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Portrait: © Rob Stevens, KU Leuven, Belgium - Illustration Figure: © Paul Beck, KU Leuven, Belgium - Caption: Starquakes (measured with the NASA satellite Kepler) allowed to discover the spin rate of the cores of red giant stars.
What is the lifespan of a sun-like star? Well, it may not be quite what we thought. The outcomes of EU-funded asteroseismology research conducted by Professor Conny Aerts and her team show that the cores of red giants don’t spin nearly as fast as expected – and this, in turn, means that our understanding of the future of our sun was flawed.
“Asteroseismology is the study of starquakes. It is very similar to the study of earthquakes, in the sense that we use the same methods to get information on the interior of a star, rather than about the interior of our planet,” says Conny Aerts of the Belgian university KU Leuven. In the PROSPERITY project, which was backed by the European Research Council (ERC), she used this technique to check aspects of the theory of stellar evolution, and delivered new insights on the secret life of stars.
There is, of course, one major difference: asteroseismologists can’t take readings in situ. “We have to try to mimic what seismologists studying the Earth get from their seismographs,” Aerts explains. “What we actually measure is the impact of starquakes on the light we receive from the stars.”
PROSPERITY, a five-year project completed in 2013, seized an exciting opportunity to take the technique to new heights: the space-borne observatories CoRoT and Kepler, respectively launched in 2006 and 2009, were beaming back exquisitely precise measurements of minute changes in the light from distant stars. Ground-based observations can’t deliver the same level of detail, as they are distorted by the Earth’s atmosphere, says Aerts.
The two space missions, in contrast, enabled PROSPERITY to study variations of a mere millionth. Sophisticated mathematical methods were used to derive information on individual stars, and particularly on what goes on inside them. Soundwaves travel differently in objects depending on what they are made of, Aerts notes. “If you create a quake in an iron ball, it won’t behave in the same way as a quake in a silicon ball,” she adds.
Asteroseismologists are thus able to derive key parameters such as the size, mass and density of the star, and how long it will be able to sustain the nuclear fusion processes at its core. The speed at which the star’s core rotates is relevant in this respect. It affects the homogeneity of the material inside, and thus the efficiency of its nuclear fusion processes. “If more matter is mixed in the core, the star can live longer,” Aerts explains.
And this is one area where the accepted theory of how stars live fell flat. Analysis of the data revealed that the cores of ageing sun-like stars don’t spin nearly as fast as scientists had assumed. “What we had in the theory was totally wrong,” Aerts comments.
“We found that the core rotation was a factor 100 slower than the theory predicted,” she notes. This finding has implications for many areas of astrophysics, and consequently attracted particular attention.
Along with this unexpected insight, PROSPERITY delivered several other advances. It created new information about tidal forces on binary stars, for example, i.e. stars that orbit around the same central point as a second star. It also built a new instrument for ground-based observations.
As a further important contribution, the project enabled Aerts to involve and train a number of fledgling researchers, several of whom have now embarked on promising international careers. About a dozen young astronomers were thus given an opportunity to reach for the stars.
Aerts is excited that PROSPERITY has rocked assumptions. “The purpose was to test the theory. And luckily the outcome was that the theory was not good,” she comments. “Imagine if we had confirmed the theory – that would have been boring, right?”
Her groundbreaking work in the project won her a prestigious award in 2012: the Francqui prize, also known as the Belgian Nobel Prize. Aerts is the first – and as of 2016 still the only – woman to have received this distinction in the exact sciences, and she doesn’t intend to rest on her laurels.
She is also the first of only two researchers so far to have obtained two successive ERC Advanced Grants in astronomy. A new project is thus already under way, again with support from the ERC. The current endeavour focuses on modelling the seismic processes of massive stars.
It’s known as MAMSIE, an acronym that the full project title was constructed to support – with excellent reason. “That’s the nickname that my kids have given me,” Aerts reveals.