World Space Day | For the First Time, Hearing the Galaxy Universal

At the edge of the nine heavens, where do they rest?

How many corners and curves are there — who knows their number? Where does heaven meet the earth? How are the twelve divisions marked? Where do the sun and moon belong? How are the stars arrayed?

— Excerpt from Qu Yuan's Heavenly Questions

Look up, and the sky is strewn with stars. Countless flickers spanning millions of light-years — time frozen on a two-dimensional curtain. That faintest glimmer is the universe of millions of years ago, just now arriving at your eyes.

Only four hundred years ago, humans still believed themselves the center of the cosmos. Now we know: we may merely inhabit a speck of dust, adrift in infinite immensity.

If we were to cast humanity's relentless drive to explore the universe as a story, it would stretch back two millennia — to when Qu Yuan uttered China's first great questioning of the root and branch of all things, earth-shaking, as he searched the autumn winds for the nine heavens: in primordial chaos, where lies the origin of the universe?

This heroic exploration of the cosmos is also a story about us. Resisting cold, darkness, and distance; armed with perseverance, conviction, and courage; through observation, experiment, and conjecture — in this odyssey toward the depths of space, we seek not only answers about the universe but also the meaning of human existence.

Everything in the observable universe is but a tiny bubble in an infinite cosmic ocean. The universe was born 13.8 billion years ago, yet that time has still been insufficient for its light to reach us. Of the light blazing across the cosmos, the human eye perceives only the smallest fraction.

But science lets us see what eyes cannot. Sweep an infrared sensor across the void, and there appear: dust still spreading from creation's dawn, black holes that devour all, planets wandering through eternal night, dying stars collapsing, igniting, exploding... Now, we can even hear the universe.

Many of the cosmic vistas we see are not true photographs but visual translations — astronomers converting electronic data captured by space telescopes (streams of 1s and 0s) into images.

Astronomers synthesize different elements or types of light (visible, ultraviolet, infrared) into images, assigning distinct colors to render the universe beyond human sight visible.

By this same principle, NASA collaborated with visualization scientists, astrophysicists, and musicians to take actual observational data from observatories and instruments worldwide, assign different elements and signals, express them through distinct "instrument" sounds, and ultimately transform them into frequencies the human ear can perceive.

The unknowable, invisible, and inaudible — made knowable, visible, and audible. Facing the unknown with hearts turned toward vastness, exploration never ceases.

This is a true space opera house. Accept Source Code Capital's invitation, and together let us immerse ourselves in this symphony resounding across the Milky Way and beyond.

Galactic Center

The symphony's first movement begins with exploration of our own galactic center! The sonification process moves from left to right across the image. Sound represents the position and brightness of light sources: objects toward the top of the image are rendered in higher pitches, while light intensity corresponds to volume.

Stars and compact sources become individual notes; diffuse clouds of gas and dust produce a sustained, rumbling bass. As we reach the bright region at the lower right, the climax arrives — the location of the supermassive (four million solar masses) black hole at the galactic center, known as Sagittarius A* (Sgr A*).

The image above combines "solos" from the Chandra X-ray Observatory, Hubble Space Telescope, and Spitzer Space Telescope into an opening movement spanning roughly 400 light-years across the Milky Way.

Supernova Remnant Cassiopeia A

The second movement features a supernova remnant called Cassiopeia A (Cas A).

Sound maps to four elements found in the exploded star's debris, along with other high-energy data. Given Cas A's shape, this time we "listen from the center outward": starting from the remnant's center — the neutron star's location — and moving along four different directions, we find silicon (red), sulfur (yellow), calcium (green), and iron (purple). Again, intensity controls volume.

Pillars of Creation Nebula

In the "Pillars of Creation" movement, as with the galactic center, we listen left to right: objects toward the top register in higher pitches, light intensity in louder volume. But this time they are not discrete notes — they vary across a continuous tonal range. Particularly fascinating: in the arc from low to high pitch and back, we can "hear" the pillars' structure.

Crab Nebula

In 1054, the Crab Nebula first appeared in Earth's sky. Modern telescopes have captured its engine — a rapidly spinning neutron star formed when a massive star collapsed. The combination of rapid rotation and intense magnetic field produces jets of matter and antimatter streaming from its poles and winding outward from its equator.

To translate this data into sound, sonification scientists paired each wavelength of light with a different instrument. Chandra's X-rays (blue and white) become brass; Hubble's optical data (purple) become strings; Spitzer's infrared data (pink) emerge through woodwinds. In this movement, light toward the top of the image sounds higher in pitch; brighter light corresponds to greater volume.

Perseus Cluster

The fifth movement involves actual sound.

Pressure waves emitted by a black hole created ripples in the Perseus Cluster's hot gas. These ripples can be translated into genuine sound — but pitched 57 octaves below middle C, far below human hearing.

There is no sound in space because most of the cosmos is essentially vacuum. But the Perseus galaxy cluster contains vast quantities of gas enveloping hundreds or thousands of galaxies, providing a medium for sound waves to propagate.

In this symphony, astronomers extracted previously identified sound waves from Perseus, making them audible for the first time.

The sound waves are extracted from center to edge, then resynthesized into the human hearing range by shifting the signal upward 57 and 58 octaves — increasing their frequency by 144 quadrillion and 288 quadrillion times their original rate. (One quadrillion is 1,000,000,000,000,000.)

