Twinkle, Twinkle, Giant Star

Art Courtesy of Luna Aguilar.

A dying star shimmers and twinkles as its inner core, formerly a churning dynamo, sputters out its final breaths. Nuclear fusion combines atomic nuclei to birth new heavy elements that will soon occupy the cosmos. And then it happens: a fiery explosion, where the insides of the star fly everywhere. Left in the aftermath of the chaos is either a neutron star or a black hole.

For astrophysicists back home on Earth, understanding the characteristics of these explosive dying stars is the key to understanding our past and current universe—everything from star formation to galaxy evolution to the very beginnings of our universe. But to really understand the characteristics of these stars, we need to start from the bellies of the beasts: the processes within these stars. This is done with asteroseismology. Asteroseismology applies the techniques of seismology—which uses waves to understand the interior of the Earth—to stars. In order to understand what is left behind after a star dies, you have to understand its internal structure back when it was still alive. This structure includes everything from the eddies of rotating plasma that twist deep within the furnace of the core to the light and heat that jostles from its surface.

So, how do we peer inside stars? We must turn an eye to their light. How bright is it? Does it change over time? Is it high-energy like an X-ray or is it low-energy like a radio wave? The answers to each question reveal a wealth of information on the energy released by the star: its temperature, its stability, and much more. By making predictions for what light outputs should look like, we can test the actual light of stars against our assumptions to see how well our physics match up with the real world. Any discrepancies reveal new avenues for future scientific exploration.

For astrophysics postdoctoral researcher Evan Anders and his research group at Northwestern University, one particular real-world signal caught their attention: red noise. Red noise is a ubiquitous, low-frequency twinkling in the light signals from massive, stable stars—much like the static on a blank TV channel. These stars are called main sequence stars, a category which most stars, like our Sun, belong to. Real-life observations about a star allow researchers to eliminate possibilities and therefore gain a more precise understanding of the internal mechanics of the star.

“We hoped this red noise was gravity waves because gravity waves give you a lot of information about the structure of the star,” Anders said. “They’re telling you about how big the core is.” Gaining a more lucid understanding of the inside of the star, such as the size of its core, allows us to better understand the energy that the star releases and define the pressure it uses to create new elements. While scientists do have simple models for this task, these models fail to align with real-world data. They need a more detailed model built on empirical evidence, and gravity waves could provide that evidence.

But what is a gravity wave? We can see an example here on Earth: storms and winds push the ocean waters up, creating waves. But the Earth’s gravity resists this upward movement, causing the wave to be pulled downwards, creating an oscillatory displacement. A similar thing happens in stars. “[In gravity waves], you displace this [fluid] from where it wants to be and gravity pushes it back down—and then you get this wiggly pattern,” Anders said.

Deep within the centers of stars, nuclear fusion of hydrogen into helium creates an inferno, generating immense amounts of bright hot plasma that have nowhere to go but out. When this material reaches the very edge of the core, it breaks free in a fourteen-day-long ripple before sinking back into the heart of the star. As fusion continues, the core roils with these cycles of hot to cool to hot to cool, churning gravity waves across the core. These waves propagate through the rest of the star, reverberating at different frequencies like guitar strings.

Modeling these waves with a supercomputer is extremely difficult, but Anders and his team designed a clever way of mimicking the red noise. Thanks to earlier models by one of the researchers (Northwestern fluid dynamicist and assistant professor Daniel Lecaonet), Anders and his team had previous models of wave formation and propagation that could be tweaked for higher accuracy. 

Anders’ simulations can be compared to a music studio—recording raw music before passing it through a filter to create a specific effect. Anders’ ‘filter’ consists of code that translates the waves created by core convection—showing what they would look like distorted by the rest of the star outside of the core—and thus what the waves actually look like leaving the star and reaching our eyes as light. 

The scientists created their filter based on a simpler model of how stars worked. The idea was that it would be easy to predict what the light signals from this rudimentary filter should look like. If the output of the program matched their predicted output, the scientists could go back and painstakingly craft a more advanced filter with all the physical complexities of the star—a filter with enough refinement to see unique signals such as the red noise. 

Their basic filter demo passed with flying colors, and it was then time for the real deal. Anders’ team set to work crafting a more accurate filter, one that captured the intricacies of the star’s mechanics outside of the core, reflecting the true polyphony of physical effects of a star rather than just a few. If the glimmer of signal leaving the filter matches the hum of the red noise, then gravity waves are the source of the red noise.

However, when Anders combined the waves generated by convection and the echo of gravity waves outside the core, the difference between the output signal and red noise was glaring. Their simulation revealed that gravity waves are far too muted to match the high-amplitude signal of red noise. But for scientists, a definitive no is just as exciting as a definitive yes. Knowing what the red noise isn’t brings astronomers closer to understanding what it really is. The next theory in line is that the red noise comes from motions closer to the star’s surface. 

Anders has two directions he might take his future research. He might want to take the elaborate programs he developed here to further explore the waves within stars. “We use the amplitude of the wave to learn something about the process that’s driving it,” Anders said. The second direction, on the other hand, would be refining his simulation further. “[We add] rotation, because stars, well, rotate,” Anders said.

There are several possibilities for improvement in the team’s research. They did not factor in the rotations that affect the cycles of plasma, nor did they include the effects of magnetic fields. However, their work still proves to be an important step in understanding the inner workings of stars. Listening to asteroseismology’s music of the spheres brings us closer to understanding the massive stars that churn in our universe.