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Asteroseismology is the seismology of stars and helioseismology (solar seismology) is specifically the seismology of the Sun. Study of the spectra of vibration of the Sun and other stars reveals information about their structure. Vibrations are classified using three types (pulsation modes):
Stars also can exhibit radial modes of vibration: compression and decompression analogous to air (sound) resonances in a pipe. Characteristics of the star may be determined by such study, including radius, mass, metallicity and age. Oscillations similar to those of the Sun are termed solar-like oscillations. Stellar models are developed that include the resulting oscillation. Observed stars' asteroseismological characteristics are also compared with those of other stars with known radius, mass, etc. Stellar radius determination by asteroseismology is considered more accurate than other methods usable at similar distances: it is thought to be accurate to about 2-to-3 percent. Sound speeds within stellar material can be estimated with enough accuracy to help determine radius from the resonances apparent in the vibrations. The acoustic frequency (time for a p-mode to travel its wavelength) depends upon the state of the material, and thus generally varies with the distance from the center of the star. Space missions to study asteroseismology include: