Asteroseismology
Encyclopedia
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s by the interpretation of their frequency spectra
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
. Different oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
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
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
study the interior of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and other solid planets through the use of earthquake oscillations.
The oscillations studied by asteroseismologists are driven by thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
converted into kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of pulsation. This process is similar to what goes on with any heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
, 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
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
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 oscillations.
Helioseismology
Helioseismology
Helioseismology is the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun. Unlike seismic waves on Earth, solar waves have practically no shear component . Solar pressure waves are believed to be generated by the turbulence in the convection zone near...
, also known as Solar seismology, is the closely related field of study focused on the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Oscillations in the Sun are excited by convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
in its outer layers, and observing solar-like oscillations
Solar-like oscillations
The term solar-like oscillations refers to oscillations in other stars that are excited in the same way as those in the Sun, namely by convection in its outer layers.-See also:* asteroseismology* helioseismology* variable stars* Procyon...
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 give 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) modesGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
, driven by buoyancy, - Surface gravityGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
(f) modes, akin to ocean waves along the stellar surface.
Within a sun-like star, such as Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
, the p-modes are the most prominent as the g-modes are essentially confined to the core by the convection zone. However, g-modes have been observed in white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
stars.
Space missions
A number of active spacecraft have asteroseismology studies as a significant part of their mission.- MOSTMicrovariability and Oscillations of STars telescopeThe Microvariability and Oscillations of STars telescope, better known simply as MOST, is Canada's first and only space telescope. It is also the smallest space telescope in the world...
- A Canadian satellite launched in 2003. The first spacecraft dedicated to asteroseismology. - COROTCorotCorot may refer to:* Jean-Baptiste-Camille Corot, French landscape painter * COROT, a space mission with the dual aims of finding extrasolar planets and performing asteroseismology* COROT-7, a dwarf star in the Monoceros constellation...
- A FrenchCNESThe is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...
led ESA planet-finder and asteroseismology satellite launched in 2006 - WIREWide Field Infrared ExplorerThe Wide Field Infrared Explorer was a satellite launched on 5 March 1999 on the Pegasus XL rocket into a polar orbit between 409 km and 426 km above the Earth's surface...
- A NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
satellite launched in 1999. A failed infrared telescope now used for asteroseismology. - SOHOSolar and Heliospheric ObservatoryThe Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
- A joint ESA / NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
spacecraft launched in 1995 to study the SunSunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. - KeplerKepler MissionThe Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
- A NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
planet-finder spacecraft launched in 2009 that is currently making asteroseismology studies of over a thousand stars in its field, including the now well-studied subgiant KIC 11026764.