The term asteroseismology comes from a combination of three words :

• aster : from the classical Greek which means star.
• seismos : also from Greek meaning tremor.
• logos : meaning reasoning or discourse.

An exaggerated example of one of the many possible ways in which a star can pulsate is shown in a little animation which can be seen by clicking on the image. Of course stars are so far away that in our MONS telescope it is merely a point. Despite of this there are two main ways in which it is possible to observe this quaking or pulsation of stellar surfaces.

• As parts of the surface are expanded away from the stellar center or contracted towards it, the brightness of each part changes. This causes the brightness of the star to fluctuate around its mean value. Rømer will measure stellar brightnesses with a very high accuracy.
• As parts of the surface are moving in or out, they are also moving towards us and away from us with some velocity. Because of the Doppler effect it is possible to measure this velocity. Rømer will not have this capability.

Rømer will measure the variations in the brightness of a large number of stars during a long period. In this way it will be possible to measure the periods of pulsations with various surface patterns with high accuracy.

The oscillation patterns that we observe on the surface of stars are caused by sound waves that also propagate into the interior of these stars. If we follow where a sound wave goes, starting from the surface, it first moves into the star almost straight towards the centre. Its path then slowly bends around, because of the increasing sound speed, so that it misses the centre of the star. How exactly it moves therefore depends on the details of the sound speed inside the star. The point of closest approach is known as the turning point of the mode. After the turning point the wave moves out again until it reaches the surface. At the surface it is reflected as if by a mirror and it goes back in again.

If two bells have exactly the same shape and are made of the same material, but one is much bigger than the other, we know that the bigger bell will make a lower dominant sound than the smaller one when it is struck. In other words the bigger bell has a set of longer periods. Something similar is true for stars. What matters here is the mean mass density of the star : the lower the density, the longer the pulsation periods of the star. This means that if the radius of the star is known its total mass can be determined easily. For all of the stars that are relevant for Rømer the radii are known : they are determined indirectly because their distance has been determined with the Hipparcos satellite.

Since each of the modes follows a slightly different path through the interior of a star it senses the sound speed in slightly different parts of the interior. Using a complicated mathematical analysis called inversion it is possible to use very slight differences in frequencies of different pulsation modes to deduce the sound speed at various depths inside the star. Deducing the sound speed as a function of depth is one of the first steps along the path of a better understanding of the internal structure of the star.

The modern age of attempts to model the structure and the evolution of stars can be said to start in the early parts of the 20th century, and it took a big leap in the 1950s when the first generation of electronic computers became available. One can wonder why, after some 50 years of constructing ever improving computer models of stars, it is still necessary to look at stars in such detail.

The answer to this question goes back to a fundamental concept in any modern science which is that scientific theories can never be proven right, they can only be proven wrong (K. Popper, 1934). By carefully comparing the predictions that a theory makes with what nature actually does, one can either obtain consistency within the measurement errors (the theory has passed a test) or one obtains a discrepancy : the theory has failed a test and is therefore incomplete or simply wrong. It is the task of scientists to devise ever more rigorous tests for their theories. Rømer is a mission which contributes to such a test.

The theory of stellar structure as applied the the Sun is already the subject of very rigorous tests. With helioseismology it is possible to reconstruct the sound speed inside the Sun as a function of radius with very high precision, and compare it to the best models. Although the models are very close it is clear that there are specific areas which clearly we do not understand very well yet.

• Models of stars are usually spherically symmetric : all physical quantities (gas temperature, density, sound speed etc.) only depend on the distance from the center. We know that even in the Sun, there are effects that make these quantities different at different places in the Sun which are at the same distance from the center : the solar poles differ from the solar equator. We know that many stars are much more aspherical than the Sun, for instance because they rotate much faster. Therefore, even if our models might be reasonably accurate for the Sun, they are probably not nearly as good for stars that rotate much faster.
• Although astronomers are well aware that stars are born, gradually grow old and then die, we tend to assume that the structure of stars changes so slowly with age that we can ignore changes with time when calculating their internal structure : models of the internal structure of stars are quasi-static. However, looking at the surface of a perfectly ordinary star like the Sun shows that in fact it is not static at all. The visible surface of the Sun is part of a convection zone, which extends about 28 % of the solar radius down. Some other stars have a convective core instead but all stars have some region(s) in which the material is in violent motion. Through helioseismology we already know that our models of the Sun are not very good especially at the boundaries of convection zones. This is one example of dynamic effects which are not properly taken into account. By studying where the convection zones of other stars begin and end we can learn how to do better.
• The Sun has dark spots on the surface, which are places where the magnetic field is particularly strong. These spots appear and disappear and sometimes the Sun has more spots than others. There is a solar cycle where roughly every 11 years the number of spots reaches a maximum. The cause of the solar cycle is still a puzzle. We know that the Sun is not particularly magnetically active. There are stars with much stronger magnetic fields, and with much stronger variation of these fields. In these stars their internal structure may well be quite different from what a non-magnetic model predicts, and the course of their evolution may well be very different. We need to study other stars even to understand the magnetism of the Sun.
• The Sun is only one star in one particular stage of its life. Even if we had a model of the Sun at this point in its life that was perfect, we still could be completely wrong about how the Sun was in the past or will be in the future. The theory of stellar evolution can only be tested by investigating the internal structure of many different stars.
• Modern models of stars require input from many areas of fundamental physics. We need to know the properties of gas mixtures over a wide range of temperatures, densities and chemical compositions : the Equation of State. We need to know how that gas absorbs and emits radiation : the opacity. To some extent these quantities can be measured in laboratories on earth, but not for all conditions. If they cannot be measured they are calculated from theory, and of course these theories can also be incomplete or wrong. With helio- and asteroseismology we also test such theories of fundamental physics because if they are wrong then our models will not match the real Sun and stars.

In Denmark astronomers have access to some of the biggest telescopes on earth for observing in visible and near-infrared light, such as those of the European Southern Observatory, and the Nordic Optical Telescope. Some of these have been used to do asteroseismology so one can wonder why there is a need to go to space. There are two main reasons why doing asteroseismology from Earth is in fact quite problematic :

• The kind of oscillations in stars similar to those the Sun experiences produce very small fluctuations in brightness. Any other source of variations can easily drown out the signal that we are looking for. Unfortunately the Earth's atmosphere interferes with light : it makes stars twinkle. The scientific name for this is scintillation. One can attempt to correct for this in various ways and by using the largest telescopes but it is extremely hard to reach the levels of sensitivity that are necessary to do asteroseismology. With the small 32 cm telescope of MONS from space one can do easily what is almost impossible with an 8 meter telescope on Earth. By clicking on the pictures one can see a demonstration of scintillation using a glass of water (the atmosphere) and an overhead projector (the star) by Hans Kjeldsen (Danish language).
  
• An essential aspect of asteroseismic observations is to obtain a time series : very frequent observations, for instance every minute, during a very long period such as a month. First of all it would not be possible to reserve an 8 meter telescope for this much time. Second of all : stars rise and set because the Earth rotates. Observations from a single site on Earth therefore always have gaps during times that the star is below the horizon. The only way to solve this problem would be to observe with several telescopes spread around the globe, so that there would always be one or a few telescopes that could observe the star. So a single 8 meter telescope would not even be enough : we would need a number closer to 6 ! A single satellite can point to a given star for almost any length of time.