach the end of their lifetime.
Stars generate energy in their core through nuclear fusion, by forcing hydrogen atoms together to form helium. This process heats up the star and supplies it with energy for billions of years.
The energy generated in the core can be released to the surface of the star through large, hot bubbles of gas. These bubbles then sink as they cool, similar to what happens in a lava lamp.
This process, called convection, mixes elements created in the core, such as carbon and nitrogen, throughout the star, the study authors said. Convection is also likely to trigger stellar winds, or fast winds that can hurl elements created by the star into space and thus contribute to the formation of new stars and planets.
When a star’s life ends, it runs out of hydrogen to convert into helium, causing the star’s core to collapse. This pressure on the core also increases the star’s temperature, which causes it to swell up and become a red giant, according to NASA.
Towards the end of their lives, the upper layers of stars are blasted away and eventually the stars collapse or explode, releasing the elements created within them into space.
“We are all made of stardust, and much of the matter around us is created in stars,” said Vlemmings. “How this matter is ejected from old stars and incorporated into new stars and planets is not yet fully understood.”
View of old stars
The team chose R. Doradus because it is one of the closest and largest red giant stars and therefore easier to observe. The telescope allowed them to collect high-resolution images of the star’s surface over the course of a month.
“Convection creates the beautiful granular structure seen on the surface of our Sun, but which is difficult to see on other stars,” said study co-author Theo Khouri, a researcher at Chalmers University, in a statement. “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.”
The Sun’s outermost layer, called the photosphere, is made of gas that is so hot that it forms bubbles. The Sun’s photosphere is full of millions of bubbles created by convection. The gas bubbles, also known as convective granules, are about 1,000 kilometers in diameter and move at a speed of a few kilometers per second, so they only survive for about 10 minutes.
However, the convection cells on the surface of R. Doradus extend over 100 million kilometers (about 62 million miles), move at speeds of several tens of kilometers per second, and last for about a month.
“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,” said Vlemmings.
Although convection bubbles have been discovered on the surface of stars before, the new observations tracked the motion of the bubbles in a way that was not previously possible.
“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 largely only observable in our Sun,” study co-author Behzad Bojnordi Arbab, a doctoral student at Chalmers University, said in a statement.
The new study includes longer observations than previous ones that captured the evolution of the bubbles, said Dr. Claudia Paladini, associate astronomer at the European Southern Observatory in Chile. Paladini authored a study on observing bubbles on the surface of the star pi1 Gruis. Although she was not involved in the new research, she wrote a paper accompanying the study in Nature.
“You can see the bubbles rising, expanding and disappearing, just like you see on the Sun. That’s remarkable considering the distance we’re talking about,” Paladini said. “Now we just need to observe a lot more of these stars!”
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