Despite being unable to listen to it with our ears, the stars in the sky keep a melodious and constant concert. Large stellar bodies emit low and deep sounds, similar to those produced by terrestrial tubas and double basses. While small stars take pride in their high pitched voices, which resonate like heavenly flutes. But this cosmic orchestra is not limited by the touch of a single note at a time. Instead, stars have thousands of sound waves, different from each other, that bounce in their nuclei at any given time.
This fact represents a huge astronomical revolution. Especially when considering that, by “listening” to these sound waves with our telescopes, it is possible to obtain a lot of valuable information, such as: the material, age and size of the stellar bodies studied. These details are obtained from the vibrations or “star earthquakes” that reveal their internal functioning; a process similar to that produced by the seismic waves of the Earth and due to which this technique is named asteroseismology.
So, knowing this phenomenon, it is worth asking: how do these waves work?
Thanks to the NASA’s Kepler Space Telescope and Exoplanet Probe Satellite (TESS), we know that sound waves move through the interior of a star due to changes in temperature. These begin in the zone of convection of the star, where hot gas arises to then move towards the stellar surface to cool down. But this gas doesn’t remain there, for it soon returns to its original zone in a violent and turbulent fall. Such a movement of heat that rises and falls generates the different waves that bounce off the star.
For their part, convection-driven waves cause the entire star to expand and contract; a process that produces a sound similar to that of a bell. In fact, the propagation of these waves is so great that the stellar surface pushes itself like a jelly. Although it is relevant to note that, while this is an abrupt movement, it is also incredibly subtle and, ergo, invisible to the human eye. That is why in images of the Sun we observe the effects of these sound waves as areas of localised brightness; which should not be confused with the dark spots we know as sunspots.
Thereafter, certain sound waves spread around the entire circumference of a star just as others furrow its nucleus. The larger the star, the longer its sound waves take to travel inside of it. On the Sun, for example, a typical sound wave completes a cycle in five minutes. But certain sound waves of stars comparable in size to the Sun can take several days to finish their cycles. Red giants, on the other hand, have low frequency waves that can spread over weeks and even months. And since new waves emerge all the time, stars are always vibrating and emitting their particular symphony.
This is the reason why the sound waves present in all star acquire their importance in the fields of space physics. Being thanks to them that the secrets that stars keep in their nuclei are revealed to us, eliciting astonishment and sparking our curiosity.