Conny Aerts: "Each star has its symphony, its musical tones"

Space is a continuous symphony of melodies, even though, unfortunately, we cannot hear them from Earth. For a couple of decades, however, space telescopes have allowed us to hear for the first time the unique songs emanating from the hearts of stars, which tell us what has so far been invisible to science and human knowledge: the inner life of these celestial bodies, fundamental building blocks of galaxies and the Universe, and true factories of the materials we are made of. As astrophysicist and science communicator Carl Sagan said, "we are all stardust."

Conny Aerts, director of the Institute of Astronomy at KU Leuven University and holder of the Astrosismology Group Chair at Radboud University in the Netherlands, is a benchmark and one of the brilliant minds who laid the groundwork for listening to these stellar symphonies, key to determining the size, mass, and age of stars. Aerts visited Barcelona this week to give a lecture at CosmoCaixa.

Every night we look at the sky and see small points of light, but we hear nothing. And yet, you claim that we can hear the stars.

— There are sound waves, in the stars, but we don't hear them from Earth. This is because in space there is the vacuum between these celestial bodies and us, and sound cannot travel in a vacuum. And also because the frequency of stellar sound waves is not in the audible range of the human ear. What seismologists like me do is to reveal the symphony of each star.

How?

— At its core, seismic waves are produced that propagate through the interior and have a frequency. We take these musical tones from the waves and sonify them, meaning we convert the data into sounds and shift the frequencies of stellar sound waves to a range audible to humans, always respecting the symphony of each star, because each one has its musical tones.

Who composes this symphony like the star?

— Stars, like our Sun, are spheres of hot gases, real balls of fire. In some of the layers that form them, a truly turbulent movement occurs. We have to imagine it like a pot of water that we put on the fire to boil, and which heats up from below until, finally, it starts to boil. The same happens in a star: heat is generated in the core through nuclear reactions, nuclear fusion occurs; then this heat spreads to other layers and the gas begins to boil, causing the surface of the star to move up and down, as happens in the pot of water. And this vibration of the surface generates a sound.

Sound waves are generated by boiling gas, therefore.

— And they travel through the star, which is the sound cavity. The bigger the star, the more time sound waves need to travel from one end to the other and bounce around the cavity. Therefore, by measuring the time it takes them to do so, we can know if the star is small or large. And this also corresponds to frequencies: the bigger it is, the lower the frequency, and the smaller it is, the higher. Often in talks I give, even with primary school children, I explain that I will help them hear the music of the stars by placing them in the center of one and then sonifying the entire symphony of waves towards their ears, and then they will be able to hear it. I often make them listen to different frequencies and... they don't fail! They know how to identify if the star is bigger or smaller.

They become seismologists for a few minutes.

— Even people with blindness can be, because they also have a very good hearing ability. So, even though they are small dots in the sky at night, frequency is what really tells us if a star is big, bigger than the Sun, or the same as the Sun or smaller. And this frequency changes over time. The Sun is now a yellow dwarf, but it will become a red giant, and when that happens its sound frequencies will lower and we will only hear a kind of cosmic hum.

How do you hear the stars?

— We use satellite data that observe their brightness, and brightness depends on temperature. As in a pot of water, boiling gas has an upward and downward movement: when the gas rises, it cools slightly and the star is less bright. In some areas of the outermost stellar layer, there is also gas that falls towards the core and heats up, which causes its temperature and brightness to increase. They are tiny vibrations, but nevertheless, they change the brightness. If we look at the graph of these vibrations, it resembles the seismic signal that a seismograph would record during an earthquake on Earth. Satellites are the instruments that allow us to observe these variations in brightness, imperceptible to the human eye, which scientists decipher and which allow us to distill the frequencies of sound waves.

Why is it important to know what happens inside a star?

— The life of a star depends on what happens in the inner core, in the deep layers where the temperature is millions of degrees and nuclear reactions occur. These reactions are what will determine how the star will live its life, how it will die. It is very valuable information, because, in the end, stars are the elemental building blocks that make up our universe. Astrosismology allows us to delve deeply into stellar physics, very similarly to how seismologists do on Earth, and to determine the age, mass, and volume of stars with unprecedented precision. It also allows us to answer one of the most fundamental questions: where does the matter we are made of come from.

