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The childhood of stars shapes the evolution of stars

Scientists now show that the biography of stars is indeed shaped by their early stage.

In classical models of stellar evolution, little importance has so far been attached to the early evolution of stars. Scientists now show that the biography of stars is indeed shaped by their early stage.

From babies to teens, stars in their “young years” are a major challenge for science. The process of star formation is extremely complex and difficult to map in theoretical models. One of the few ways to learn about the formation, structure, or age of stars is to observe their oscillations. “Similar to the exploration of the Earth’s interior using seismology, we can also make statements about their internal structure and therefore also about the age of stars based on their oscillations,” says Konstanze Zwintz. The astronomer is considered a pioneer in the young field of asteroseismology and heads the “Stellar Evolution and Asteroseismology” research group at the Institute of Astro- and Particle Physics at the University of Innsbruck. The study of stellar oscillations has evolved significantly in recent years as the possibilities for accurate observation by space telescopes such as TESS, Kepler and James Webb have improved on many levels. These advances now also shed new light on decades-old theories of stellar evolution.

Stars are called “children” as long as they don’t burn hydrogen into helium in their cores. At this stage they are in the pre-main sequence; after inflammation, they mature and move to the main sequence. “Star research so far has focused mainly on mature stars — like our sun,” said Thomas Steindl, a member of Konstanze Zwintz’s research group and lead author of the study. “Even if it sounds counter-intuitive at first glance, little attention has been paid to the evolution of the pre-main sequence so far because the phase is very turbulent and difficult to model. It is only the technological advances of recent years that will allow us a closer look at the infancy of stars — and thus the moment the star begins to fuse hydrogen into helium.” In their current study, the two Innsbruck researchers now present a model that can be used to realistically represent the earliest stages of a star’s life, long before they reach maturity. The model is based on the open-source stellar evolution program MESA (Modules for Experiments in Stellar Astrophysics).

Inspired by a lecture by astronomer Eduard Vorobyov of the University of Vienna at a meeting in 2019, Thomas Steindl spent months refining the method for using this stellar evolution code to mimic the chaotic phase of early star formation and then predict specific oscillations. “Our data shows that pre-main sequence stars have a very chaotic course in their evolution. Despite its complexity, we can now use it in our new theoretical model.” said Steindl. For example, the astronomer shows that the way the star is formed influences the oscillation behavior even after the ignition of nuclear fusion on the main sequence: “Childhood influences the later pulsations of the star: this sounds very simple, but there are was strongly doubted. The classical theory assumes that the time for ignition is simply irrelevant. This is not true: Similar to a musical instrument, even subtle differences in composition lead to significant changes in tone. Thus our modern models are better describe the vibrations in real stars.”

Konstanze Zwintz is delighted with this discovery and is very optimistic about the future: “I was already convinced about 20 years ago, when I first saw the oscillation of a young star in front of me on the screen, that I might one day be able to prove the importance of early stellar evolution on the ‘adult’ star Thanks to the great work of Thomas Steindl, we have now passed: Definitely a eureka moment for our research group and another milestone for a better understanding of the growth steps of stars.

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