In the dark heart of the Perseus galaxy cluster, 300 million light-years from Earth, a supermassive black hole has been singing the same note for 2.5 billion years. Its tone registers 57 octaves below middle C and, according to scientists at NASA's Chandra X-Ray Center, is a resounding B-flat. Yet, how is this possible in the vacuum of space?

Sound requires a medium, such as water or air, to travel. Here on Earth a sound wave moves from its origin by causing the surrounding air molecules to vibrate. The vibrations pass from one molecule to another; when they hit an ear, they are understood as noise. But because neither air nor water nor much of anything else exists in the majority of vast reaches of space, it is difficult for sound to travel there.

It takes a supermassive black hole—like a robust opera diva—to sing a resonant note in space. These monstrous celestial objects range from hundreds of thousands to tens of billions times our sun's mass and are commonly found in the center of active galaxies. For example, Sagittarius A*—a supermassive black hole—sits at the center of our own galaxy, the Milky Way.

Black holes are notorious for their gravitational might, which is so strong that nothing can escape, according to conventional wisdom. But this isn't quite correct—some matter does. A black hole's gravity pulls a mishmash of matter and energy into its surrounding accretion disk—a ringlike structure formed by gas and dust. But some of this matter is violently expelled from the black hole's poles as "relativistic jets." These jets surge into the scorching gas surrounding the hole and generate pockets in the otherwise uniform cloud.

"Sound waves are pressure waves. And black holes, or at least their relativistic jets, can generate enormous sound waves, which then propagate through surrounding galactic gas," explains astronomer Steven Allen, a professor of physics at Stanford University who studies the Perseus galaxy cluster. "When relativistic jets, which contain material moving at close to the speed of light, slam into the hot gas that pervades giant elliptical galaxies and clusters of galaxies, they beat a 'galactic drum,' as it were." The jet acts as the "stick," whereas the surface of the gas is the "drum."

Although people can't hear these waves (because sound can't travel through the vast vacuum separating this "drum" and us), we can "see" them using x-ray observations. As sound waves spread through the scorching gas in galaxies and galaxy clusters, regions of greater pressure (sound wave peaks) tend to appear brighter in x-rays; fainter regions (troughs) are dimmer.

Chandra x-ray telescope observations of the Perseus Cluster show roughly concentric ripples of brighter and fainter gas, which indicate sound waves. "We can't see the waves moving," Allen says. "The relevant timescales are too long, since the period of the waves is about 10 million years—but we have a clear 'snapshot' of them."

Perseus' black hole is not the universe's sole galactic vocalist. M87, a galaxy that holds one of the universe's most massive black holes, is also known to croon. Although its song isn't as steady as Perseus', it is more involved, with notes as deep as 59 octaves below middle C.

"There's no reason for black holes to sing the same note," says Peter Edmonds, an astrophysicist at the Chandra X-Ray Center. Galaxies that have more matter may provide a deeper sound, because this matter could lead to bigger, but less common eruptions from the black hole. There are bound to be other important factors contributing to a black hole's specific sound, such as the temperature of the gas and its location, but the details aren't well understood, says Edmonds.

Other interstellar objects and events produce sound waves as well, he adds. In fact, the echoes of the big bang have been humming and hissing since shortly after the universe's birth.

5 Comments

1. 1. vimalshashi 11:25 PM 10/7/08

   In this article, it is quiet intriguing to assume that sound will travel in vacuum. There is possibility that these sounds have traveled into the space through photons. Photons are the supposed elementary matter of light. It might be possible that there exist different types of photon-like matters, which are still not understood by human ( For the time being we call these matters as subphotonic matters.) . In this light, we can think black-hole-sound to consist of photons oscillating in uniform intensity pattern. This hypothesis sounds will be consistent with rational thinking.

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2. 2. Stathve1 08:59 PM 9/15/09

   I dont get it. Sound is the definition of vibration patterns hitting the ear, so without this vibration, how does the "sound" exist?
   

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3. 3. bertwindon in reply to Stathve1 02:42 PM 5/7/10

   Our auditory systems can detect a range of frequencies. Some folks are deaf. A deaf person realises that sound is real even though they cannot detect it. Maybe they used to hear ok, or a bit. The existence of anything is not dependant upon our ability to detect it. e.g. if you shut your eyes - or bury the head in sand - the world still exists.
   "Sound waves" are a physical phenomenon which - unless we are deaf, of the waves are of a frequency outside the range of our hearing - we "hear".
   And it appears, from Chanra's pictures (in X-ray), that sound waves of these mind-gnumingly low frequencies, do in fact propagate through the incredibly low-pressure gas in the interstellar space ?. That's what it seemed to say to me.
   57 octaves below the freq. of middle c is middle c (244 Hz ? ) divided by is 8 to the power 57. Shall I risk it on my pocket calculator ? will you buy me a new one if it breaks ?!

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4. 4. bertwindon 02:45 PM 5/7/10

   Sad lack of "concentric rings" in the photo supplied. I'm quite ready to believe it. Is it still April 1st ?

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5. 5. Peymael 11:21 AM 8/18/10

   I guess this black hole is the "heart" of the Universe

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