Why is Space a Vacuum

[Suicide Junkie]

The amount of matter in the universe is just very small compared to its volume.
On the order of 10^-28 kg per cubic meter.
The mass of a proton is 1.67 x 10^-27.

Now, most of the mass is also concentrated in stars, planets and whatnot, so the matter left to spread over deep space is even less.
Interplanetary space has a bit more than deep space, mot not too much.

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The oxygen molecules from your tank are all travelling quite fast in random directions, and since there are no longer any walls to hold them in, the random motions cause them to spread out quite quickly. Gravity will deflect them somewhat towards the nearby large bodies, but the velocity of those oxygen molecules is surprisingly high.

After they rapidly leave your immediate vicinity, they'll spread out around the solar system and the concentration drops to undetectable levels again.

A kilogram of oxygen has 2x10^25 molecules, but a cubic AU (radius of earth's orbit) has 3x10^33 cubic meters in it.
Evenly spaced and at orbital speeds, you'd bump into those lost molecules rarely enough to count each meeting.

[Renegade 13]

Gases such as oxygen exist in nebula's due to the nova's or supernova's that created the nebula's. During the final years of a star's life, progressively 'heavier' elements are combined via fusion to create even more, "heavier" elements. That is to say, at first hydrogen is fused to form helium. Then, once the hydrogen supply becomes a little short, the helium at the core of the star begins to fuse together to form other elements. Through this chain of events, the "heavy" elements are formed, elements like oxygen, lithium, etc.

So when the star goes nova, those elements are dispersed into interstellar space, forming a nebula. However, these nebulae are constantly expanding due to the forces that created them in the first place. So eventually, the elements that form them are dispersed until the elements are spread as evenly as possible.

So basically, what a vacuum is is all the atoms that form the universe trying to spread themselves as evenly as possible throughout the universe, trying to create a uniform pressure. So the gas escaping from the tank is simply trying to equalize its internal pressure with the pressure of space. However, the pressure of space is very low, and space is so huge, so the tank becomes virtually empty, except for a few molecules that remain.

Also, gravity does overcome the drive to equalize pressure on both large and small scales. Planets on the small scale, and galaxies on the large. And galaxy clusters and superclusters if you want to go that far.

But I've probably confused you enough already, so I'll stop there.

[Suicide Junkie]

The densest nebulae have about 10,000 molecules per cm^3.
The air we breathe has 10^19.

If they get too dense or cool, then they'll collapse into star clusters.

[Paul1980au]

Fully understand the vacuum is because there is a lack of any heavier elements hydrogen and helium making up 99.999% of the volume of interstellar space.

[Fyron]

Umm... invert that number and then divide some really huge exponentials into it and you have a more accurate number on the volume of space taken up by hydrogen and helium. Now, the vast majority of matter is made of those 2 elements, but the vast majority of space is empty.

[narf poit chez BOOM]

When molecules move and collide with something, they exert force on it, transfering some kinetic energy. This is what pressure is, a measure of the average force that the matter is exerting upon a surface, an object, a probe, etc.

ah, that explains it. so, if you froze something to near 0 kelvin, you could compact it alot?

[Phoenix-D]

Originally posted by Paul1980au:
Fully understand the vacuum is because there is a lack of any heavier elements hydrogen and helium making up 99.999% of the volume of interstellar space.

Fyron is correct..if this was true, letting oxygen out into space and then lighting a match would result in an explosion. It doesn't.

Hydrogen and helium make up the majority of elements because they are the simplest and take the least mass. For every, oh, carbon atom you could have TWELVE hydrogen atoms. For the metals and heavy radioactive elements, it gets even worse.

EDIT: and yes, things near 0K can be more compact. Note the can. Because as you compact them you decrease the volume and increase the pressure. The result of -that- is an increase in heat.

[ January 28, 2004, 05:13: Message edited by: Phoenix-D ]