"This behaviour can occur only at very early stages, when the system is dominated by its initial state."

An experimental work by a team of researchers at the Mumbai-based Tata Institute of Fundamental Research (TIFR) may have led to the discovery of the sound of stars.

The results were published recently in the journal Physical Review Letters.

In a table-top experiment, scientists at TIFR were able to produce a very hot (millions of Kelvin), solid density plasma, much like that found in stars. A femtosecond laser of very high intensity was used to produce the star-like object.

“We were able to produce the star-like object for hundreds of picoseconds (10) to nanoseconds (10) using the laser,” said Amitava Adak, Research Scholar at the Ultrashort Pulse High Intensity Laser Lab, TIFR. He is the first author of the paper.

The sound was inferred by using a very simple technique called the pump-probe. The pump is the main laser that produces the star-like object and the probe, which is a tiny part of the laser itself, helps in studying the evolution of the object.

“We measured the intensity and spectrum of the reflected probe and we could find very high frequency sound being generated in the hot, dense object,” Mr. Adak said. “It was a chance discovery.” The frequency of the sound is way beyond 20,000 Hertz, which is beyond the outer limit of human hearing, and hence cannot be heard.

The sound is produced by hydrodynamic flows of differential velocities down a density gradient in the plasma. In other words, the plasma flowed from a region of high density to regions of low density. Hydrodynamic instability is caused when the “rapidly moving plasma encounters a stagnant flow in the plasma corona.”

When this happens, a localised buildup of pressure results and this moves further upstream. “Plasma piled up at the interface between the high and low density regions generates a series of pressure pulses: a sound wave,” a release said.

“This behaviour can occur only at very early stages, when the system is dominated by its initial state,” notes the paper. “The picosecond-scale observations helped the team to detect the high frequency oscillations,” Mr. Adak said.

“One of the few locations in nature where we believe this effect would occur is at the surface of stars. When they are accumulating new material, stars could generate sound in a very similar manner to that which we observed in the laboratory — so the stars might be singing — but, since sound cannot propagate through the vacuum of space, none can hear them,” Dr. John Pasley, of the York Plasma Institute in the Department of Physics at York said in the release.