B♭ is not A♯

Astronomers say they have heard the sound of a black hole singing. And what it is singing, and perhaps has been singing for more than two billion years, they say, is B flat — a B flat 57 octaves lower than middle C.

The notes appear as pressure waves roiling and spreading as a result of outbursts from a supermassive black hole through a hot thin gas that fills the Perseus cluster of galaxies, 250 million light-years distant. They are 30,000 light-years across and have a period of oscillation of 10 million years. By comparison, the deepest, lowest notes that humans can hear have a period of about one-twentieth of a second.

The black hole is playing ''the lowest note in the universe,'' said Dr Andrew Fabian, an X-ray astronomer at the Institute for Astronomy at Cambridge University in England.

Dr Fabian was the leader on an international team that used NASA's Chandra X-ray Observatory to detect the black hole's notes as ripples of luminosity in the X-ray glow of the cluster. The discovery, announced last week at NASA headquarters in Washington and in a paper in the journal Monthly Notices of Royal Astronomical Society, might help solve longstanding problems regarding the structure of galaxy clusters, the largest, most massive objects in the universe, and the evolution of galaxies within them, astronomers said.

Far from being ''just an interesting form of black hole acoustics,'' as Dr Steven Allen of the Institute of Astronomy said in a news release, the sound waves might be the key to figuring out how such clusters grow.

Black holes, as decreed by Einstein's general theory of relativity, are objects so dense that neither light nor anything else, including sound, can escape them. But long before any sort of material disappeared into a black hole, theorists have surmised, it would be accelerated to near-light speeds by the hole's gravitational field and heated to millions of degrees as it swirled in a dense doughnut around the gates of doom, sparking X-rays and shock waves and squeezing jets of energy and particles across space.

Such furiously feeding black holes are thought to be the engines responsible for the violent quasars and other phenomena in the cores of galaxies. The new work suggests that such black holes can exert influences far beyond their host galaxies.

The biggest clusters, like the one in Perseus, can contain thousands of galaxies and trillions of stars. Paradoxically, most of the ordinary matter in them resides not in stars, but in intergalactic gas that has been heated by the fall into the cluster to temperatures of 50 million degrees or more. The gas glows brightly enough in X-rays to be seen far across the universe. Cosmologists use this X-ray glow to find clusters in the deep of spacetime.

It has long been a puzzle what keeps the cluster gas hot. Without a continuing input of energy, the gas at the centre would radiate its heat, lowering its pressure, and cooler gas would flow in from the outskirts, providing fresh fuel to make stars.

The key, or rather tone, to this is B♭.