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.

1. The quantity n/V is a concentration of particles, so we plug in the 100,000 part. per cm^3 we looked up
2. We use Avogadro’s number to convert the particles to moles, because problems are easier to do in moles
3. R is the gas constant, which we look up: 8.314 J/mol/K
4. T is the temp, 10 kelvins
5. 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.”)

We talked about how there is a lowest possible temperature, and that it’s absolute zero: -273 °C. You may wonder: how did they discover that? They must have had a bunch of fancy equipment and huge rooms full of machines to achieve low temperatures, right?

No. They couldn’t get anywhere near absolute zero when they discovered it. (Even though now we can, and yes you have to have lots of fancy equipment.) What they did was realized that PV=nRT was a good relation between properties of a gas. Think about this: what if P became 0 in that equation? Then T would also have to be zero. So a gas in a pure vacuum will have zero temperature.

Well, a pure vacuum doesn’t really exist, because if the gas is there, then there can’t be a pure vacuum. But this is still useful, because what they did is measured the P of several gases at 2 T values. That’s easy enough. Then you notice that they all extrapolate back to the same “zero” temperature. This is absolute zero.

Lots of people in the history of science contributed to this, but Joseph Louis Gay-Lussac was the first person (in 1802) to use the number -273.

To consider absolute zero, let’s mention an ideal gas. If you’ve ever taken chemistry, you’ve probably heard of something called an ideal gas. An ideal gas is a simplified model to describe how gas molecules act. It treats them as essentially like billiard balls. So if you’ve played pool (or taken physics) you’ve got a handle on that. (BS Alert: they’re like billiard balls with zero size. But we digress …)

The equation you derive for this is PV=nRT, which kind of shows you that “pressure and temperature of an ideal gas are proportional to each other.” If T goes up, P goes up. If you don’t like math or equations, you can still totally understand this:

• Temperature makes the balls move faster
• Pressure is cause by them hitting the walls of the box they’re in

So, totally non-math: if they go faster, they hit harder. Easy, right? If T goes up, P goes up.

Don’t do this, but if you throw an old canister in a fire it will explode. Because the billiard balls inside go so fast they blow the canister open. (And the billiard balls are the gas molecules, like air molecules.)