Many people know that stars twinkle because our atmosphere bends starlight as it reaches Earth. But stars also have a natural "scintillation" - caused by ripples of gas on their surface - that is currently undetectable by telescopes on Earth. In a new study, a team of researchers led by Northwestern University has developed the first three-dimensional simulation of energy rippling from the core of a massive star to its outer surface. Using these new models, researchers have determined for the first time how much stars should naturally twinkle.
Scientists at Northwestern University have developed for the first time three-dimensional simulation technology to study energy ripples from the core to the outer surface of a massive star, providing a new perspective on the inherent "scintillation" of stars. The team also converted these waves into sound, allowing listeners to "hear" the interior of the star and its natural flickering. Source: E.H.Andersetal.
The team also converted these gas ripples into sound waves for the first time, allowing listeners to hear the sounds of the star's interior and "shimmers." It's so fascinating. The research was published in the journal Nature Astronomy.
"The motion of a star's core creates waves like an ocean," said Evan Anders of Northwestern University, who led the study. "When the waves reach the star's surface, they cause the star to flicker, and astronomers may be able to observe this flicker. For the first time, we have developed a computer model that allows us to determine how much the star flickers due to these waves. This work allows future space telescopes to detect the central region where stars forge the elements we live and breathe."
Anders is a postdoctoral fellow at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Daniel Lecoanet, co-author of the study report, assistant professor of engineering science and applied mathematics at Northwestern University's McCormick School of Engineering and CIERA member, provided guidance.
A three-dimensional simulation of how turbulent convection in a large star's core (centre) creates ripples that ripple outward and resonate near the star's surface. By studying changes in a star's brightness caused by vibrations, scientists may one day better understand processes deep within the cores of large stars. Image source: E.H.Anders et al./"Nature-Astronomy" 2023
All stars have a convection zone, a volatile, chaotic region where gas churns, pushing heat outward. For massive stars (at least 1.2 times the mass of the Sun), the convection zone is located at the core of the star.
Convection currents inside stars are similar to the processes that fuel thunderstorms. The cooled air falls, heats up, and rises again. This is a turbulent process that transports heat. It also creates waves - small streams that cause the starlight to dim and brighten, creating subtle twinkles. Because the cores of massive stars are obscured, Anders and his team tried to simulate their hidden convection currents. After studying the properties of convection in the turbulent core, the characteristics of the waves, and the observational features these waves might have, the team's new simulation incorporates all the relevant physics to accurately predict how the star's brightness changes in response to the waves generated by the convection.
After convection creates waves, these waves bounce inside the simulated star. Some waves end up on the star's surface, creating a scintillating effect, while others get trapped and continue to bounce around. To isolate the waves that are emitted to the surface and create the scintillation effect, Anders and his team built a filter that describes how the waves bounce inside the simulated star.
"We first put a damping layer around the star - like the padded walls in a recording studio - so we could measure exactly how the core convection creates the waves," Anders explains.
Anders likens it to a music studio, using soundproof padded walls to minimize the acoustics of the environment so musicians can extract the "pure sound" of the music. The musicians then apply filters and engineer these recordings to achieve their desired effect.
Play Gustav Holst's "Jupiter" through three sizes of massive stars. Source: Northwestern University
Likewise, Anders and his collaborators applied their filter to the pure waves they measured emanating from the convective core. They then tracked the waves bouncing around a model star and ultimately found that their filter accurately described how the star altered the waves coming from the core. The researchers then developed a different filter that describes how waves bounce inside real stars. After applying this filter, the resulting simulation shows how astronomers expect the waves to appear when observed through powerful telescopes.
"The brightness or dimming of a star depends on various dynamic changes occurring within the star," Anders said. "The flicker caused by these waves is so subtle that our eyes are not sensitive enough to see it. But powerful telescopes in the future may be able to detect it."
Anders and his collaborators took the recording studio analogy one step further and next produced sounds using analog. Because these waves are beyond the range of human hearing, the researchers evenly increased the frequency of the waves, making them clearly audible.
Depending on the size or brightness of the massive star, the waves produced by convection correspond to different sounds. Waves from the core of a large star, for example, sound like a twisting ray gun blasting across the alien landscape. But when these waves reach the star's surface, the star changes these sounds. For large stars, the ray gun-like pulses turn into low echoes that reverberate through empty rooms. Waves on the surface of medium-sized stars, on the other hand, are reminiscent of the constant buzzing sound produced by wind-driven earthquakes. And the waves on the small star's surface sound like the bland sirens of a weather siren.
The visuals of Little Star play through three sizes of massive stars. Source: Northwestern University
Next, Anders and his team played the song through different stars and listened to how the stars changed the song. They passed short audio clips of "Jupiter" (a movement from composer Gustav Holst's orchestral suite "The Planets") and "Twinkle Twinkle" through massive stars of three sizes (large, medium and small). As they travel through the stars, all the songs sound distant and lingering -- like songs from "Alice in Wonderland."
"We were curious about what a song would sound like if it traveled through a star," Anders said. "The star changes the music and, in turn, changes what the waves would look like if we saw them flashing across the star's surface."