Helio- and Asteroseismology



The reason why we want to do so much detailed work on observing oscillations on the Sun and on other stars is that we want to improve our models and our understanding of the physics of stellar interiors. For this reason we have been constructing models of stars for a very long time, in the last few decades using extensive computer programmes -- the physical effects that are included in the models are now so extensive and varied that we can no longer do the calculations using just paper and pencil. The more details we get from the observations, the better the models that we can construct, and the greater is our understanding of the stars. Making stellar models involves a number of physical principles :
  • Thermodynamics : The Sun and stars shine, emitting energy into the rest of the Universe and they therefore lose energy from their interior. From the colour of the light coming from the Sun (or a distant star) we know the temperature on the surface. That temperature is so high that we immediately know that stars cannot be anything other than plasma : highly ionized gas. The physics that applies to stars is therefore the physics of hot gas and radiation.
  • Hydrostatic equilibrium : At every point inside the Sun and in stable stars (and most stars are) the force of gravity must be balanced precisely by the gas pressure. Just as the pressure on your body increases as you dive deeper and deeper into the sea, so does the pressure increase as you go deeper and deeper into the Sun.
  • Nuclear physics : The Sun has been shining steadily for at least as long as the earth has existed. The output of energy (light) is enormous, and so we know of only one source of energy that can sustain that output for so long : nuclear fusion of light elements. In the Sun the energy is generated by four hydrogen nuclei fusing into one helium nucleus.
  • Atomic physics : The energy that is produced in the central parts of the Sun is transported outward by radiation throughout most of the solar interior. To describe this transport of energy we need to be able to calculate how radiation is absorbed and emitted by the matter inside the Sun, which requires a detailed knowledge of atomic physics.
  • Hydrodynamics : In the outer parts of the Sun the transport of energy occurs through the rising of elements of hot gas and the sinking down of cold gas : this process is called convection. The details of the gas motions are very complicated and not at all well understood, but we can make extensive computer models of the processes using hydrodynamical simulations.
To construct a model of the Sun as it is today, we must follow its evolution. The reason for this is that the structure of the Sun changes due to the fact that hydrogen is converted into helium. The Sun is now about 4.5 billion years old and has, according to our models, used up about half of the available hydrogen in its central parts. We can therefore count on the Sun to live another 4 billion years before the hydrogen is completely used up.

One of the predictions of the computer models is the way the gas density and temperature behave inside the Sun as a function of radius. Some results of the model calculations are shown here. Especially the temperature is important for us because there is a direct relation between the temperature and the sound speed in gas -- a quantity we can measure using seismological methods. This means that we can compare directly our computer models with the Sun throughout its interior, and so gain in our understanding of the physical conditions. How well we do is shown on the Results page.

By comparing the models with reality we can see whether we did the calculations correctly. However it also shows whether the physical assumptions we used in the calculations are correct. In other words we can use the observations to learn something about fundamental physics. With sufficiently accurate observations we can in fact use the Sun as a kind of big laboratory, in which we can study the properties of matter under conditions that we cannot possibly reproduce in laboratories on Earth.