Asteroseismology
Asteroseismology (also astroseismology or stellar seismology ) is the science of inferring their internal structure from the frequency spectrum of pulsating variables . When applied to the sun , one speaks of helioseismology .
Pulsations
The vibrations in stars caused by the conversion of heat energy into kinetic energy , eg. B. by the kappa mechanism . The density waves propagate in the star and are reflected by a density jump on the star's surface. If a wave runs into the star, the density and thus the speed of sound increase with increasing depth . The result is an angle change in the direction of travel of the shaft, which is directed back towards the surface. Astroseismology tries to analyze the resulting frequency spectrum and compare it with models of the star structure. This is complicated by the non-rigid rotation of the stars, non-linear effects, and the deviation of the stars from spherical shapes.
With sun-like stars
The Helioseismology examined the structure of the sun from the observed oscillations of the Sun's surface . With the sun and sun-like stars , the oscillations are stimulated by the convection in the outer layers. This leads to a large number of oscillation periods , although several thousand could be detected in the case of the sun. In order to resolve these many frequencies , the observation of satellites (e.g. COROT ) is carried out in order to avoid interruptions due to daylight and bad weather. The oscillations are verified by measurements of radial speed and / or brightness .
Methods
The fluctuations in brightness are usually so small that their detection in normal stars other than the sun from the ground has never been really convincing because of the unrest in the air. Hopes are therefore directed towards brightness measurements from the much more stable observation position of satellites in space (see WIRE , MOST , COROT , Kepler , PLATO ). The stronger fluctuations in brightness of pulsating variable stars , on the other hand, could already be measured by ground-based observatories .
Spectroscopic methods are also used from Earth . In doing so, the spectra of the stars are searched for red- or blue-shifted spectral lines that indicate a change in radial velocity. This method is mainly used for the detection of extrasolar planets , as these cause a periodic change in the radial velocity due to gravitational disturbances ( radial velocity method ). However, it can also be used to investigate the faster, internal vibrations of the star.
literature
- Helmut Scheffler, Hans Elsässer : Physics of the stars and the sun. 2nd revised and expanded edition. BI Wissenschaftsverlag, Mannheim 1990, ISBN 3-411-14172-7 .
- R. Kippenhahn , A. Weigert: Stellar Structure and Evolution. Corrected 3rd printing. Springer, Berlin et al. 1994, ISBN 3-540-58013-1 ( Astronomy and astrophysics library ), (Reprint: Study edition. Ibid. 2008, ISBN 978-3-540-58013-3 ).
- Frank P. Pijpers: Methods in helio- and asteroseismology. Imperial College Press, London 2006, ISBN 1-86094-755-7 .
- DW Kurtz: Asteroseismology: Past, Present and Future In: Journal of Astrophysics and Astronomy. 26 / 2–3 / 2005, ISSN 0250-6335 , p. 123.
- Conny Aerts , Jørgen Christensen-Dalsgaard , DW Kurtz: Asteroseismology, Springer 2010