2022 Kavli
Prize in
Astrophysics

Citation from the Committee

The study of observed oscillations on the Sun’s and on stellar surfaces, helioseismology and asteroseismology, couple mathematical modeling with space science technology. This study involves data-analysis methods such as time series analysis, pattern recognition, and statistical modeling, while relying on various fields of physics and chemistry such as thermodynamics, nuclear and atomic physics, and quantum mechanics. The bridging of these scientific fields, starting from the appropriate observational input, allows the extraordinarily precise determination of the physical properties of stellar interiors. Roger Ulrich led the theoretical foundation of the field, while Conny Aerts and Jørgen Christensen-Dalsgaard have extended its reach to stars of all masses in various evolutionary stages, using both ground- and space-based observations.

Roger Ulrich was the first to show that the oscillations observed on the solar surface can be used to make precise measurements of the characteristics of the interior of our nearest star. Ulrich derived the equations of helioseismology and interpreted the oscillatory surface phenomena as the surface response to interior eigenmodes. He furthermore predicted that discrete eigenfrequencies would be discovered, deriving the general behaviour of the relation between frequency (ω) and spatial wavenumber (k): To quote his original paper “…the 5-minute oscillations are acoustic waves trapped below the solar photosphere and that power in the (k,ω) diagram should be observed only along discrete lines.” Solar, and indeed stellar, oscillations contain information on stellar structure. The excellent agreement between solar calculations and observations was amongst other things a key ingredient in convincing physicists that the solar neutrino problem could only be solved by revising standard electroweak theory. Ulrich continued to lead the field with groundbreaking theoretical and observational programs over several decades.

Jørgen Christensen-Dalsgaard further developed the topic of helioseismology and played a major role in the “first generation” development of asteroseismology. Through the 1980s and 1990s, he determined the sound speed profile throughout the Sun, its 2-dimensional rotation map, its helium content, and the level of helium settling at the base of its convection zone. Christensen-Dalsgaard saw the great potential of applying this science to other stars in the Milky Way, fully realized with the launch of the CoRoT, Kepler, and TESS planet finding missions. Christensen-Dalsgaard participated actively to the preparation and exploitation of later missions. Asteroseismology gave rise to sizing, weighing and age-dating tools that have been applied to thousands of stars in the galaxy, including exoplanet hosts. Asteroseismology of cool stars revealed that the core rotation of subgiants and red giants required major fixes in the theory of angular momentum in stellar interiors.

Conny Aerts is a leading figure of the “second generation” of asteroseismology. Her remarkable involvement in observational approaches to asteroseismology, both from the ground and from space, using data from CoRoT, Kepler, and TESS, has had a very strong impact. While Christensen-Dalsgaard’s work is mainly concerned with low mass cool stars and the Sun, Aerts is widely known for her work on massive hot stars, extending the impact of asteroseismology across the Hertzsprung-Russell diagram. She is a leader in probing the interaction between pulsation, rotation, and stellar winds through an integrated approach, taking advantage of changing spectral line shapes, as well as photometric variations. In particular, she developed clever methods to identify pulsation modes in massive stars, opening the door to the modeling of their interiors. Aerts also pioneered a rigorous methodology to identify and model gravito-inertial modes in rapidly rotating stars, allowing estimates of rotation and mixing in stars with masses between 1.3 and 40 solar masses. Thus, her work has enabled the first quantitative estimates of near-core and envelope mixing, leading to significant improvements in stellar evolution theory.

Stellar astrophysics has been revolutionized by the data made available from planet hunting spacecraft - but this has only been possible given the extensive theoretical and observational groundwork done by these individuals and their collaborators.

Finally, Conny Aerts, Jørgen Christensen-Dalsgaard, and Roger Ulrich have also spent vast amounts of energy training and inspiring the next generations of helio- and asteroseismologists. All three will undoubtedly be remembered as founders of this extraordinarily effective tool that has transformed our understanding of stellar structure and evolution.

Sound waves allow us to probe the depths of stars

Just as seismologists measure the seismic waves produced by earthquakes to better understand our planet's interior, scientists are now able to study the insides of the sun and other stars thanks to their observations of stellar vibrations. Those investigations, known as helioseismology and asteroseismology, have been pioneered by the three prizewinners of this year’s Kavli Prize in Astrophysics – Conny Aerts, Jørgen Christensen-Dalsgaard and Roger Ulrich.

