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.”)

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.

We talked about the antiparticles, which form antimatter atoms when combined in the same way as regular particles and regular atoms. Hydrogen atoms are one electron orbiting one proton. Antihydrogen is one positron orbiting one antiproton.

Antihydrogen was produced at CERN in 1995. This was done by making antiprotons using a particle accelerator and shooting them into xenon clusters (a bunch of xenon atoms). Only a very small number of antihydrogen atoms can be made this way.

Theoretically, there would be a lot of antimatter in the universe, and therefore a lot of antihydrogen floating out there in space. This could result in higher antimatter atoms (helium, lithium, etc), and even antimatter stars and planets. This appears not to be the case, or at least it cannot be detected if it is.

So since the planets all came from our flattened protoplanetary disc, they all ended up in the same plane, meaning the solar system is flat: it looks like a frisbee, instead of like a ball. Except they’re not exactly in the same plane, but they’re close. The drawing above shows the solar system as seen edge-on.

The plane of the Earth’s orbit is called the plane of the ecliptic (because eclipses happen in this plane). The plane of the ecliptic is 1.6 degrees off of the invariable plane, which is a sort of average orbital plane of the entire solar system. If they were exactly the same, it would be 0 degrees off.

The other planets are also fairly close to the invariable plane, with Mercury being the furthest off by far, at 6.3 degrees to the invariable plane. The drawing above is not to scale of course, but you can see how close to flat variations in degrees this small are. Asteroids, comets, and other objects do not adhere to the invariable plane like the planets do, however.

It is thought that solar systems form when part of a diffuse cloud of gas (mostly hydrogen and helium) begins to collapse due to its own gravity. This is called a pre-solar nebula. At the center, a star forms, and much of the remaining material ends up rotating around the star in a flattened disc. This so-called protoplanetary disc eventually agglomerates into orbiting planets, which build up from the material in the disc over time.

Protoplanetary discs have been observed by astronomers around young stars close by. The sun, the Earth, and our fellow planets likely formed the same way. Nearly everything in our solar system turns in a counter clockwise motion, which means our protoplanetary disc and the nebula that lead to it must have been spinning this direction.

Why rotation at all? Anytime a lot of mass ends up in one place (the protostar), gravity there gets very strong. This pulls in the surrounding material (the disc). Anytime something spinning a little bit gets pulled in, it speeds up. (Watch a spinning ice skater pull their arms in—or try it in your desk chair!) So, the disc ends up rotating quite a bit.

### theclearscience@gmail.com

Alan Russell asks: Is the solar system really flat, as shown in every diagram? As in, do the planets really orbit in one two-dimensional plane? Why is this?

Fantastic question, Alan. The short answer is basically yes, although not exactly. Let’s elaborate …

We saw that at the beginning of the universe, the only elements present were the most simple ones: Hydrogen, Helium, and some Lithium and Beryllium.

Over billions of years, nuclear reactions in stars made heavier elements. Some of those include Carbon, Nitrogen, and Oxygen. These elements make up most of life on Earth.

They came from stars, and ended up here. They became part of the cycle of life. Some of them became you. You are made of star stuff.

We said the periodic table right after the Big Bang (about a minute after) was very simple, and had H and He (and a dash of Li and Be).

You might be skeptical (the Clear Science staff always are!): How can you possibly measure that? It’s just a guess right?

You actually can measure it. To study the early universe, you either look at things that are very old or are very far away. In fact, if you look very far away with a telescope, you are also looking back in time. Do you know why?

“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.”

— Carl Sagan

One of Carl Sagan’s favorite things to say was “We are made of star stuff.” And it’s true. For a minute, should we dwell on what he meant by this?