For a well defined group of stars there exists a self-excitation mechanism which is capable of generating steady standing waves inside the distant suns. These waves are analogous to the waves inside organ pipes or other wind instruments. These stars are called (pulsating) variable stars. However, there is no dense-enough interstellar matter in which the stellar sounds could travel to Earth, i.e. there is no way to “hear” these stellar waves directly by observation. However, the brightnesses of these heavenly bodies are modulated by the oscillations. This makes it possible for earthlings to observe the acoustic waves of stars by measuring the light variation of heavenly bodies.
The period of stellar oscillations extends from minutes to years, e.g. the range of frequencies is about 20 octaves wide compared to the 10 octave range of the audible sounds. Furthermore, even the highest pitch of the stars is approximately 15 octaves lower than Middle A! This makes it necessary to transpose the wide range of stellar pulsations to make them audible.
The pitch of stellar pulsations depends on the size of the star (as for real musical instruments), but also on the density of its matter. During stellar evolution the stars expand and contract very slowly, providing very slow changes in their acoustic spectra. Interestingly, not only the periods of pulsations vary, but also the pitch ratios in the overtone spectrum shift.
Contrary to most real musical instruments, stellar acoustic spectra do not consist of harmonic pitches. The partial tones of stellar instruments are not in integer frequency relation with the fundamental oscillation, i.e. the overtones are not harmonics. This gives a significant difference compared to organ pipes or most of the classical musical intruments.
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