The MONS project aims to give Danish scientists a unique possibility to be the first to apply accurate seismological methods to stars. The primary idea is to construct a detailed picture of the stars from their youth until their old age. MONS will on the one hand see stars as laboratories for the study of physical processes that cannot be investigated on Earth, and at the same time function as an instrument for the measurement of stellar ages and chemical composition.
The MONS project (on the Rømer satellite) is a very ambitious satellite project, which will radically change our understanding of the conditions in stellar interiors. MONS is an abbreviation of Measuring Oscillations in Nearby Stars, and the scientific part of the project is being headed by scientists from Aarhus University.
The aim of the MONS project is to give a detailed picture of stellar structure and to contribute to the understanding of the life of stars from soon after they appear out of the interiors of enormous dark clouds of interstellar gas, until soon before they are laid to rest, when all the fuel for the production of starlight is exhausted.
In order to achieve this goal, it is necessary to improve substantially on the precision of the measurements that we take of the stars. The MONS project therefore relies upon observations of incredibly high accuracy, and this will enable Danish as well as foreign scientists to improve the theoretical description of the processes which control the stars. It is expected that MONS will create the possibility of a small revolution in our understanding of the most common stars.
The scientists of the MONS project will make use of seismological methods to achieve the necessary improvement of the quality of the collected data. In fact, the precision of the measurements will produce data which are around a factor of 100 better than what has been available up to now, primarily collected through measurements from the Earth's surface.
Main ideas of the MONS project :
Seismic investigation
The seismological technique, which the scientists of the MONS project will use, relies on the same principles which are used every day by geologists for the investigation of the interior of the Earth and by astronomers for the investigation of the solar interior.
When an earthquake occurs, waves are emitted and propagate into the Earth's interior. By making use of seismographs that are set up on all large landmasses on the Earth, geologists can register waves from any given earthquake, and by detailed analysis of the times the waves take to propagate from their origin to the seismographs, it is possible to determine the physical conditions in the Earth's interior. Although we are not able to perform direct measurements of the Earth's interior, seismological measurements can nevertheless give very detailed information concerning the conditions below the Earth's surface.
For the Sun and the stars something quite similar is true. By direct measurement one has the possibility only for studying their surface, while it is impossible to measure the conditions in the interior of the stars. Already since the 1960's it has been possible to construct good models of stars by making use of computers. These models have for the most part matched well the conditions that can be observed on the surface of stars. The big problem is that it has not been possible to test or verify these computer simulated models of the interior of stars. Therefore seismological investigation in this astronomical field is an extremely important tool to deepen our understanding of the structure and evolution of the stars.
More than 25 years ago it was discovered that the Sun is continually oscillating - the so-called solar oscillations or sunquakes - which can be seen on the solar surface as a complicated system of waves. It requires very sensitive measuring equipment to even see these waves. By measuring thousands of such sunquakes it has been possible - thanks to seismological methods - to investigate the solar interior. In the same way that geologists have investigated the Earth, astronomers have in the course of the past 20 years built up a very detailed picture of the conditions under the solar surface, conditions which are not accessible to direct observation.
Quakes on the solar surface have been studied in great detail both from the Earth's surface as well as from space - most recently by using the SOHO satellite. This work has been carried out by scientists from the Theoretical Astrophysics Center in Århus, among others.
The results of these seismological investigations have led to a revision of the understanding of the solar interior, which of course has led to speculations about to what extent our insights into the conditions inside the stars are valid.
For this reason astronomers have for the past 15 years attempted to find waves and oscillations (starquakes) on other stars than the Sun, in order to acquire the possibility to undertake detailed investigations of the interior of stars - something which has never been done before. But this is not so straightforward. The periodic wave motions that arise from starquakes and sunquakes, are almost impossible to measure. Even for the Sun, which is more than 10 billion times brighter than even the brightest star, it requires very special equipment to measure the relatively small wave motions. Attempts to observe starquakes by direct measurement of motions on stellar surfaces have therefore failed up to now.
An example of a starquake : periodic wave motion on the surface as well as in the interior of the star.
The small waves on the surface also make the stellar luminosity and temperature vary. Despite the fact that these variations are also very small, nevertheless even a small telescope can in principle perform measurements sufficiently sensitive to see the variations in luminosity caused by the starquakes. From the Earth's surface the attempts have failed because the measurements of the luminosity variations are strongly affected by the disturbing motions in the air of the atmosphere, the so-called atmospheric scintillation. In principle, this can be suppressed by using extremely large telescopes, but in practice this is not a realistic option.
Simulation of the seismic signal from a quaking star. Luminosity variations are indicated in units of one millionth of the stellar luminosity. The time scale is given in units of hours. It is clear that these are very small variations which require very sensitive equipment in order to measure them.
An analysis of a simulated seismic signal is shown here in a diagram where the light variation is determined for various oscillation periods. The peaks in the diagram show at what periods (in minutes) starquakes occur. The precise periods of the quakes themselves contain information about the conditions deep inside the stars. The luminosity variations are given in units of one millionth of the stellar luminosity. MONS will determine such diagrams for a considerable number of stars.
However, if we get out into space - above the Earth's atmosphere - there is no need for a big telescope if we want to measure starquakes. The idea is therefore that even a small satellite, with a small telescope on board, can perform the measurements, and give scientists the data they for so long have longed to get their hands on. In the MONS telescope the diameter of the mirror which collects the light is 34 cm. The design provides for a very compact telescope of which the length is only ca. 55 cm. Besides this main telescope MONS is equipped with two smaller telescopes, each with a diameter of just 24 mm. The small compact telescopes make it possible to "see" a large part of the sky simultaneously. In this way MONS can undertake measurements on many hundreds of stars with a somewhat lower precision than is the case for the main telescope.
It is thought that MONS as a part of the Rømer satellite can, for a period of two years, undertake measurements of starquakes on roughly 25 nearby stars, as well as several thousand stars further away. The main emphasis is on stars of the same type as the Sun. The MONS telescope will point at each star uninterrupted for about a month and in this period MONS will see the luminosity variations generated by 40-100 different types of quakes on each star. When the analysis of the data from MONS is concluded, the scientists involved will hopefully have answers to a long list of important questions.
In this way MONS will at the same time see the stars as physics laboratories for the study of processes which cannot be investigated on Earth, because the conditions inside the stars are too extreme, as well as work as an instrument for the measurement of stellar ages and chemical composition.
It is, therefore, not surprising that not just Danish scientists are standing in line to participate in the MONS project. From the international side there has been overwhelming interest to participate both in the development of the satellite as well as in the analysis of the expected high-precision data. 150 scientists from more than 50 universities, research institutes, and observatories in 19 countries are already collected in a big science-team - the MONS Science Consortium - and a part of this large number of scientists met on 24th-26th of January for a conference in Århus with the single purpose of planning the science to be done with MONS.
The technical parts of MONS are developed in close collaboration between the Institute for Physics and Astronomy at Aarhus University, the Danish Space Research Institute in Copenhagen and the companies TERMA Elektronik A/S and Auspace Limited in Australia.
The science centre for MONS is placed at Aarhus University, where the initial analysis af all data will take place.
The project leader (PI) is professor Jørgen Christensen-Dalsgaard and the technical leader (Project Scientist) is Hans Kjeldsen, both working at Aarhus University.