The temporal analysis of a signal with the Foiurier method (spectral analysis) allows us to highlight the main components of a signal. In the musical world, the spectral analysis allows us to identify different instruments : each instrument has its own particular tone.
We can "observe" the sound, which is nothing other than a movement of matter. The resonance frequencies in particular are very characteristic. A resonance vibration (or oscillation) is characterized on a chord by a succession of antinodes and nodes, i.e. of points where the vibrations have a maximal or null displacement respectively.
When studying a signal, a sound for instance, the spectrum allows us to determine the characteristics of this signal : frequency and intensity.
In the case of the Sun or stars, which are located millions of kilometers away, how can we observe the interior of these objects, for which the temperature exceeds one million degrees?
We must listen, not try to only see, and use the sound waves.