A tale of stellar dances: the formation of Barium stars

Ana Escorza Santos · 29-06-2021 10:00 · Winners "Your research in one image or 1000 words" - 1st Edition

Stars are the chemical factories of the Universe. Look around, feel around, breathe. The majority of the chemical elements that you have just interacted with were once made in the hot interior of a star. The calcium in the milk you put in your coffee, the iron in your blood and the oxygen in the air we breathe are nothing more than stardust. Stars are born from material left behind by previous generations of stars, and just like humans, stars are born, live and die. During their lives, stars work to enrich the Universe with freshly made chemicals. And also, just like humans, stars dance.

Yes, you read correctly, more than half of the stars that you see in the night sky currently have a dancing partner. This happens when two stars are born close enough to each other that they become bound by gravity. These stellar couples are called double-star systems and the two members of such systems spend their lives together dancing, orbiting around each other like the Moon does around the Earth. Since we live in a stellar system with a single star, our Sun, we are biased towards the idea of lonely stars with their planets. However, believe me when I tell you that double-star systems are just as common as single stars for stars like the Sun and much more common among hotter stars. A few examples of well-known stars that have dancing partners are Polaris, the North star; Sirius, the brightest star on the night sky; and Proxima Centauri, our closest neighbour.

You can imagine that, just like with humans again, the presence of a partner can have a big effect on the life of a star. Stars in double-star systems interact with each other, exchanging material and changing each other’s behaviour when compared to single stars. And this is what makes this story interesting: some of the most beautiful astronomical objects, some of the biggest stellar mysteries and some of the most energetic explosions in the Universe result from the interactions between these stellar dancing partners.

Today, I bring you one of those mysteries. I bring you a tale of stellar dances that started in the 50s and continued to amaze astronomers until now. I bring you the story of Barium stars. This chronicle started with a scientific discovery that like many other scientific discoveries began with a group of scientists that found something peculiar in their data. They found something that their knowledge about stars could not explain at the time. I mentioned before that stars are chemical factories continuously forging new elements in their interiors. This is a good time to clarify that not all the stars make the same elements. For example, a star like the Sun will never make iron, but a star thirty times heavier will.

Additionally, a star will make different chemical elements at different stages of its life. For example, our Sun is currently transforming hydrogen into helium in its core, but when it becomes older and the core runs out of hydrogen to burn, the Sun will be able to make carbon, oxygen and some heavy metals. Knowing this, the chemical elements found in a star can give an indication of the type of star we are looking at and of its age.

The scientists we were talking about before knew all this, but their data showed something that did not fit in this picture. They identified a few stars that showed weirdly large amounts of the chemical element barium in their atmospheres. This was puzzling back then because large amounts of barium were only expected in much older and much more luminous stars than those they were observing. They called these weird stars Barium stars.

Rather than being solved, the mystery grew over the next few decades. More and more Barium stars were discovered, some of them as young as our own Sun. Additionally, it was noticed that barium was not the only chemical element present in these stars in anomalous quantities, and the current understanding of stellar astronomy at the time still could not provide scientists with an explanation for these chemical peculiarities.

Given my introduction about double-star systems, you can probably already guess the key to the mystery. It was only in the mid-eighties that another group of scientists could prove, after pointing their telescopes at Barium stars for several years, that all of them were dancing around an invisible partner. All Barium stars seemed to have a companion, but one so small and so faint that it could not be directly detected with their telescopes. However, the periodic dance of the Barium stars could be measured, shedding light on their long-standing mystery.

The fact that Barium stars were part of double-star systems was an important piece to solve the puzzle of their formation. You might remember I said that barium and the other chemical elements found to be weirdly overabundant on the atmospheres of Barium stars were only expected in older and more luminous stars. Do you know how those stars look like when they burn out and slowly die? They expel their atmospheres to the Universe and leave behind only their cores, which cool down and dim, becoming invisible to most telescopes.

Chemistry was telling scientists that Barium stars used to have luminous dancing partners, however, just like humans, stars die. Those luminous companions made heavy elements in their interior, barium included, and just before turning off their engines and becoming almost invisible, they gave their lifelong dancing partner one final gift. They transferred a little bit of this enriched material, polluting their partner’s atmosphere with unexpected elements and forming the mysterious Barium stars.

I would love to bring you a closed happy ending, but scientific tales are hardly ever complete because there is always more to learn. Further research is still needed to fully understand how the transfer of material happened from the former luminous star to its partner, the current Barium star, and to understand how this interaction affects the pace and path of their dance. However, the formation of Barium stars is a perfect example of a long-lasting mystery solved by pushing further, trying to understand the effect of what we do not see. Solving any long-lasting mystery deserves a celebration, so let’s honor double-star systems and celebrate with a dance.

About the author: 

Ana Escorza Santos is a Spanish scientist currently working in Chile as a postdoctoral fellow at the European Southern Observatory (ESO). There, she spends part of her time in the Paranal Observatory, pointing some of the most amazing telescopes and astronomical instruments in the world to the sky, to learn about stars and their lives. Ana defended her PhD in Astronomy in March 2020 in Belgium, in a dual program between the KU Leuven and the ULB and funded by the Research Fundations Flanders (FWO). Her scientific interests focus on the products of interacting (or dancing) double-star systems, especially those systems that include old evolved stars.



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