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They orbit each other, swallow the other, or tear off their partner’s shell (or “envelope”) – stars have turbulent relationships with each other. The universe is full of multiple star systems in which the celestial bodies come so close together that they end up influencing each other. Astronomer Stephan Geier explores what these stellar connections look like in aging stars.
As soon as it gets dark and the sky is clear, they slowly become visible. At first, only a single point shines but eventually more and more start revealing themselves. "Can you count the stars that brightly twinkle in the midnight sky?" says a well-known German lullaby. But even if we were able to count all the points in the sky, we still would not know how many are really there. Appearances are deceiving: A single luminous point is rarely only one star but usually two, three, or even more.
"Stars are usually not alone," Geier says. The reason for this phenomenon lies in star formation. The cradle of the luminous celestial bodies are gas clouds that are gradually condensing. The gas molecules attract each other, and the distance between them decreases. Ultimately, a star is formed out of this gas cloud, with hydrogen fusing in its core. The centrifugal forces within the rotating cloud ensure that this usually happens in several places. This is how multiple star systems come into being. Binary stars can sometimes be seen with the naked eye: The light flickers when the orbiting stars cover each other.
Geier has long explored the relationship governing such constellations. The astronomer, who was recently appointed Professor of Stellar Astrophysics at the University of Potsdam, focuses on one category in particular: double star systems that are nearing the end of their relationship, because one of them has started engulfing the other and losing its envelope in the process. The mechanisms behind these events can reveal a lot about the formation and evolution of stars.
Aging stars have quite a few surprises in store. Once they have exhausted all of their hydrogen through fusion, they expand to giant stars, which range from about 10 to 100 times the size of our sun. When two stars very tightly orbit each other, the envelope of such a giant star can absorb its partner. Two cores orbit in one envelope. The researchers call this phenomenon a "common envelope". "In this common envelope, the nuclei of the two stars lose energy and slow down. They continue to approach each other and become very, very close binary star systems," explains Geier. It takes only a few hours or even minutes for these stars to orbit each other.
Researching these systems poses some difficulties for astronomers. "These interactions cannot be observed live," Geier explains. What he sees in the night sky is always just a snapshot. "We are able to observe certain phases of stellar evolution but not others. Our problem as astronomers is that the lifespan of stars is much longer than ours," he says. Geier, therefore, searches for remnants of stellar interactions in the vastness of the universe – the "world’s largest laboratory", he says, which can reveal more about their evolution than has previously been known.
Hot subdwarfs are just the kind of remnants Geier is looking for. This hot, yet compact type of star is very rare. To date, only a few thousand hot subdwarfs have been identified in the Milky Way: an insignificant number considering that there are billions of stars in it. They are the remnants of red giants that have lost their outer layers. While the reasons for this are unclear, Geier surmises that this event takes place in the common envelope phase of a binary star system. In theory, which is plausible but not yet sufficiently documented by measured data, the envelope of the two star cores heats up due to their interaction. At some point, the envelope dissolves, leaving behind the star cores, which are still closely orbiting each other.
With enormous ground telescopes and space probes, astronomers are now able to observe the rare hot subdwarfs. Some of them were discovered by chance, others tracked down and observed systematically. Geier and his team travel to the Chilean Atacama Desert, Argentina, and southern Spain. The observation times for the telescopes in these places are highly coveted among astronomers and strictly limited. If you are lucky, your proposal will be granted a few days of that valuable time. Then you need to do night shifts in front of the many monitors of the observatory; coffee consumption is high. "We start our observations as soon as night falls. We record the spectra and light curves of the stars we are interested in," Geier explains. "It is somehow meditative," he adds, laughing.
Once the data for a particular star is available, the real work begins. Astronomers use the spectral lines of the celestial body to analyze how hot the star is, how it moves, and which chemical elements comprise its atmosphere. The spectra of the stars are compared with models on the computer. There is a problem though: hot subdwarfs still lack suitable models, which the researchers first have to develop. The analysis is, therefore, very individual and difficult and can take months or even years.
The astronomers also look eagerly at the data of a current mission of the European Space Agency (ESA). Since 2013, the Gaia spacecraft has been systematically scanning the entire sky, capturing approximately one percent of all stars in our galaxy: the Milky Way. It is a mammoth project that measures over two billion stars with high accuracy and is the first to do so. The Potsdam researchers hope that the mission will also provide information on the number of binary systems in our galaxy, how they move, how many hot subdwarfs exist, and which star interactions take place.
Stephan Geier is an astronomer with heart and soul. Apart from stars, the 40 year old has a completely different, earthly passion: history. Due to "youthful foolishness", he studied this subject in addition to physics, he says with a wink. This youthful spontaneity led not only a degree, but also a second doctorate on German foreign policy after World War II. Even today the researcher is very enthusiastic about history. He is sure that some of this will be incorporated into his teaching. "There are many examples of how science and technology intervene in history; just think of nuclear fission," Geier says. "I would like to take up these examples." He has already reached out to historians at the University of Potsdam but will not reveal any concrete ideas yet. "We will cross that bridge when we come to it." His students have every right to be excited.
Prof. Stephan Geier studied Physics and Ancient History, Classical Archeology and Modern History. Since April 2018, he has been Professor of Stellar Astrophysics at the University of Potsdam.
Text: Heike Kampe
Translation: Susanne Voigt
Published online by: Alina Grünky
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