Astronomers detect echoes from the depth of a red giant star
On March, 17, 2011, an international team of astronomers reported the discovery of waves inside a star
that travel so deep that they reach the core. The discovery was published in the renowned
journal Science, and was possible thanks to precise measurements with the Kepler space
Waves traversing stars,
similar to sound waves here on Earth, were already
known to exist, but up to now only waves in the outer part of the star were observed. They
travel hundred thousands of kilometers deep, and at a certain depth the stellar material
gets too dense to penetrate so that the waves bounce back to the surface. The team now
unexpectedly found the signature of waves that run all the way to the center of the star.
Astronomers love this kind of waves, or stellar oscillations as they call them. Just like a
doctor listens to the sound of your heart to make a diagnosis, or like seismologists use
earthquakes on Earth to probe the inside of our planet, the scientific discipline of
asteroseismology studies stellar oscillations to draw a detailed picture of stellar interiors.
The detection of waves that are able to sense the conditions in the core of a star opens a
window to an inferno which otherwise would remain unreachable and hidden.
The discovery was made in a red giant star. These are elderly stars which our Sun will
become in about 5 billion years. By that time our Sun will have inflated more than 10 times
its current size, and will be about 50 times brighter. At the same time its color will have
changed from yellowish to reddish, hence their name.
'Having a view into the core of these red giants will teach us exactly what will happen to
our Sun when it grows older', says Paul Beck, a PhD student at Leuven university in
Belgium. Paul is one of the many young researchers that are given the opportunity to work
with Kepler data. He, Tim Bedding of The University of Sydney, and Marc-Antoine Dupret
from the University of Liège were the first to notice that some oscillations seemed to
behave differently, 'out of key'. After comparing the observations with theoretical models,
they soon realized that they were looking at waves that feel the conditions in the heart of
At the starʼs surface the oscillations manifest themselves as patches where the
temperature changes slightly, more or less periodically over time. Overall, this causes tiny
variations in the brightness the star, and after the light traveled for hundreds of years
through empty space these changes are now carefully recorded by NASA's space
The team that made the discovery is part of KASC, the Kepler Asteroseismic Science
Consortium, currently one of the largest consortia in astronomy, consisting of more than
440 astronomers specialized in probing stellar interiors. Its headquarters are located in
Aarhus, Denmark. 'Astronomers of all over the world are taking part in this huge effort to
exploit Kepler data to better understand the interiors of stars', says Hans Kjeldsen of
Aarhus university, the coordinator of KASC. 'The measurements provided by Kepler are so
incredibly precise that we see things we never saw before. It's like traveling in a whole new
The spacecraft is expected to operate for at least another 2 years, and will continue to
measure the same stars, making the datasets better every day. For sure to be continued.
Press release and supplementary information
This computer generated animation illustrates waves, or "stellar oscillations" as astronomers
call them, in red giant stars. The amplitude has been exaggerated to make the waves better
visible, and the oscillations are sped up. In reality, one oscillation takes several hours.
In the first scene a crosscut from the surface to the core of the red giant is given.
Colors were chosen so that the eye can easily follow the waves. In the outer region of the star
the gas particles are moving back and forth, compressing and decompressing the gas, quite
similar as for sound waves on Earth. Most waves inside red giant stars fade out towards the core,
but the waves detected now intensify again in the core of the giant star. The second scene
zooms in on the core to illustrate this. Note that the waves inside the core behave quite
differently from the ones in the outer region.
The animation can be downloaded in the following formats:
For a version of the animation and the figures without copyright information please contact Paul Beck.
| Scene 1: Cross cut through the star
|| Scene 2: Zoom into the near core region \
Links, Figures and art work
We acknowledge the work of the team behind Kepler. Funding for the
Kepler Mission is provided by NASA's Science Mission Directorate. Paul
Beck received funding from the European Community's 7th Framework
Programme, ERC grant number 227224 ( PROSPERITY). Joris De Ridder acknowledges
support from the FWO-Flanders under grant No. G.0728.11. Saskia Hekker
acknowledges financial support from the Netherlands Organisation for
Scientific Research (NWO).
Joris De Ridder