You can't see a black hole, but can you hear it?

Yes. NASA's Chandra X-ray Observatory has detected sound waves produced by a supermassive black hole. The sound is too low for our ears to sense, but we can hear this symphony of extreme gravity with special instruments.

Supermassive black holes lurk in the center of most galaxies, including our Milky Way. Some are more active than others. That is, some supermassive black holes are actively pulling in stars and gas within their vicinity. This process, called accretion, releases terrific amounts of energy. Matter falling into a black hole gets so hot that it shines in X ray light, thousands to millions of times more energetic than the optical light our eyes can detect. So while we can't see a black hole proper, we can see matter around it as it falls toward the void.

But wait, this week's question is about sound, not sight. Dr. Andrew Fabian of the Institute of Astronomy in Cambridge, England, led a team of scientists observing a supermassive black hole in the Perseus galaxy cluster, which is located 250 million light years from Earth and contains thousands of galaxies. The team found that the black hole, in the center of this cluster, is bellowing a B-flat.

You may have heard that there is no sound in space. That's not entirely true. Yes, there is no sound in a vacuum. Those loud explosions in movies like Star Wars are from Hollywood's special effects crew. Sound travels by compressing a medium, such as the air on earth. Yet space isn't a complete vacuum. In regions rich in gas, sound can travel, albeit quietly.

The sound from Perseus is a result of pressure waves from the central black hole's outbursts, which churn through the hot gas in the Perseus cluster. Each wave is about 30,000 light years across with a period of oscillation of 10 million years. That's one low sound. How low can you go? Not as low as this. Humans cannot detect sound with a period lower than one-twentieth of a second. The black hole music is 57 octaves below middle C.

But wait, for the scientists, this finding is about X rays, not sound. The sound was "seen" as ripples in the abundant X-ray-hot gas in between galaxies. Most of the matter in Perseus resides between galaxies, not in stars. Scientists have long wondered why cluster gas is so hot, and now they have one possible answer: Black holes might be energizing the region with pressure waves.

Scientists estimate that the black hole has been ringing for about 2.5 billion years. This Perseus Gravity Opus in Bb may not by the prettiest symphony, but it is the most enduring.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.