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Artistic impression of illuminated gas, Sagittarius A*
Artistic impression of a material disc with illuminated gas around Sagittarius A*Via Wikimedia Commons

The sounds of space: why NASA makes music out of black holes

The space agency has dropped a new track by Sagittarius A*, the black hole at the centre of the Milky Way

The supermassive black hole at the centre of the Milky Way (AKA Sagittarius A*) is a relatively quiet one, which is good news for our home galaxy. But every now and then, it appears to “wake up” for short periods of time, during which it goes goblin mode and swallows up anything in its vicinity. Gas clouds, asteroids, and even stars can be pulled in by the huge gravitational forces of Sagittarius A*, which is 4.1 million times the mass of the Sun.

According to new astronomical observations, one of these periods occurred about 200 years ago (ie very recently, on an astronomical timescale). How do we know? Well, its “echoes” can still be seen in the interstellar clouds around the centre of the Milky Way, and thanks to NASA, we can even take a listen for ourselves.

First, though: how did scientists discover what Sagittarius A* was up to 200 years ago? According to a team led by Frédéric Marin of the University of Strasbourg, unusual activity has been observed in the star-forming clouds, or “stellar nursery” (aw, cute), for some time. Specifically, the clouds shine brighter than expected in X-ray images. While the universe is filled with bubbles of X-ray emitting gas, it was theorised that this X-ray light wasn’t coming from the clouds themselves but was bouncing off them from another source.

Using NASA’s Imaging X-ray Polarimetry Explorer (IXPE) satellite, Marin and his team found strong evidence to support the reflection theory. The satellite also managed to pin the X-ray light to an outburst from Sagittarius A* around the beginning of the 19th century, by measuring the direction of X-ray “echoes” emitted by a violent event that lasted a little less than one and a half years (which must have been when the black hole woke up from its beauty sleep).

Summarising the findings in a scientific paper, Marin’s team said: “Our work presents the missing piece of evidence that X-rays from the giant molecular clouds are due to reflection of an intense, yet short-lived flare produced at or nearby Sgr A*.” The precise object that flew too close to the black hole and triggered the flare is unknown.

“What did it sound like though?!” we hear you cry. OK, OK, we’re getting to it. Although, as any high school science teacher or Ridley Scott enthusiast will tell you, you can’t actually hear anything in space, because sound typically travels via the vibration of atoms in a medium like air or water. In space, there’s no air, so there’s no way for sound to reach our ears – except in rare cases where it’s carried on dense clouds of gas.

That being said, researchers have used combined images of Sagittarius A* – captured by the IXPE and Chandra X-ray Observatory – to give us a hint of what the black hole’s cosmic “burp” might have sounded like. In the “sonification”, an arced line ripples across the glowing data points captured amid the intergalactic clouds. “As it passes over the orange-tinted IXPE data, sounds like digital winds are triggered, particularly where those orange areas are brightest,” explains NASA. “When the travelling line passes the blue-tinted Chandra data, the resulting notes resemble steel drums.”

The result? Something like a warped collaboration between Mario composer Koji Kondo and William Basinski (whose 2019 album On Time Out of Time did, in fact, incorporate data from the merging of two black holes). NASA isn’t going to be putting out any cutting-edge ambient LPs any times soon, though, which begs the question: why do they bother with creating these “data sonificiations” – is it just because they sound cool and spooky, or is there something more to find among the shimmering synths?

According to the space agency itself, it’s partly a case of expanding access to the (often beautiful) data it gathers from across the universe. “Sonifications allow the audience, including blind and visually impaired communities, to ‘listen’ to astronomical images and explore their data.” The same has previously been done with spider webs, as well as COVID-19’s genetic code.

Assigning sounds to the elements of a cosmic image, such as position or brightness, also provides a new way to conceptualise data. In meaningful research, it has helped uncover previously-hidden phenomena including micrometeoroids that pepper spacecraft, or lightning strikes on alien planets. 

For the moment, it’s unclear what the applications of the Sagittarius A* sonification might be, but it is clear that further studies on the black hole’s activity could have big implications. “We know change can happen to active galaxies and supermassive black holes on a human timescale,” says Steven Ehlert, IXPE project scientist at NASA’s Marshall Space Flight Center. “We’re learning more about this one’s behaviour over time, its history of outbursts, and we’re eager to observe it further to determine which changes are typical and which are unique.”

In the meantime, enjoy the otherworldly tunes.