A massive black hole releases sound waves in a deep bass pitch, astronomers have found.
Researchers used NASA's orbiting Chandra X-ray Observatory to "listen" to wavelengths coming from the heart of the Perseus A Cluster, a giant clump of galaxies 250 million light-years from Earth (A light-year is the distance light travels in a year in a vacuum, about 9.5 trillion kilometres.)
They found the pitch of the sound is about 57 octaves below middle C, which is roughly the middle of a standard piano keyboard.
The tone frequency is more than a million, billion times deeper than the limits of the human ear, the researchers said.
Prof. Andrew Fabian of the Institute of Astronomy (IoA) in Cambridge, England, led the 53-hour Chandra observation. The discovery was described at a NASA news conference in Washington on Tuesday.
It is "the lowest note known in the universe," said Fabian.
The researchers presumed a "supermassive" black hole lies in the cluster. Some astronomers believe most galaxies, including ours, may house black holes in the centre.
Scientists can't observe dense black holes directly because the gravitational pull of the drain is so strong. Nothing, not even light, can escape it.
Instead, researchers watch what happens on the edges of black holes.
By analysing the pattern of X-rays coming from the superheated gases, Fabian and his colleagues were able to see jets of extremely hot matter shooting out from a single point.
The compression of the jets causes the sound waves, said Fabian's colleague, astronomer Steve Allen of the IoA.
The gravitational pull of the black hole is thought to squeeze the hot gases. Extra sound waves add heat energy, causing a slight change in the observed X-ray pattern.
"We've known that a black hole can give off energy as light and heat and now we are seeing a third way sound," said Bruce Margon, associate director for science at the Space Telescope Science Institute.
Detecting cosmic sound waves may help solve the long-standing mystery of why the hot gas in the middle of the Perseus cluster hasn't cooled over the past 10 billion years to form trillions of stars.