Race cars circling a track? A demon groaning? Or perhaps a heavenly chorus?
Whatever it is, the sound of a black hole is extremely, extremely freaky.
NASA's exoplanet team, which looks for worlds around other stars, posted a fascinating audio file to Twitter on Sunday with a "sonification" of sound waves from a black hole at the center of the Perseus galaxy cluster.
The 34-second track has to be heard to be believed - and as of Monday morning it already had more than 6.4 million spins. (NASA actually first released the file in May of this year, but the weekend tweet brought the audio back to the top of the Outer Space charts.)
"Since 2003, the black hole at the center of the Perseus galaxy cluster has been associated with sound. This is because astronomers discovered that pressure waves sent out by the black hole caused ripples in the cluster’s hot gas that could be translated into a note – one that humans cannot hear some 57 octaves below middle C," NASA scientists wrote in May.
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To make the sounds audible to humans, NASA had to scale them up -- 288 quadrillion times their original frequency.