Sagittarius A*

The sixth movement comes from the supermassive black hole at our galaxy's center, Sagittarius A* (Sgr A*). Using a radar-like scan beginning at the 12 o'clock position and sweeping clockwise, volume variations represent brightness differences observed by the Event Horizon Telescope (EHT) around Sgr A*'s event horizon. Matter closer to the black hole and moving faster produces higher frequencies.

Listeners will experience this movement in 3D surround sound.

Southern Ring Nebula

NASA's Webb Telescope reveals two views of the Southern Ring Nebula — near-infrared light (left) and mid-infrared light (right).

This scene was created by a white dwarf: the remnant left when a sun-like star shed its outer layers and ceased nuclear fusion. Over tens of thousands of years, before becoming a white dwarf, the star periodically ejected mass — visible as shells of material. Like a repeating playback, it collapsed, heated, then exploded again and again.

At the center of this planetary nebula, two stars orbit each other. The smaller, dimmer red star in the right-hand mid-infrared image is in its final life stage — it has been expelling layers of gas and dust for millennia. Its companion, the brighter, larger star in the image, has stirred these ejecta. Now listeners can clearly hear the stars and surrounding material shells in each image.

In the Southern Ring Nebula chapter, light frequency converts directly to sound frequency; different colors map to different pitches. The initial near-infrared light is represented by higher-frequency tones. Midway through, mid-infrared light and longer wavelengths are rendered in lower bass tones.

Listen carefully at 15 and 44 seconds, where the stars appear. In the opening near-infrared image, only one star's sound is clearly audible, emitting a louder tone. In the latter half of the track, you will hear a low note followed by a higher one — indicating two stars detected in mid-infrared light. The lower note represents the redder star that created this nebula; the second note is the brighter, larger star.

Carina Nebula — "Cosmic Cliffs"

The Carina Nebula is a stellar nursery within the Milky Way. Its sharp boundary between dust below and gas above has earned it astronomers' nickname "Cosmic Cliffs."

Brighter light in the image sounds louder; higher position means higher frequency. Bright light near the top sounds loud and high-pitched, while bright light near the middle sounds loud but lower in pitch. Darker, dust-obscured regions in the lower portion of the image are rendered in lower frequencies with distinct notes.

Perhaps you can identify the wandering melodic line representing the nebula's "mountain range," traversing left to right through the image's center, rising and falling with the terrain's contours. This jagged line between dense and thin regions of gas and dust traces the arc of the audio melody. All stars are represented by a set of pitches and processed piano notes, but the brightest stars and longer diffraction spikes are accompanied by cymbal crashes and chimes.

M51 — The Whirlpool Galaxy

Messier 51 (M51) may be better known by its nickname "Whirlpool Galaxy." Its tightly wound spiral arms face Earth, allowing its magnificent form to be captured more intuitively by human eyes. It resembles our own Milky Way — a structure we cannot directly observe from within.

In the symphony's finale, sound processing begins at the top and moves radially clockwise; the galaxy's radial extent maps to different notes of a melodic minor scale.

Each wavelength of light captured by NASA's space telescopes (infrared, optical, ultraviolet, and X-ray) is assigned to a different frequency range. The sequence begins with sounds from all four light types, then moves separately through data from Spitzer, Hubble, GALEX, and Chandra. Where spiral arms are prominent, pitch gradually rises as the arms extend farther from the core.

You can hear a sustained low hum — the galaxy's bright core — intermittently interrupted by brief sounds from compact sources within the galaxy.

About the Sonification Project

What is this project? In 2020, experts at the Chandra X-ray Center and the System Sounds team launched NASA's first sustained "sonification" project converting astronomical data into sound. It was a rapid response to the coronavirus pandemic, which had disrupted our 3D modeling/printing project with the visually impaired community. During physical isolation, sonification became a purely digital alternative for our continued collaboration with community partners.

Who is involved? The Chandra X-ray Center's project lead is Dr. Kimberly Arcand, Chandra visualization scientist; System Sounds is led by Dr. Matt Russo (astrophysicist/musician) and Andrew Santaguida (musician/sound engineer). Accessibility expert, podcast producer, and visually impaired community member Christine Malec also joined the project.

How are sonifications made? We obtain actual observational data from telescopes including NASA's Chandra X-ray Observatory, Hubble Space Telescope, or James Webb Space Telescope, and convert it into frequencies the human ear can hear.

Are all sonifications the same? No, each sonification is different because we employ different techniques depending on the object and available data. Each is designed to best present the scientific data, accurately representing and narrating the data's story in the most meaningful way, while simultaneously offering a new mode of meaning-making through sound.

What are the project's results? Has it succeeded? Success can be measured in many ways, but testing with diverse audiences — from students to adults, especially the visually impaired — has yielded very positive responses. Dr. Arcand conducted sonification studies with blind and sighted users, showing high learning gains, enjoyment, desire to learn more, and strong emotional responses to the data. Statistically, visually impaired users showed slightly higher engagement than sighted users, while sighted users also gained understanding of how others (such as visually impaired users) more commonly access data. Overall, the project has achieved notable success in increasing audience engagement, advancing scientific knowledge, and expanding inclusivity. Sonification not only provides a new pathway for visually impaired individuals to understand and experience astronomical data, but also brings entirely new modes of science communication to sighted people. Thus, the project can be considered a success.

(Materials for this article from https://chandra.si.edu/sound/; visit the official website for more information~)