By Edwin Cartlidge, science writer

Roger Ulrich made the key theoretical breakthrough that led to the field of helioseismology in 1970, a few years after it was observed that the sun's surface moves up and down by a few kilometers about once every five minutes. Researchers at the time thought that these oscillations were restricted to the solar atmosphere, but Ulrich came to a different conclusion after modelling the sun as a ball of hot gas and realizing that they were the result of sound waves bouncing around inside the solar sphere.

Musical instrument
Ulrich showed that the sun behaves a bit like a musical instrument, ringing with large numbers of acoustic waves each having a resonant frequency – just as an organ pipe or a plucked violin string acquires its distinctive sound from the combination of many overtones. Those waves are generated by the motions of convection that arise in the solar interior as heat from the nuclear reactions in the sun's core makes its way slowly to the surface, where it is radiated into space. We can't hear the oscillations directly since sound isn't able to traverse the vacuum that separates us from our star, but we infer their existence via the frequency shifts and intensity variations they impart in solar spectral lines observed with periods of roughly five minutes.

These sound waves can tell us plenty about the sun itself. Vibrations with different horizontal wavelengths reach different depths within the sun – some bouncing back and forth along the underside of the surface while others get close to the center – and thereby become encoded with information about the respective solar regions. The key parameter is a wave’s speed, which depends chiefly on the local temperature of the sun. With the help of numerical models, this information in turn reveals several things about the sun’s hot plasma, including its density, original chemical make-up, and the fusion reactions that have given rise to its current state.

New thinking on neutrinos
Beyond solar physics, however, helioseismology has also proved crucial to more fundamental research. When scientists first intercepted the exceptionally inert subatomic particles known as neutrinos produced by solar fusion reactions, they detected far fewer particles than they had expected. The very close agreement between solar theory and the results of helioseismological measurements convinced physicists that their understanding of the sun was not to blame for the mismatch. This led them to overhaul their thinking on neutrinos themselves – concluding that the particles must have mass, since this would allow them to change form as they travel to Earth.

Space missions
Beyond his theoretical studies, Ulrich worked hard in the 1970s and early 1980s to get the Solar and Heliospheric Observatory (SoHO) mission approved by the American and European space agencies. The goal of making ever more precise measurements of the sun's oscillations was also shared by Christensen-Dalsgaard, who played an important role in developing SoHO. Over the following two decades, he helped set up many programs to this end – looking to exploit both widely distributed terrestrial observatories and space missions to guarantee continual observations of the solar surface. With the ensuing data, he was able to make advances in several areas, among them improved calculations of the sun's rotation at various depths, thanks to the fact that sound waves travelling with the rotation are slightly sped up while those propagating in the opposite direction slow down.

What's more, Christensen-Dalsgaard realized that such measurements could be made for stars other than the sun. In particular, he saw the great potential of data from satellites funded with a headline-grabbing objective – the search for planets orbiting other stars. Starting with the French CoRoT satellite and moving on to NASA's Kepler and more recent TESS observatories, these missions have provided very precise measurements of intensity variations from thousands of stars in the Milky Way –
yielding seismic information about the structure and evolution of our galaxy from the properties of its stars.

Understanding stellar structure
While Christensen-Dalsgaard has focused mainly on the output from cooler, lighter stars similar to the sun (such as yellow and red dwarfs), Conny Aerts, in contrast, has switched attention to heavier, hotter (bluer) objects – and in doing so extended the study of asteroseismology across the full range of observable stars. These studies have shed new light on the functioning of rapidly rotating heavier stars, allowing estimates of internal rotation and the mixing of elements in stars more than 10 times the solar mass – yielding significant improvements to the understanding of stellar structure.

In particular, Aerts has studied what are known as g-modes. In contrast to sound waves (p-modes), which propagate thanks to differences in pressure, g-modes respond to the force of gravity – being essentially waves of buoyancy. They are harder to detect than p-modes since they are generally confined to stars’ inner regions near their cores and require a large stellar mass to yield visible surface oscillations. Aerts and her colleagues discovered the two types of waves together in heavy blue stars and in red giants – and in doing so deepened our understanding of stellar evolution.

Further Reading

Astrophysicist Roger Ulrich Honored for Unlocking New Ways of Peering Inside Our Sun (AAAS)