Until recently, it was impossible to study the insides of stars, but studying starquakes is changing that. “In classical astronomy we had to make do with the radiation that comes to us from the surface of the star, but by studying the properties of starquakes we can see directly inside the star,” says Conny Aerts (55).
Aerts is a professor at KU Leuven and Radboud University and one of the pioneers of asteroseismology, the field that deals with these complex vibrations in stars in order to get to know the inside of the stars. It has only been about fifteen years since we have been able to measure and process these quakes on any scale, but the field has already identified fundamental shortcomings in star physics and chemistry. Aerts’ contribution to this was awarded in the Netherlands this year with a foreign membership of the KNAW.
How do those star tremors come about?
“They arise in different ways. Firstly, by convective movements inside stars and the pressure waves, a kind of sound waves, that are created by them. We mainly encounter this shape in stars such as the sun. For these kinds of waves I often envision a concert hall in a sphere, where someone then strikes a gong. The convective movements are then the gong.
“There are also other ways to create a starquake, which you mainly see in more massive stars. Stars have nuclear fusion deep inside them, which creates light particles, photons, that radiate outward. Some stars absorb this radiant energy in certain layers and then convert it into kinetic energy. Those are gravitational waves. They have much longer periods, and they also move differently. They are more horizontal movements, while the sound waves move from the inside out. I often compare gravitational waves with waves at sea. If you put a cork in those waves, it will mainly move horizontally.”
I often compare it to a concert by an orchestra. You can also immediately recognize individual instruments in this as a person
How do we measure those quakes?
“The waves cause up and down movements on the surface of the star. Suppose you are a gas particle on a star and a tsunami passes by, then you go up and down. In the largest earthquakes, those variations are up to 10 percent of the star’s radius, but in the Sun it’s only a few thousandths of the radius. The satellites we use do not measure up and down movements, but the variations in brightness caused by those movements. And that’s actually the surface temperature, because the star’s brightness is mathematically determined by its temperature to the fourth power.
“If a gas particle moves slightly outwards, it becomes slightly colder and when it moves inwards, it becomes slightly warmer, because it then comes closer to the core of the star. As a result, the brightness of the star always varies. So we measure the fluctuations around the average brightness.
“That is the net effect composed of all the individual vibrations. We use time series measurements with the brightness of a star as a function of time. From that very long series of measurements we then obtain the frequencies of the various earthquakes that occur simultaneously.
“In a star we measure a very complex pattern in brightness. In this way we recognize the different frequencies of the individual quakes. I often compare it to a concert by an orchestra. As a human being, you can immediately recognize individual instruments in this too. Those individual frequencies are determined by a star’s internal physics. The starquakes are therefore not the goal in themselves, but that is my way of studying the insides of the stars.”
For this kind of research we need very long measurement series
What do the quakes say about that?
“The value of the frequency of those vibrations is determined by the size of the star, by the amount of material it contains and by the chemistry of the star. We can learn a lot from it.”
The field is still quite young, how did it come about?
Proud: “It originated in Belgium. On a theoretical level, the field is already about 70 years old. The Liège professor Paul Ledoux is the godfather of the starquakes. He made calculations purely with pencil and paper of how a sphere would behave if there were up and down movements in it.
“But we have only recently been able to properly measure the star quakes. We’ve been doing that for about 15 years now from space using scientific satellites.
“The measurements of observatories are always interrupted, because you cannot measure the stars during the day and the stars are not visible all night. For this kind of research, we need very long series of measurements, because the tremors of the stars that I find interesting occur with periods of one day to two days. If you can only measure a few hours each time, you will not be able to unravel them. You really need years of continuous measurements for some types of stars.”
The image of the starquakes shows us that the star physics and chemistry we have now is not sufficient
What are the successes so far?
„An important discovery has to do with the internal rotation of the stars, which is a Leuven discovery from 2012. As a star gets older, she gets bigger. You’d expect her to spin slower on the outside, similar to a figure skater extending his arms out for a pirouette. The inside would then spin faster. Based on that theory, we have predicted the life of stars for 30 years.
“We now see, based on starquakes, that the slower spinning on the outside is correct, but the spinning on the inside of red giants is a factor of a hundred slower than we thought. We saw that based on shifts in the frequencies of the quakes. We are still looking for the mechanism that explains the slower rotation.
“That difference in rotation changes the age stars can be, because rotation causes the chemicals in the star to mix. If you’re wrong on that spin, you’re wrong on chemistry. So the image of the starquakes shows us that the star physics and chemistry that we have now is not sufficient.”
ESA’s Plato mission in 2026 will be the first major mission truly designed for asteroseismology. What are your expectations?
Aerts beams when she talks about the new mission. “I sometimes call that mission my third child. I have been involved in the project from the very beginning. To properly measure starquakes, you must have a high cadence in terms of measurements. For example, starquakes on the sun rise and fall in 5 minutes, then measuring every half hour is not enough. Kepler [een eerdere ruimtetelescoop van NASA] could measure every minute at five hundred stars simultaneously. Plato can measure hundreds of thousands of stars at once every 25 seconds. That will generate a flood of new data.”