Humans have been in awe of the harmony of the heavens since times immemorial. Ancient Greeks believed that celestial bodies made music. In the clinging of hammers Pythagoras heard “a clue from God”, or so a folk myth goes. Stretching strings and plucking them, he discovered an intimate connection between mathematics and music, and that objects produced sound when in motion. He was thus convinced that planets moving in orbit should be humming a heavenly tune, and he sought to find the astronomical harmony of the cosmos.
In our modern times, another polymath longed for a similar fulfilment. In 1926 Arthur Eddington, an English astronomer lamented in his book The Internal Constitution of the Stars how the star’s deep interiors are further beyond the reach of human exploration than any other region in the Universe. As telescopes probe deeper and deeper into space, he beseeched to know how we can look beyond the barriers of the stellar surface. What instrument can pierce through and probe its hidden secrets, he wondered. Scientists now have the means to pierce through and see into a star. It’s called asteroseismology, the science of studying the music of the spheres. Pythagoras would have jumped in joy and elation.
Listening to the stars
Stars are not quiet, but rather giant musical instruments brimming with sound waves. High pressure inside the star ploughs through, compressing the gas as it propagates at the speed of sound. These pressure or sound waves wildly bounce inside the gaseous interiors, making stars quite noisy places. However, to us stars are mute because their sounds cannot travel in the vacuum that separates us.
These bouncing waves make the star quiver or “pulsate”. As it throbs, the swelling and contraction make the star cooler and hotter, causing periodic changes in its brightness which we can detect with our telescopes. Using basic physics and mathematics, these vibrations reveal secrets about the star’s interior in exquisite detail such as its rotation, magnetic fields, nuclear burning as well as its stage in life, mass, radius and age.
You are well aware, perhaps without realising, that the speed of sound is different according to the chemical medium it travels in. The hilarity of your voice as you breathe in helium at parties demonstrates exactly that. This is because sound travels three times faster through a vocal tract full of helium than it would through the heavier nitrogen-rich air we usually breathe. Thus the quality or timbre of your sound changes. The same thing happens in stars. As sound travels from a hydrogen-rich to a helium-rich medium, its speed -or the star’s voice- changes. This change tells us what the chemical makeup of its deep layers is. Just like your voice now is not the same as your voice when you were a toddler, a star’s voice changes too as it ages and its hydrogen transforms into helium.
A rhythmical throbbing
We see pulsating stars of different masses at essentially all stages in life. As a star swells and contracts, energy is damped and lost. So what feeds this relentless pulsation?
One driver of the continuous throbbing is heat. When a layer inside the star gets compressed by pressure, it heats up. It then converts its thermal energy to mechanical energy, acting as an engine that powers the pulsations.
Another driver is opaqueness. If a region in the star is particularly opaque, it blocks radiation from seeping through, so pressure builds up and the star swells. Its rising temperature reduces the opaqueness, allowing radiation to be released and the star deflates. Deflation increases opaqueness again and the same process repeats, quite periodically.
A third driver is resonance. It is thought that this type drives the Sun’s pulsations. Turbulent motions in its surface layers generate acoustic noise that can set it throbbing. Some scientists devote their research careers studying such oscillations. The Birmingham Solar Oscillations Network at the University of Birmingham, for example, runs a set of remote telescopes which monitor the oscillations of the Sun around the clock.
NASA’s Kepler mission, a highly accomplished planet-hunting machine, revolutionised asteroseismology by observing the minuscule dimming of the light of a wide variety of stars. It was retired only last October, when its science work was done and it ran out of fuel.
What do stars sound like?
That’s how we see the sounds of the stars, but can we actually hear them? You and Pythagoras would be thrilled to know that yes, we can. Just like we can’t normally hear bats but with the right detectors or “ultrasound ears” we can, shifting the sounds in the star by several octaves would make them audible. As intriguing as it is to eavesdrop on the stars, it remains an exercise for pleasure, not for science. You can hear a stellar music composition here.
Eavesdropping on Mars
Earlier this month, we experienced how deeply humanity resonates with the sounds of the Universe when NASA InSight lander picked up the eerie low rumble of the Martian wind. Its UK-developed ultra-sensitive seismometer includes sensors that can detect fluctuations at unimaginably small scales, shorter than the diameter of a hydrogen atom, or less than one-millionth the width of a human hair. Thus it could hear the wind on Mars, which is barely within the lower range of human hearing. The public heard it almost unadulterated, and it caused a stir. By stimulating a familiar sense, it establishes a human connection to this distant and vastly different environment.
Auditory stimulation, together with visual experiences, evoke feelings and make us aware about our position and movement in the spaces that we occupy. This is how we make meaning of ourselves, of each other and the Universe. This awareness informs our decisions and thus formulates our beliefs. Beliefs collectively shape our identity and our identity drives our behaviour. Perhaps we simply need to hear in order to thrive, believe and explore.