# Temperature of SPACE

## Main Question or Discussion Point

Temperature of SPACE!!

Temperature is a thermodynamic property of a substance and space is absolute vaccum, no substance. So does that mean temperature of space cant be defined? and it does not have any temperature.

What will happen if a red hot iron ball is kept in space? if we stop its heat loss by radiation then will it reatin its high temp. forever? 'coz in that cas ethere will be no conduction, convection and radiation.

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the temperature of space is very close to -273

In space, heat gain / loss only occurs by radiation. The temperature is determined by radiation in / out (which is higher).

The average temperature of space is ~2.7K. This would be 'empty' space outside the influence of stars.

You can never stop radiation loss totally, only slow it down. If you could stop radiation loss than yes, an object would retain its temperature. However, this is no different to saying "what if we could stop conduction, convection and radiation losses on earth?".

russ_watters
Mentor

Temperature is a thermodynamic property of a substance and space is absolute vaccum, no substance. So does that mean temperature of space cant be defined? and it does not have any temperature.
Correct. Space has no temperature per se. When people say "temperature of space" they really mean the temperature indicated by the background radiation traveling through space.

Correct. Space has no temperature per se. When people say "temperature of space" they really mean the temperature indicated by the background radiation traveling through space.
Wow, that's a lovely way to put it - I like that. I was going to say the same (probably in half a page!) but nowhere near as elegantly.

jambaugh
Gold Member

Temperature is a thermodynamic property of a physical system, whether you consider it a substance or not. If you were only considering fluctuations of the space-time geometry in the absence of any other matter-energy you can invoke thermodynamics and possibly define a temperature.

Also space is not an absolute vacuum. Even if you pump out the sparse interstellar hydrogen you have a "photon gas" which can only be "removed" by cooling a region to 0 degrees K. And even then it will have a temperature, namely 0deg K.

As far as your red hot iron ball is concerned, yes, if you say surround it with a perfect reflector and pump out any trace gas atoms, and do a couple of other things to prevent any interaction with the environment, then it will stay hot forever.

russ_watters
Mentor

There is no need to quibble over whether space is a complete vacuum or not: you can easily grab a parcel of space of a few cc that has no atoms in it because the density of space is pretty low. But it will still have quite a bit of radiation traveling through it.

....and if your detector (say, an infrared thermometer) is facing the sun, it may come to the conclusion that space is very hot. But that's because it's measuring the temperature of the object that emitted the radiation, not the temperature of the space that radiation was traveling through.

turbo
Gold Member

There have been calculations and estimates of the temperature of space for a long time. Even back into the 1800s. IIR, most have been pretty close to the CMB number.

Remember, this was back when most astronomers thought that everything we could see in the night sky was our "universe" and they had no idea that various "nebulae" were galaxies, similar to the one that we live in.

So, the arguement is - what is space? then. If we consider it to be the vacuum, then it has no substance to hold internal temperature - it is merely a void through which particles of various types 'wander' through. If we take it as an area of space and count everything in it - i.e. take the average thermodynamic temeperatures of the particles et al, then it has its own temperature.

Now do we say a tin can left in the sun on Earth is the temperature of the metal (as heated by the sun's radiation, the warm air's convextion and the road surfaces conduction) or do we include the temperature of the sun's rays streaming down on it too?

To me, the temperature of space - as asked by the op - is the temperature of the "substance" that is the vacuum - and therefore can have no temperature as it has no mass/energy in itself.

If the question was how hot is such an such an area of space, then we look at ambiant temperature (those sparse particles carrying their own temp) plus the radiation passing through.

Problem is, through no fault of his own I'd guess, the op's question is too vague and thus we all apply our own understanding of the answer required.

Smile folks only 311 days to Christmas :D

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I think I'd like to clarify something here. When I said the "temperature of space" I was referring specifically to the equilibrium temperature of an object left in space - space as I defined in my post.

D H
Staff Emeritus

Vacuum is a relative term. Space is permeated by a very tenuous gas. Even the space between galaxies is not completely void of substance. As a substance, this interplanetary / interstellar / intergalactic medium has a temperature. How to measure it?

A macroscopic thermometer will never be able to measure the temperature of this medium. Radiative processes dominate over heat transfer to / from this medium because it is so tenuous. If a macroscopic thermometer is shielded from nearby sources of radiative heat (e.g., the sun), the thermometer will not come to thermal equilibrium with the local interplanetary / interstellar / intergalactic medium. It will instead come to thermal equilibrium with the CMBE, ~2.7 K. In the sense of the reading from a macroscopic thermometer, the temperature of space is very low.

The temperature of the medium can be measured by looking at the radiation from the medium itself. In this sense, the temperature of the medium can be extremely high. Millions of degrees high.

Thanx guys!!!