Who knew: big black holes sing bass. One particularly monstrous black hole has probably been humming B flat for billions of years, but at a pitch no human could hear, let alone sing, astronomers revea

Who knew: big black holes sing bass. One particularly monstrous black hole has probably been humming B flat for billions of years, but at a pitch no human could hear, let alone sing, astronomers revealed yesterday (Sept. 9).

"The intensity of the sound is comparable to human speech," said Andrew Fabian of the Institute of Astronomy at Cambridge. But the pitch of the sound is about 57 octaves below middle C, roughly the middle of a standard piano keyboard. This is far, far deeper than humans can hear, the researchers said, and they believe it is the deepest note ever detected in the universe.

The sound is emanating from the Perseus Cluster, a giant clump of galaxies some 250 million light-years from Earth. A light-year is about 6 trillion miles (10 trillion km), the distance light travels in a year.

Fabian and his colleagues used NASA's orbiting Chandra X-Ray Observatory to investigate X-rays coming from the cluster's heart. Researchers presumed that a supermassive black hole, with perhaps 2.5 billion times the mass of our sun, lay there, and the activity around the center bolstered this assumption.

Black holes are powerful matter-sucking drains in space, and astronomers believe most galaxies, including our own Milky Way, may contain black holes at their centers. Black holes have not been directly observed, because their gravitational pull is so strong that nothing, not even light, can escape it.

So researchers have concentrated on what happens around the edges of black holes, just before matter is pulled in. When scientists trained the Chandra observatory on the centre of Perseus last year, they saw concentric ripples in the cosmic gas that fills the space between the galaxies in the cluster. "We're dealing with enormous scales here," Fabian said. "The size of these ripples is 30,000 light-years."

Fabian said the ripples were caused by the rhythmic squeezing and heating of the cosmic gas by the intense gravitational pressure of the jumble of galaxies packed together in the cluster. As the black hole pulls material in, he said, it also creates jets of material shooting out above and below it, and it is these powerful jets that create the pressure that creates the sound waves.

To scientists, he said, pressure ripples equate to sound waves. By calculating how far apart the ripples were, and how fast sound might travel there, the team of researchers determined the musical note of the sound. Fabian said the notion of singing black holes might well be extrapolated to other galaxies, but not necessarily to the Milky Way.


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