What is the composition of outer space?

I'm curious to know the atomic/molecular composition of outer space, excluding the celestial bodies suspended within it and theoretical "dark matter".

I'm also interested in knowing the approximate temperature of space that isn't being affected by a star or other nearby body, or if measurements like that have even been taken.

<edit> Sorry, I just realized that this should probabally be in general astronomy/cosmology, I'll look out for that in the future.</edit>

I'm curious to know the atomic/molecular composition of outer space, excluding the celestial bodies suspended within it and theoretical "dark matter".

The interstellar medium is about 75% hydrogen gas, 25% helium gas, and 1% dust.

http://www-ssg.sr.unh.edu/tof/Outreach/Interstellar/index.html?what1.html

I'm also interested in knowing the approximate temperature of space that isn't being affected by a star or other nearby body, or if measurements like that have even been taken.

Presently, it is 2.725&plusmn;0.002 degrees above absolute zero, the temperature of the cosmic microwave background radiation.

Thanks for the concise and informative answer.

I actually had someone on another forum tell me space was a complete vacuum and thus, being devoid of matter, at absolute zero...

Originally posted by wasteofo2
Thanks for the concise and informative answer.

I actually had someone on another forum tell me space was a complete vacuum and thus, being devoid of matter, at absolute zero...

the person who told you that is crazy. the fact that space is at approximately 3 kelvin(due to radiation) is the most evident proof of Big Bang.

It is impossible to have a ideal condition where there is nothing(including radiation).

-benzun
All for God.

Originally posted by benzun_1999
the person who told you that is crazy. the fact that space is at approximately 3 kelvin(due to radiation) is the most evident proof of Big Bang.

It is impossible to have a ideal condition where there is nothing(including radiation).

-benzun
All for God.

Mmm, even before I got an answer here, I knew that it couldn't possibly be a total vacuum or at absolute zero and I told this guy. He typed a smily with its eyes rolling and said "ok, absolute zero + about 4 degrees, It was just an estimate..."

i think the density of "stuff" in deep interstellar space is around 1 particle per cubic centimeter, where "stuff" is dust, hydrogen, etc.

ISM? IGM?

It's a good deal more complicated than ~1 atom/molecule per cubic cm, ~3K.

At the '0-th level':

"... the region between the stars in a galaxy like the Milky Way is far from empty. These regions have very low densities (they constitute a vacuum far better than can be produced artificially on the surface of the Earth), but are filled with gas, dust, magnetic fields, and charged particles. This is commonly termed the interstellar medium."
Source: http://csep10.phys.utk.edu/astr162/lect/milkyway/ism.html

Even within the Milky Way, the interstellar medium (ISM) varies by at least eight orders of magnitude in its density, and ~5 in its temperature. None of it is as cold as 3K; there's too much high energy EM about for it to get that cold, except in the hearts of dense, opaque clouds.

Some introductions/overviews:
http://www.stormpages.com/swadhwa/stellarevolution/lecture9.htm
http://www-ssg.sr.unh.edu/tof/Outreach/Interstellar/
http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/Html/Lec04/Lec04_pt7_interstellarMedium.htm

A concise summary of the VLISM ("very local interstellar medium), in section 2:
http://web.mit.edu/space/www/helio.review/axford.suess.html

Interestingly, the intergalactic medium (IGM) is thought to comprise ~30%+ of the baryonic matter in the universe (that's the stuff from which all stars, gas, dust, planets, etc is made up of; together amounting to ~4% of the universe). It's pretty rarified stuff, ~10^-23 particles per cubic cm, with a temperature of 10⁴ to 10⁸K, though values these are poorly constrained.

IIRC, the lowest density parts of the universe are in the bubbles evacuated by the jets from quasars. If the gas is flowing hypersonically, it may also be the lowest temperature natural material in the universe, maybe.

here is a best guess (my best anyway)

97.8% H
2.1% He
.003% H-II
.001% O2
.001% De
.001% Silica dust
.00001% C
.00001% N
.00001% Fe
.00001% M

and... that is a quess. this 95%/4%/1% is out of most froshman astronomy books. Try spectrographing and it is not that easy.

Let's just say there is a whole bunch of stuff in trace amounts..oh, and a bunch of Nuetral H.

Laters:

Dr. Bill