The Sun is unfathomably enormous. I mean, can you contemplate that you can fit 64.3 million of our moon inside the Sun? It’s too much for our tiny minds. So, what about the question: how loud would the Sun be if we could hear it?
Fortunately, this ‘counterfactual’ question was posed to science-friendly redditors, and while more than one solar astrophysicist took a break from the telescope, ‘Dr Zowie’ aka Dr. Craig DeForest, a solar astrophysicist at the Southwest Research Institute and PhD from Stanford, nailed the question. And there’s some really interesting facts and figures that give us an idea of how loud the Sun would be from over 149,600,000 kilometres away.
First fact: The Sun is immensely loud. Why?
DeForest: Heat gets brought to the surface of the Sun by convection – hot material rises through the outer layers, reaches the surface, cools off (by radiating sunlight), and sinks.
The ‘typical’ convection cell is about the size of Texas, and is called a ‘granule’ because they look like little grains when viewed through a telescope. Each one (the size of Texas, remember) rises, disperses its light, and sinks in five minutes.
That produces a hell of a racket. There are something like 10 million of those all over the surface of the Sun at any one time.
Most of that sound energy just gets reflected right back down into the Sun, but some of it gets out into the solar chromosphere and corona. None of us (professional solar physicists) can be sure, yet, just how much of that sound energy gets out, but it’s most likely between about 30 and about 300 watts per square meter of surface, on average.
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But wait, how do we know the Sun makes a huge racket? Well, we know what the Sun sounds like. Sort of.
DeForest: Instruments like SDO’s HMI or SOHO’s MDI [Helioseismic and Magnetic Imager for Solar Dynamics Observation] or the ground-based GONG observatory measure the Doppler shift everywhere on the visible surface of the Sun, and we can actually see sound waves (well, infrasound waves) resonating in the Sun as a whole!
Since the Sun is large, the sound waves resonate at very deep frequencies – typical resonant modes have 5 minute periods, and there are about a million of them going all at once.
How loud does all that at once sound?
DeForest: The surface generates thousands to tens of thousands of watts of sound power for every square meter. That’s something like 10x to 100x the power flux through the speakers at a rock concert, or out the front of a police siren. Except the “speaker surface” in this case is the entire surface of the Sun, some 10,000 times larger than the surface area of Earth.
But what does that mean for me on Earth, listening to the Sun?
DeForest: In terms of dBA, if all that leaked sound could somehow propagate to Earth, well let’s see… Sunlight at Earth is attenuated about 10,000 times by distance (i.e. it’s 10,000 times brighter at the surface of the Sun), so if 200 W/m² of sound at the Sun could somehow propagate out to Earth it would yield a sound intensity of about 20 mW/m².
0dB is about 1pW/m², so that’s about 100dB. At Earth, some 150,000,000 kilometres from the sound source. Good thing sound doesn’t travel through space, eh?
Now, for reference, 100 decibels is something like what a chainsaw, jackhammer, tractor or a noisy subway sounds like at about five metres away. Eight hours of listening to this would cause hearing loss.
You can imagine a very different world. The noise would pretty much drown everything out except for jet engines, gunshots and thunder from lightning.
NASA’s pushed some sounds from the Sun into YouTube from the Solar Dynamics Observatory (SDO), combined with some amazing images of the sun taken across different wavelengths.
Here’s the description: New video released by NASA from the SDO which was launched February 11, 2010. The Atmospheric Imaging Assembly (AIA) takes these images across 8 different wavelengths (out of the 10 available wavelengths) every 10 seconds. NASA SOHO recorded sounds of the sun added.
Imagine that low hum at the noise level of a chainsaw from just five metres away.
Footnote: Techly requested Dr DeForest’s permission to run this article.