I apologize if this question is confusing, I will try to clarify a little... so, this is talking about 'space' in the form of space that exists outside of our atmosphere. Outer space.

I once read that the temperature of outer space varies, but is almost always above absolute zero with the theoretical exception of the eridanus supervoid (and relating voids), where the amount of heat sources present is few to none. But why is outer space above absolute zero in the first place?

If space is defined as the absence of everything, why is the ambient temperature of the absence of everything above absolute zero?

EDIT: For example, the region of space immediately surrounding a star (past the atmosphere). Why is the temperature of this space 'warm' even though there are no particles to heat up or excite?

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    $\begingroup$ "But why is outer space above absolute zero in the first place?" Because there is stuff in it, its not completely empty. Dust, radiation, you name it. $\endgroup$ – Polygnome Jan 17 '17 at 17:28
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    $\begingroup$ Isn't this more of a physics question as opposed to SEx? $\endgroup$ – Everyone Jan 17 '17 at 17:48
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    $\begingroup$ Two points: 1) Your starting premise, "If space is defined as the absence of everything," is faulty; 2) Temperature is an intrinsic property of matter. An absence of matter ipso facto cannot have a temperature. Matter that is located in space near a star will get warm because of radiative heat transfer from the star, not because the "space" itself is warm. $\endgroup$ – Tristan Jan 17 '17 at 18:06
  • $\begingroup$ @Tristan +1 except you can talk about the temperature of the cosmic microwave background (CMB) - basically a very large collection of photons, and I'm not sure if everyone calls photons matter. "...the thermal radiation left over from the time of recombination in Big Bang cosmology. ...or relic radiation". $\endgroup$ – uhoh Jan 18 '17 at 7:28
  • $\begingroup$ There's also the solar wind slowing to subsonic speed despite no sound :) These terms are somewhat redefined in vacuum of space. $\endgroup$ – SF. Jan 18 '17 at 20:25

The short answer is that heat is transmitted through space by radiation, but space has a temperature because it isn't truly empty. Or, better, it is not space that has a temperature at all, but the (very sparse) light and matter found in space.

Transmitting heat is not the same thing as having a temperature. You may remember learning in school about convection, conduction, and radiation as the three ways of transmitting heat. Convection, which is stuff that has heat moving around and carrying the heat to new places, does not work in perfectly empty space. Conduction, which is what you call it when rapidly jiggling atoms knock against their neighbors and make them jiggle faster, spreading the heat through the material, also doesn't work in empty space. But radiation, which basically means photons, can carry heat through perfectly empty space from one object to another. (By "perfectly empty", I mean "perfectly empty of matter", which I think is what you mean too.)

But you also ask a different question about how people can talk about the temperature of the space outside the atmosphere of a star. And the answer is that space is never completely empty, if you look on a large enough scale. Even between galaxies, there is estimated to be about an atom every cubic meter. And those atoms are moving. Since temperature is a measure of the energy of how much random motion there is, you can meaningfully assign a temperature to even the sparsest collection of atoms. You can also assign a temperature to the electromagnetic radiation, since photons have kinetic energy too.

Even though temperature has meaning in this case, it would do almost nothing to heat you up if you were out there. The interstellar medium can be immensely hot, but there is so little of it that the few atoms hitting you have essentially no effect, and you end up cooling off to near absolute zero by emitting radiation. This is similar to the way air at 450 degrees coming out of an oven is harmless to you, but you would burn yourself touching glass at the same temperature.

There are a lot of subtleties here. In a normal gas, atoms collide all the time, and so their speeds tend to get averaged out to be roughly similar. Some are slower and some are a good amount faster, but there's a formula for how many particles would be moving at a given speed that's different from the average. In space, they hardly collide at all, so those formulas don't apply. That changes the rules a bit, but people still find it useful to use the concept of temperature.

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    $\begingroup$ Radiation in space (the CMB for example) has a (very) well defined temperature, and it actually represents a reservoir of heat. There's definitely real, stored energy in all those photons. You could even extract a (very) little bit of it as an academic exercise if you had something colder than 2.7K handy and something to block radiation from all the other sources out there. In fact, that's roughly how (some of) the CMB detectors work - bolometers, little tuned pieces of metal that absorb the CMB photons and convert their energy back to heat, then measure it's temperature rise. $\endgroup$ – uhoh Jan 18 '17 at 7:46
  • $\begingroup$ This is a good point. When I have read about the temperature of a radiation flux I have always interpreted that expression in terms of the notional black body that emitted it. But I see your point that it is legitimate to consider it to be the temperature of the photons themselves. Is it correct to think that a body in a finite universe (e.g., perfectly reflective walls) would reach thermal equilibrium with the EM flux? $\endgroup$ – Mark Foskey Jan 18 '17 at 23:15

The concept of temperature started as a term in classic macroscopic physics. When dealing with steam engines, you transfer heat by steam convection and conduction through the walls of a heat-exchanger. These are the major mechanisms in a high pressure system.

Outer space is near vacuum. Convection and conduction do not play a significant role. It would be impossible to bring a sensor (which is a thermal reservoir of its own) into equilibrium with the few particles in outer space. The concept of temperature as mean kinetic energy of particles is meaningless in space.

Each body radiates "black body radiation" with a (Planck-) spectrum depending on its temperature. At the same time, it receives radiation from other bodies. This mechanism also establishes an equilibrium without the bodies being in contact or exchanging particles.

In space, if you are far from any star, not much of your field of view may be filled with matter to exchange radiation with. Most will be void. But even the cosmic background is not at absolute zero Kelvin; it has a radiation spectrum which can be translated into a temperature. This is the temperature of space. The radiation is not constant in all directions, but these inconsistencies don't relate to particles: a "cold spot" from earth's perspective in the cosmic background does not imply any particles in that direction are less thermally excited.

The question of background radiation is interesting for physicists and astronomers (including space-based telescopes). It it totally unimportant for space travel, because spacecraft never reach thermal equilibrium with the cosmic background.


Particles are heated up because of the presence of heat, but they are not the medium through which heat is necessarily transferred. Heat is radiation, which travels through vacuum just as well as (actually, better than) it does through matter.

  • $\begingroup$ "Heat is radiation" is not always true - heat can certainly be molecular or lattice vibrations, and probably other things as well. Thermal radiation (in the form of electromagnetic radiation) does indeed travel better in space than through matter - depending on what "better" means, but trying to get instruments and electronics to cool by radiation is a very big problem in space, compared to using convection in Earth's atmosphere. If you are building a satellite - a relevant concept here - you would really wish you had some matter that could take the heat for you. $\endgroup$ – uhoh Jan 18 '17 at 7:31

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