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.
