A team of researchers led by Northwestern University has made groundbreaking discoveries about the innate “twinkle” of stars caused by rippling waves of gas on their surfaces. While stars are known to appear to twinkle due to the bending of starlight as it travels through Earth’s atmosphere, this new study focused on the natural “twinkling” of stars.
In this study, the researchers developed the first 3D simulations of energy rippling from the core to the outer surface of massive stars. These simulations allowed them to determine, for the first time, the extent of stars’ innate twinkling. Additionally, the team converted these rippling gas waves into sound waves, providing listeners with an auditory experience of what the inside of stars and their “twinkling” should sound like.
The study’s findings are set to be published in the journal Nature Astronomy on July 27. According to Evan Anders, a postdoctoral fellow in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the leader of the study, motions in the cores of stars create waves similar to those on the ocean. When these waves reach the surface of a star, they cause it to twinkle in a way that astronomers may be able to observe.
Understanding how stars naturally twinkle has significant implications for future space telescopes, enabling them to explore the central regions of stars where crucial elements are formed, essential for sustaining life on Earth. This research opens up new possibilities for investigating the processes that forge the elements we depend on to live and breathe. Anders was advised by study coauthor Daniel Lecoanet, an assistant professor of engineering sciences and applied mathematics in Northwestern’s McCormick School of Engineering and a member of CIERA.
Exploring Chaotic Convection
In all stars, there exists a region called the convection zone, a dynamic and chaotic space where gases churn to transport heat outward. However, for massive stars, which are at least about 1.2 times the mass of our sun, this convection zone is located at their cores.
Evan Anders, a researcher leading the study at Northwestern University, compares convection within stars to the process that fuels thunderstorms. In this turbulent process, cooled air descends, warms up, and rises again, creating a mechanism for heat transfer.
As a consequence of this convection, small rivulets or waves form, causing the starlight to flicker subtly, resulting in a twinkle. Given that the cores of massive stars are not directly observable, the team of researchers, led by Anders, endeavored to model the hidden convection. Their simulations built upon previous studies that explored the properties of turbulent core convection, wave characteristics, and potential observational features of those waves. By incorporating all relevant physics, the new simulations accurately predict how a star’s brightness changes depending on the waves generated by convection.
Understanding Convection-Driven Twinkling Phenomenon
Once convection generates waves inside a simulated star, these waves exhibit intriguing behavior – some emerge to the star’s surface, creating the twinkling effect, while others get trapped and continue bouncing around. To distinguish the waves responsible for the surface twinkling, Evan Anders and his team devised a specialized filter that characterizes how waves move within the simulations.
Analogous to the padded walls in a recording studio that dampen acoustics, the researchers placed a damping layer around the star to precisely measure how core convection generates waves. This setup enabled them to extract the “pure sound” of the waves. Drawing inspiration from the process of recording music, where musicians apply filters to engineer their recordings, the team applied their filter to the pristine waves coming from the convective core. By tracing the path of these waves within a model star, they discovered that their filter accurately described how the star altered the waves originating from the core.
Subsequently, the researchers developed another filter to represent how waves should move inside an actual star. By applying this filter, they were able to simulate how astronomers might observe these waves using a powerful telescope. Stars experience slight fluctuations in brightness due to dynamic internal phenomena, and the twinkling caused by these waves is exceptionally subtle, beyond the sensitivity of our eyes. However, future telescopes equipped with greater capabilities may be able to detect and study this elusive twinkling phenomenon.
Exploring the Possibility of Music in the Stars
Delving deeper into the recording studio analogy, Evan Anders and his collaborators embarked on a fascinating exploration, converting their simulations into audible sounds. Although the waves generated by convection in stars lie beyond the range of human hearing, the researchers ingeniously adjusted the frequencies to make them perceptible.
Depending on the size and brightness of a massive star, convection gives rise to waves that produce distinctive sounds. For instance, waves emerging from the core of a large star evoke the image of a warped ray gun, firing through an otherworldly landscape. However, as these waves reach the star’s surface, the sounds are altered, with the ray gun-like pulses transforming into a low echo reverberating through an empty room.
In contrast, surface waves on medium-sized stars conjure images of a persistent hum resonating through a windswept terrain, while those on small stars sound like a plaintive alert from a weather siren.
To further explore the interplay between music and stars, Anders and his team passed well-known songs through various sizes of massive stars. Songs like “Jupiter” (from Gustav Holst’s “The Planets” orchestral suite) and “Twinkle, Twinkle, Little Star” were propagated through the stars. The result was haunting and distant melodies, akin to something out of “Alice in Wonderland.”
Anders expressed his curiosity about the transformation of songs when heard as propagated through a star. As the stars alter the music, they simultaneously influence how the waves would appear as twinkling on the star’s surface, presenting an intriguing relationship between sound and stellar behavior.
This groundbreaking study, “The photometric variability of massive stars due to gravity waves excited by core convection,” was made possible with support from CIERA, NASA, and the National Science Foundation.
Story Source:
Materials provided by Northwestern University. Original written by Amanda Morris. Note: Content may be edited for style and length.
Journal Reference:
- Evan H. Anders, Daniel Lecoanet, Matteo Cantiello, Keaton J. Burns, Benjamin A. Hyatt, Emma Kaufman, Richard H. D. Townsend, Benjamin P. Brown, Geoffrey M. Vasil, Jeffrey S. Oishi, Adam S. Jermyn. The photometric variability of massive stars due to gravity waves excited by core convection. Nature Astronomy, 2023; DOI: 10.1038/s41550-023-02040-7