Astronomers Observe a Massive Star Releasing Cosmic Bubbles

Scientific Asia: 180 light-years away, on the surface of the massive star R. Doradus, astronomers can observe boiling plasma.

Boiling processes on the red giant star R. Doradus have produced enormous bubbles that are 75 times larger than the Sun. However, the speed at which the bubbles “pop” is what astronomers found fascinating—not the magnitude of these formations.

A study led by Wouter Vlemmings (Chalmers University of Technology, Sweden) discovered that the bubbles form and fade in less than a month during five observations that span over four weeks in the summer of 2023. That’s around three times faster than anticipated, and the team’s Nature publication of the results offers no apparent explanation.

It’s important to note that although the image above seems to show bubbles on the star’s surface, it doesn’t accurately capture the star’s visible surface or the boiling motions themselves.

To reconstruct radio images of R. Doradus, the researchers employed dozens of dishes from the Atacama Large Millimeter/Submillimeter Array (ALMA). The wavelengths of those photographs reveal heated gas slightly higher up in the star’s atmosphere rather than the visible surface, also known as the photosphere. On the other hand, the boiling motions, or convection, in the star itself are what create the shock waves that radiate from the heated gas. Therefore, even though what we observe at these short radio wavelengths occurs in the star’s atmosphere, it provides a reasonable estimate of what’s occurring a little bit below the photosphere.

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Heat causes the motion of convective cells in R Doradus, just like it does in a pot of water on the stove. Heat is carried by water (or plasma in the case of the star) to the surface, where it cools and descends once more in a single cell. These boiling motions, which are just 1,000 kilometers (600 miles) across, seem to us as granules on the Sun. The granule-like features are slightly larger on R. Doradus: The largest one covers almost 25% of the star and is 100 million km across or 0.72 astronomical units.

This chart highlights the position of the nebula R Doradus within the Southern Hemisphere’s Dorado constellation, also known as the Swordfish. It features most of the stars visible to the naked eye under ideal viewing conditions, with R Doradus indicated by a red circle.

Given their distance, it is nearly impossible to see details on other stars in the same manner that we do on our own Sun. However, using a method known as interferometry, astronomers have made progress in recent years toward witnessing the much larger granule-like formations that exist on huge stars.

Astronomers have resolved large-scale convection in a few supergiant stars using interferometry. For instance, Betelgeuse, the well-known star in Orion’s shoulder, has convective cells spanning half of its stellar disk, according to these studies. Therefore, it is not surprising that R. Doradus has equally enormous structures, given that it is also a huge star and that our current technology is unable to detect the existence of much smaller cells.

The nature of the convection accounts for the result’s perplexing character. The researchers predicted that the bubbles would emerge and vanish after a few months, based on the projected rate at which hot gas should ascend and fall through the star. However, in the shorter observation period of one month, they were already able to observe bubbles emerging and disappearing.

In a perspective piece published in the same issue of Nature, Claudia Paladini (European Southern Observatory) exclaims, “I think these are very exciting times!” A team led by Paladini was able to capture a single image of a red giant star’s convective cells back in 2018. A full movie demonstrating the development of these features is now available.

She continues, “One can see the bubbles rising, spreading, and disappearing like it’s seen in the sun.” “Considering the distance we are talking about, it’s remarkable.”

As Paladini points out, there’s more going on here than just basic convection; thus, these are the kinds of data we need to start understanding the intricate dynamics of big stars.

She states, “We must remember that these stars are pulsating and that they are near the surface.” Convection and pulse interact. I suspect that the convective cells rising to the surface could accelerate if there was a right push from a pulsation shock passing through the stellar interior.

Astronomers will have a better understanding of what causes and how big bubbles to burst with further observations such as these of other giant stars.