Stars make sounds too high for us to hear

Just as the elusive dog whistle is beyond our mere human auditory capabilities, there is an entire reality out there full of waves beyond our visual spectrum, patterns beyond our recognition and dimensions beyond our reach.

However, even the most avid science fiction fans may be surprised by recent findings published this March in Physical Review Letters.

According to reports from a team of researchers funded by the United Kingdom’s Engineering and Physical Sciences Research Council and India’s Tata Institute of Fundamental Research, sound waves are being produced by stars in terahertz frequencies six million times higher than the hearing of any mammal. Even if the sound could navigate the vacuum of space — and it can’t — we would be incapable of perceiving the celestial hum surrounding us. The stars are effectively “singing,” but no one can hear their song.

In less romantic terms, Amitava Adak and colleagues discovered that acoustic disturbances are emitted from stars due to the hydrodynamics, or fluid motion, of hot plasma flowing across their surfaces.

Dense or “crowded” regions of plasma rush toward regions of lower density, or less crowded areas. In doing so, the plasma applies pressure to itself when plasma flows “bump” into other plasma flows. These alternating patterns of high and low pressure are, by definition, a sound wave.

Since the surface of a star was a tad out-of-reach, the researchers used a different technique to model and study the effects — lasers. By employing high-powered lasers to super-heat the plasma in a calculated fashion, researchers, such as Alex Robinson of the Plasma Physics Group at Central Laser Facility, were able to generate, on command, sound waves with certain desired properties, including sound waves to mimic the acoustics of the stars.

Like radar on a submarine, Robinson’s mathematical model predicts the hydrodynamic movement of plasma by measuring the Doppler effect of laser pulses against the moving plasma. By measuring the reactions of the plasma, the speed and direction can be calculated and then used to determine the overall pattern of pulses, or the sound wave.

The implications of this discovery go beyond stars and astrophysics and have prompted scientists to consider this new phenomenon of hydrodynamics. While it is possible that the effects extend beyond star surfaces, John Parsley, one of the authors of the study, believes that star surfaces are one of the few places where this effect would occur.