From the November 2012 issue

If you could get close enough to the Sun and hang out a microphone that wouldn’t melt, would it pick up any sound from our star?

Susan Newman, Richfield, Minnesota
By | Published: November 26, 2012
The Sun has millions of internal oscillation modes, but they have frequencies too low for humans to hear. In this computer model of one solar oscillation mode, receding regions appear in red tones and approaching ones in blue. Astronomers measure the frequencies of such modes and use theoretical models to learn about the internal structure and dynamics of the Sun. // Sun: SDO/NASA/AIA Consortium; Oscillations: NSO/AURA/NSF
Astronomers have been “listening” to the Sun’s “humming” since 1960. But, if the solar system is a recording studio, keep in mind that it resides in a vacuum chamber. The Sun’s music reaches us not as sound but as light. To record these solar “sounds,” you must replace the microphone with a photometer or a spectrograph, which measures subtle changes in the brightness or wavelengths of sunlight due to solar vibrations.

The Sun vibrates like a musical instrument, similar to an organ. Instead of the room-temperature air in an organ pipe, though, hot hydrogen and helium gas, whose temperatures vary with depth, fills the Sun. Instead of a cylindrical pipe, our star is a sphere, its shape molded by gravity, with no solid container. And while an organist forces pressurized air into a pipe to make it resonate, natural turbulence in the gas near the Sun’s surface causes its vibrations. This generates sound waves that travel into the Sun, are reflected when they reach its surface again, and cause the star to resonate in millions of frequencies.

Astronomers record these frequencies, not for their interstellar iPods, but to probe the Sun’s interior, the same way geophysicists use earthquake waves to study Earth’s interior. Because of this parallel, the technique is called helioseismology.

The Sun’s “loudest pitch” is about 0.0033 Hertz (Hz) — some 6,000 times below the lowest tone audible to a human ear. Music-minded astronomers take the frequencies of solar oscillations, multiply them by a million, and convert them into a soundtrack you can hear — although not quite the same as the microphone and sound system at a concert. But would the noise from the Sun’s turbulence be loud enough and in the right frequency range to hear?

Loudness depends on how much the pressure changes at the microphone (or ear). At the depth from which visible light can first escape the Sun (a few hundred kilometers), the pressure fluctuations need be only 0.1 percent to be as loud as a chainsaw. But the pitch of a chainsaw’s whine is about 4,000 Hz, a million times higher than the frequencies of the smallest turbulent eddies in supercomputer models of the solar atmosphere. Even if the Sun had waves a hundred meters from crest to crest, short enough to be the lowest audible pitch, you wouldn’t hear it because there’s no solid surface against which those waves could crash.

The shallows of the solar sea are indeed a noisy place, but not to human ears. The Sun is singing the sound of silence.

Jaymie Matthews
University of British Columbia, Vancouver