# Density of space

by 1,1,2,3,5,8,13,21
Tags: density, space
 P: 166 What is the currently accepted 'density of space'? different sources i read said it was anywhere from 1 atom/m3 and others said up to 100 atoms/m3. What is right and what is not, or is there really any way to tell? Thanks in advance, Fibonacci
 Emeritus Sci Advisor PF Gold P: 10,424 The matter density of space varies dramatically from point to point in that space. Are you asking for the average density of the entire universe? - Warren
P: 1,603
 Quote by chroot The matter density of space varies dramatically from point to point in that space. Are you asking for the average density of the entire universe? - Warren
or are you asking for the vacuum energy density?

MF

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PF Gold
P: 8,147

## Density of space

I think the numbers Fibonacci quoted are for the interstar density in the local region, the kind of thing a spaceship from Earth might have to deal with, or exploit. One hydrogen nucleus per cubic meter used to be the quick and dirty figure cited.
 P: 166 yes, i am looking for the interstar density, not one with lots of stars and crap. one seemed to be the main number, so i think selfAdjoint's answer is most likely correct Fibonacci
 P: 1,105 I didn't know space has a density.
 P: 915 The density of the interstellar medium is not uniform, there are series of bubbles where the density drops significatively. The Sun and other stars lie concretely inside the Local Bubble, and more concretely, inside a region called the Local Fluff, that has a density of 0.1 atoms/cm3. The Local Fluff is contained inside the Local Bubble
 P: 1,105 How many bubbles is our universe made up of?
 P: 915 Hi miss kitty, I don't know, but why don't you count you it by yourself in this photo? http://antwrp.gsfc.nasa.gov/apod/ap970424.html More seriously, I know that appart of the Local Bubble, there exist also the Loop I bubble, Loop II bubble and Loop III bubble. It would be nice to know the number of bubbles inside the Milky Way
P: 1,105
 Quote by meteor Hi miss kitty, I don't know, but why don't you count you it by yourself in this photo?
What? I'm confused...

 Quote by meteor http://antwrp.gsfc.nasa.gov/apod/ap970424.html More seriously, I know that appart of the Local Bubble, there exist also the Loop I bubble, Loop II bubble and Loop III bubble. It would be nice to know the number of bubbles inside the Milky Way
The picture by the way is very awesome. So what do the loops mean? Are they circles of bubbles scientists have plotted that are close to our solar system?
 Emeritus Sci Advisor PF Gold P: 2,977 The interstellar medium is composed of several "phases", in between which the density varies a lot. In the "hot" phase, densities are usually of order 0.001 atoms per cubic centimeter, while in the colder phase, it's typically 1 per cubic centimeter. On the other hand, at the centers of molecular clouds, things can get really dense (as high as a million per cubic centimeter). It all depends on where you look.
 P: 1,105 Huh, intriguing. Thanks SpaceTiger. The space doesn't seem as big as I thought it might be; then again we are talking about atomic and sub-atomic particles so 1 cubic centimeter is quite a bit. One thing I have been contimplating lately has been how can the space between the stars be so cold and the stars themselves and the space around them be so hot? How can there be such a massive temperature difference? How did the stars begin to burn in the first place?
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P: 2,977
 Quote by misskitty One thing I have been contimplating lately has been how can the space between the stars be so cold and the stars themselves and the space around them be so hot? How can there be such a massive temperature difference? How did the stars begin to burn in the first place?
The interstellar medium is actually quite hot, for the most part. Even the cold phase is about 100 times hotter than on the surface of the earth. Only in the centers of molecular clouds do things get really cold.

It depends on how you define "cold", though. Our human sensation of cold has to do with total energy deposition on our skin, which depends on both density and temperature.
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P: 8,147
 Quote by SpaceTiger The interstellar medium is actually quite hot, for the most part. Even the cold phase is about 100 times hotter than on the surface of the earth. Only in the centers of molecular clouds do things get really cold. It depends on how you define "cold", though. Our human sensation of cold has to do with total energy deposition on our skin, which depends on both density and temperature.

That defintion of hot just means the particles are moving fast. The definition of absolute temperature is just the average speed of the particles, nothing said about their density. But as Space Tiger suggested, if you had the misfortune to be immersed in such a region of space, particle speed would just mean you were irradiated while freezing to death, assuming you hadn't already exploded from internal pressure!
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P: 2,977
 Quote by selfAdjoint That defintion of hot just means the particles are moving fast. The definition of absolute temperature is just the average speed of the particles, nothing said about their density.
I can't tell if you're agreeing with me or not. The density dependence I spoke of was referring only to the magnitude of our "sensation" of hot and cold. In other words, if we're sitting in 10^4 K interstellar gas with densities of one per cubic centimeter, we won't feel "hot".
P: 1,105
 Quote by selfAdjoint That defintion of hot just means the particles are moving fast. The definition of absolute temperature is just the average speed of the particles, nothing said about their density. But as Space Tiger suggested, if you had the misfortune to be immersed in such a region of space, particle speed would just mean you were irradiated while freezing to death, assuming you hadn't already exploded from internal pressure!
Wait, whoa! WHAT?!?! Self, you have been so patient with my ignorace, but could you expand on that for me?

Ya, got my attention this time. Not that you didn't have it before.
 P: 2,223 Temperature is the measure of kinetic energy of the particles in an atom. In space, pressure is nearly nonexistant due to the lack of atmosphere, this will cause your body to explode because the inside of your body is pressurized, my guess is at a pressure of 1 atm.
 P: 1,105 So wait, if space has a lack of atmosphere, then how and why does it form around the planets and how they pull matter into them when they first start out?

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