A team of researchers led by Northwestern University in the United States has developed the first 3D simulations of energy ripples from the core of a massive star to its outer surface. Using these new models, the researchers determined, for the first time, how much stars should innately twinkle.

And, for the first time, the team also converted these gas waves into sound waves, allowing both the insides of stars and their “scintillation” to be heard, as published in the journal Nature Astronomy.

Many people know that stars appear to twinkle because our atmosphere bends starlight as it travels toward Earth. But stars also have an innate "twinkle," caused by ripples in gas on their surfaces, that is imperceptible to today's ground-based telescopes.

"Motions in the cores of stars launch waves like those in the ocean," explains Evan Anders, a postdoctoral fellow at Northwestern's Center for Interdisciplinary Research and Exploration in Astrophysics (CIERA), who led the study. "When the waves come to the surface of the star, cause it to twinkle in a way that astronomers may be able to observe. For the first time, we have developed computer models that allow us to determine how much a star should twinkle as a result of these waves. This work will allow scientists to future space telescopes will probe the core regions where the stars forge the elements on which we depend to live and breathe."
All stars have a convection zone, a wild and messy place where gases churn to push heat out. In massive stars (at least 1.2 times the mass of the Sun), this convection zone is in the core.

    "Convection inside stars is similar to the process that powers thunderstorms," Anders said. "Cooled air falls, warms, and rises again. It's a turbulent process that carries heat."

    It also creates ripples, little rivulets that dim and lighten the starlight, producing a subtle twinkle. Since the cores of massive stars are hidden from view, Anders and his team tried to model their hidden convection. Based on studies examining the properties of turbulent convection in the core, the characteristics of the waves, and the possible observable features of those waves, the team's new simulations include all the relevant physics to accurately predict how a star's brightness changes over time. function of the waves generated by convection.
After the convection generates waves, they bounce off the interior of the simulated star. While some waves end up hitting the star's surface to produce a scintillation effect, others get trapped and keep bouncing around. To isolate the waves that come to the surface and create the scintillation, Anders and his team built a filter that describes how waves bounce around inside the simulations.

    "We first put a damping layer around the star, like the padded walls of a recording studio, so that we could measure exactly how convection from the core produces the waves," Anders says. He likens it to a music studio, which takes advantage of soundproof padded walls to minimize the acoustics of an environment so that musicians can extract the "pure sound" of music. The musicians then apply filters and design those recordings so that the song comes out the way they want it to.

    Similarly, Anders and his collaborators applied their filter to the pure waves they measured leaving the convective core. Next, they followed the waves bouncing off a model star and found that their filter accurately described how the star modified the waves coming from the core. Next, the researchers developed a different filter to determine how waves should bounce inside a real star. With this filter applied, the resulting simulation shows how astronomers expect the waves to appear if viewed through a powerful telescope.
"Stars get a little brighter or a little dimmer depending on various things that happen dynamically inside the star," he says. "The flickering caused by these waves is extremely subtle, and our eyes are not sensitive enough to detect them." see it. But future powerful telescopes could detect it."

    Taking the recording studio analogy a step further, Anders and his collaborators then used their simulations to generate sound. Since these waves are out of the range of human hearing, the researchers uniformly increased the frequencies of the waves to make them audible.

    Depending on how big or bright a massive star is, convection produces waves that correspond to different sounds. Ripples emerging from the core of a large star, for example, emit sounds similar to those of a warped ray gun traversing an alien landscape. But the star alters these sounds when the waves reach its surface.

    In a large star, the ray gun-like impulses become a low echo that reverberates through an empty room. Ripples on the surface of a medium-sized star, by contrast, evoke images of a persistent hum across windswept terrain. And the surface waves of a small star sound like the plaintive alert of a weather siren.

Next, Anders and his team played songs through different stars to hear how the stars change the songs. They ran a short audio clip of “Jupiter” (a movement from composer Gustav Holst’s orchestral suite “The Planets”) and “Twinkle, Twinkle, Little Star” through three sizes (large, medium and small) of stars. massive. Spreading across the stars, all the songs sound distant and haunting, like something from “Alice in Wonderland.”

source: https://www.nature.com/articles/s41550-023-02040-7