Asteroseismology
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Asteroseismology is the science that studies the internal structure of pulsating stars by the interpretation of their frequency spectra. Different oscillation modes penetrate to different depths inside the star. These oscillations provide information about the otherwise unobservable interiors of stars in a manner similar to how seismologists study the interior of Earth and other solid planets through the use of earthquake oscillations.
The oscillations studied by asteroseismologists are driven by thermal energy converted into kinetic energy of pulsation. This process is similar to what goes on with any heat engine, in which heat is absorbed in the high temperature phase of oscillation and emitted when the temperature is low. The main mechanism for stars is the net conversion of radiation energy into pulsational energy in the surface layers of some classes of stars. The resulting oscillations are usually studied under the assumption that they are small, and that the star is isolated and spherically symmetric. In binary star systems, stellar tides can also have a significant influence on the star's oscillations. One application of asteroseismology is neutron stars, whose inner structure cannot be directly observed, but may be possible to infer through studies of neutron-star oscillatons.
Helioseismology is the closely related field of study focused on the Sun. Oscillations in the Sun are excited by convection in its outer layers, and observing solar-like oscillations in other stars is a new and expanding area of asteroseismology.
Asteroseismology provides the tool to find the internal structure of stars. The pulsation frequencies gives the information about the density profile of the region where the waves originate and travel. The spectrum gives the information about its chemical constituents. Both can be used to give information about the internal structure.
Waves in sun-like stars can be divided into three different types;
- Acoustic or pressure (p) modes, driven by internal pressure fluctuations within a star; their dynamics being determined by the local speed of sound.
- Gravity (g) modes, driven by buoyancy,
- Surface gravity (f) modes, akin to ocean waves along the stellar surface.
Within a sun-like star, such as Alpha Centauri the p-modes are the prominent as the g-modes are essentially confined to the core by the convection zone. However, g-modes have been observed in white dwarf stars.