We are all stardust.

— We are! Stars are the metal factories of the Universe. At the Big Bang, there was only hydrogen and helium, and a little lithium. Touch your hair, your hands, the earrings you wear, the mobile phone you are recording me with... All and each one of the atoms that form all these things were once in a supernova explosion. All chemical elements, the components that matter is made of, are manufactured in stars, throughout their evolutionary path, their life cycle. And that's why we want to understand everything better. Knowing the age of the star allows us to calculate how long it has been capable of producing materials. It is also important to understand how the gas rotates. And this is my main contribution to this field.

Which one?

— Imagine I am playing a melody on the piano on top of a podium that is still. The sound will be pleasant. But now imagine that the podium I am on starts to spin. The melody will be destroyed due to the rotation and the experience of listening to me will be terrible. The same happens with stars. Sound waves move with the frequency of rotation, the Coriolis effect you studied in school. And we can unravel this because we can measure the changes in sound waves created by the rotation of gas inside the star. This is unique! Only asteroseismology can provide this information.

Why is it important?

— Because when it spins, as happens when you stir coffee with milk with a small spoon, the ingredients mix much more efficiently. When the interior of the star spins, the gas mixes the atoms of the different elements very efficiently. If the rotation is fast, more hydrogen is introduced into the core of the star. If it has more hydrogen, the star lives longer, because stellar lifespan is mainly determined by the amount of hydrogen in the core, which can be transformed into helium. Thus, the more rotation, the more fuel to power nuclear fusion. And if the star lives longer, it can manufacture more materials.

His mathematical work has been crucial in deciphering the symphonies of the stars.

— When I was young there was a theory that said that if we could ever observe the frequencies of waves due to stellar vibrations we could deduce physical information from them. I was very attracted to this, and I studied mathematics, and for many, many years I dedicated myself to developing the mathematical models that in the future, if we ever obtained good data, we could use to detect star vibrations. I knew that in the future there would be space missions that would allow us to obtain this data. So I was very patient! At that time I was one of the few people in the world working in this field.

Until the Corot mission was launched, in 2006, and then Kepler and Tess, the two exoplanet hunters.

— Corot was equipped with a very refined detector that allowed it to measure these brightness variations that reveal stellar internal music to us. And Kepler and Tess were based on the transit method to detect planets outside our galaxy: when a celestial body moves in front of a star, the brightness of that star decreases slightly. These instruments measure these small changes, invisible to the human eye, with very high precision, on the scale of one part per million, which is exactly what I need. I remember a conversation with the principal investigator of NASA's Kepler mission, William Borucki. When it was launched, in 2009, I was still a young scientist, and I told him: “You will find exoplanets and it will be a great success, but there will be another, because we will manage to develop asteroseismology from stellar vibrations.” He looked at me incredulously, as if I were saying something foolish. When years later that was the case, he had to admit to me that I was right.

Was it a surprise that it worked?

— Not for me, because I was convinced of it! [Laughs.] But it was a surprise to discover that the rotation inside stars happens differently than we thought. In fact, we were completely wrong, because we assumed it was much faster than it actually is. And if stars rotate more slowly, the mixing of components is also affected. There are aspects of physics that we didn't know. Now we have much better models, because we have evolved the stellar model. We also know that magnetic fields are an additional force, and that there are tidal forces. As with the Earth-Moon system, when there is a system of two stars or a star and a heavy planet, they create tides between them. These tides deform the stars, which stop being spheres when they are close and become flattened. The mathematical description of oscillations cannot be done as if there were a sphere. And it is a huge mathematical challenge that we will try to solve. In fact, we have now been awarded a Synergy grant from the European Research Council to work on this. Together with four other international teams, we will try to improve the models that represent how flattened stars live, how their vibrations can be measured, how they can be interpreted.

You are a reference in this field.

— A domain dominated by men. It hasn't been easy at all to build a career in it; 30 years ago there wasn't as much attention to gender and diversity aspects, and there were hardly any women my age. That's why I've always tried to be a role model and convey to girls and women that they can dedicate themselves to astrophysics. Furthermore, I am a firm believer in teamwork and that the more diversity, the more creativity and the better the work. Egocentric successes do not interest me.