Granules Studied for the First Time on a Distant Star

The surface of our Sun is not as smooth as it might appear to the naked eye. Already using a small telescope, we can observe some darker areas, called sunspots, where temperatures are relatively low. Now, similar sunspots have been observed on the surface of another star. Scientists believe this discovery can offer crucial insight into the future of our own Sun.

How does a granule work?

Getting even closer to the surface, we might notice the presence of irregular, granular structures that cover it entirely. They are bubbles that originate due to convective motions. Their mechanism is the same as boiling water: due to nuclear reactions, gases at the center of a star heat up to tens of millions of degrees and move outward. As they reach the surface (the photosphere), they cool down to tens of thousands of Kelvins and sink. We can observe these convective currents in the picture below: the darker part of the granule is the colder one, moving toward the center of the star, while the brighter part is the warmer one, moving to the surface.

Convective motions seem to be crucial for the production of winds that spread heavy elements, important for the formation of new stars and planets, throughout the Universe.

Not only on our Sun!

Due to the high level of detail required, before today they have only been thoroughly studied on our star’s surface. By using the Atacama Large Millimeter/submillimeter Array (ALMA, we briefly spoke about it in this article), scientists managed to observe the surface of the star R Doradus. Its relative proximity (180 light-years from Earth) and its considerable size made these observations possible. R Doradus is a giant red star with a diameter 350 times bigger than that of the Sun. What they saw was something very similar to the granular structures described above. Just bigger. The largest observed, in fact, was as big as half of the visible hemisphere.

From visual inspection, moreover, astronomers concluded that their lifetime was of, at least, three weeks. This result, however, was not exactly what they expected and an explanation is still being sought for this.

Why is it important?

This is an observation of great relevance for several reasons. Wouter Vlemmings, the lead author of the study published today in Nature, says: “We had never expected the data to be of such high quality that we could see so many details of the convection on the stellar surface.” Theo Khouri, a researcher who took part in the study, adds: “It is hard to see [the granular structures] on other stars. With ALMA, we have now been able to not only directly see convective granules, but also measure how fast they move for the first time.”

In addition, being R Doradus similar to what the Sun is expected to be in five billion years, studies like this help astronomers understand what will be the fate of our star as it approaches the end of its life.