In order to understand our Universe, one has to start from its main
constituent which are the stars. Since the beginning of the XXth
century, scientists have been able to analyse the light coming from the
stars. They have derived with more and more accuracy the chemical
composition of the stellar matter as well as informations about the
physical conditions of the stellar surface layers.
However, the opacity of a star is so large that we can only see through a very tiny layer surrounding the star, which is called the photosphere. One of the main dreams of astrophysicists has always been to enter the star down to its center and to know what really happens there.
In the meantime, the theory of stellar structure and stellar evolution
was making giant steps and models of stars were being built which could
be confronted with the observations. The goal was to compare a whole
theoretical structure where complex physical processes are taking place
to the only observations we had consisting of some properties of its
surface. Can you possibly know what there is inside an orange by just
looking at its surface ? The one and only way to probe the
internal layers of a star is through asteroseismology.
The best way of understanding what asteroseismology means comes from musical instruments. Take a violin. The music, in all its tones and harmonics, that comes out of this instrument is caracteristic of the instrument itself. Even the best gifted violonist cannot make a violin sounds like a flute or even a cello. Each instrument has what is called a system of eigenfrequencies which is like the genetic code of the instrument. Knowing this code is like peeking through the whole instrument.
In the early sixties, following the violent earthquake of Chile,
oscillations of this type were detected for the Earth. At that time, the
internal structure could be theoretically described with some rather
crude estimations. From these models, the eigenfrequencies were then
computed and they showed that many transition limits were badly located.
This was the first victory of seismology.
In the seventies, it became clear that the Sun was also oscillating in a variety of modes with periods around five minutes. This was the starting point of helioseismology. Huge efforts have been made since then to identify these modes with ultrahigh accuracy either from ground-based observations or from space-borne experiments.
Achievements have been spectacular. Theoretical solar models were not correct. First, the adopted Helium abundance was wrong which meant that a physical process called diffusion, ignored in the theoretical computations, was acting in the Sun. Second, the convective envelope had not the right extension. And third, and probably the most important result, a description of the way all the internal layers were rotating could be found. And all this, just by knowing what the frequencies of oscillation were.
Stars can be very different from the Sun. Taking into account only their
surface properties we can put them in a diagram which is called the
Hertzsprung-Russell diagram or HR diagram.
|HR diagram with variable stars|
This is another HR diagram with the location of variable stars. In fact,
most stars do belong to a class of variable stars.
|Overshooting in a Scuti star|
Just to illustrate this point, here is an example with a
Scuti star similar to what will be
observed with the COROT experiment.
We would like to acknowledge support from an SSTC/PRODEX contract.