Horsehead Nebula
So we’re wondering if you could hear sounds in a nebula. We’ve figured out that sounds, which are waves of pressure, can be detected by humans if they are larger than 20 micro-pascals (μPa).
So what is the pressure in a nebula? The Clear Science staff looked it up, and found that a cold, dark nebula (like the Horsehead Nebula) will have at most at its core 100,000 particles per cubic centimeter, which is a box about the size of the end of your pinkie finger. Also, the temperature will be about 10 kelvins, or -263 °C.
Using a little math, we can figure out about what pressure this would mean. Don’t panic! This is using the ideal gas equation, PV=nRT, and the level of difficulty is about the same as in a high school Chemistry I class.
- The quantity n/V is a concentration of particles, so we plug in the 100,000 part. per cm^3 we looked up
- We use Avogadro’s number to convert the particles to moles, because problems are easier to do in moles
- R is the gas constant, which we look up: 8.314 J/mol/K
- T is the temp, 10 kelvins
- And the last two terms we add are unit conversions: 1: a joule is a newton meter, and 2: we convert to make sure all lengths are in meters and not centimeters
The answer we get is 14 pico pascals or pPa. This is much lower than 20 μPa, so no, there is not enough gas density in nebulae to support sound waves! (At least not the kind of waves we call “sound.”)
So we wondered, since sound travels through gases, does that mean you could hear a sound in a nebula? It will have to do with pressure. Pressure is the force per unit area exerted on any object due to the surroundings. Atmospheric pressure on Earth is about 101.3 kPa (kilo-pascals), but it varies up and down a bit depending on the weather.
Absence of pressure is called a vacuum. Outer space is pretty close to a vacuum, being about 0.1 pico-pascals, which is close to zero. Since sound is a wave of pressure, there is a threshold minimum pressure wave humans can hear, and it is about 20 micro-pascals. To hear sound, the surrounding pressure would need to be higher than that, to support a wave of that amplitude.
Wikipedia has a very nice list of significant pressure values, which the list above is partially compiled from.
Sound is technically an oscillation of pressure, i.e. waves, through a material. We generally experience sound as waves in our atmosphere, which is a gas, air. Gases are fairly separated particles moving rapidly and not well-connected to each other. There is also gas in space. Nebulae are interstellar clouds of dust and gas, often very pretty seen through a telescope.
So is there sound in nebulae? Luckily one member of the Clear Science staff moonlights at JPL when not clarifying science, and knew exactly how to approach this question. Let’s use the Horsehead Nebula as an example, which is what’s called a dark nebula. The Horsehead Nebula is in Orion, and was discovered in 1888 by Williamina Fleming at Harvard College. (You heard that right, Williamina was a woman, although science was pretty male back then.)
By the way, the visualization of sound waves shown above is from Bell Labs, and the Clear Science staff got the image from Modern Mechanix, one of the best websites on the internet. They love black and white science just like the Clear Science staff does.
