The vastness of space brings me a sense of chilliness even though I have never experienced it, although I wish to. Just how cold is interstellar space (on average)? How is this even measured? I mean you can't just stick a thermometer in space, right?

  • $\begingroup$ If you leave aside incident radiation from stars near and far, and particles impinging on your thermometer, and truly try to measure the temperature of interstellar space, in other words, the vacuum itself, you find that space has not temperature. If you're not holding the measuring device and it's not generating any energy itself, whatever heat it holds with radiate away, and eventually your thermometer drops to almost absolute zero. But even then, you're not measure the temperature of the vacuum, just the temperature of the thermometer. $\endgroup$ – Howard Miller Jun 8 '16 at 2:57

You can stick a thermometer in space, and if it is a super-high-tech one, it might show you the temperature of the gas. But since the interstellar medium (ISM) is so dilute, a normal thermometer will radiate energy away faster than it can absorb it, and thus it won't reach thermal equilibrium with the gas. It won't cool all the way to 0 K, though, since the cosmic microwave background radiation won't allow it to cool further than 2.7 K, as described by David Hammen.

The term "temperature" is a measure of the average energy of the particles of a gas (other definitions exist e.g. for a radiation field). If the gas is very thin, but particles move at the same average speed as, say, at the surface of Earth, the gas is still said to have a temperature of, say, 27º C, or $ 300\,\mathrm{K}$.

The ISM consists of several different phases, each with their own physical characteristics and origins. Arguably, the three most important phases are (see e.g. Ferrière 2001):

Molecular clouds

Stars are born in dense molecular clouds with temperatures of just 10-20 K. In order for a star to form, the gas must be able to collapse gravitationally, which is impossible if the atoms move too fast.

The warm neutral medium

The molecular clouds themselves form from gas that is neutral, i.e. not ionized. Since most of the gas is hydrogen, this means that it has a temperature of roughly $10^4\,\mathrm{K}$, above which hydrogen tends to get ionized.

The hot ionized medium

Gas that accretes onto the galaxy in its early phases tend to have much larger temperature, of roughly $10^6\,\mathrm{K}$. Additionally, the radiative feedback from the hot stars (O and B), and the kinetic and radiative energy injected by supernova explosions ionize and heat gas bubbles that expand. This gas comprises the hot ionized medium.


The reason that the ISM is so sharply divided into phases, as opposed to just being a smooth mixture of particles of all sorts of energies, is that gas cools by various physical processes that have a rather temperature-specific efficiency. "Cooling" means converting the kinetic energy of particles into radiation that is able to leave the system.

Hot gas

Very hot gas is fully collisionally ionized and thus cools mainly through free electron emitting Bremsstrahlung. This mechanism becomes inefficent below $\sim10^6\,\mathrm{K}$.

Warm gas

Between $10^4\,\mathrm{K}$ and $10^6\,\mathrm{K}$, recombinations (i.e. electrons being caught by ions) and collisonal excitation and subsequent de-excitation lead to emission, removing energy from the system. Here the metallicity$^\dagger$ of the gas is important, since various elements have different energy levels.

Cool gas

At lower temperatures, the gas is almost fully neutral, so recombinations cease to have any influence. Collisions between hydrogen atom become too weak to excite the atoms, but if molecules or metals are present, it is possible through fine/hyperfine lines, and rotational/vibrational lines, respectively.

The total cooling is the sum of all these processes, but will be dominated by one or a few processes at a given temperature. The figures below from Sutherland & Dopita (1993) shows the main cooling processes (left) and the main cooling elements (right), as a function of temperature:


The thick line show the total cooling rate. The figure below, from the same paper, shows the total cooling rate for different metallicities. The metallicity is a logarithmic scale, so [Fe/H] = 0 means Solar metallicity, and [Fe/H] = –1 means 0.1 times Solar metallicity, while "nil" is zero metallicity.


Since these processes don't cover equally the full temperature range, the gas will tend to reach certain "plateaus" in temperatures, i.e. it will tend to occupy certain specific temperatures. When gas cools, it contracts. From the ideal gas law, we know that the pressure $P$ is proportional to the product of the density $n$ and the temperature $T$. If there's pressure equilibrium in the ISM (which there isn't always, but in many cases is a good assumption), then $nT$ is constant, and thus if a parcel of hot ionized gas cools from $10^7\,\mathrm{K}$ to $10^4\,\mathrm{K}$, it must contract to increases its density by a factor $10^3$. Thus, cooler clouds are smaller and denser, and in this way the ISM is divided up in its various phases.

So, to conclude, interstellar space is not as cold as you may think. However, being extremely dilute, it is difficult to transfer heat, so if you leave your spaceship, you will radiate away energy faster than you can absorb it from the gas.

$^\dagger$In astronomy, the term "metal", refers to all elements that are not hydrogen or helium, and "metallicity" is the fraction of gas that consists of metals.

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  • $\begingroup$ +1 for detail but what exact physical processes cause the ISM to be divided into phases or at least a link. $\endgroup$ – HotSaucey Jun 2 '16 at 19:05
  • $\begingroup$ @TheCodeMan: I updated the answer, elaborating on cooling processes, and gave some references. $\endgroup$ – pela Jun 3 '16 at 9:12
  • $\begingroup$ Thank you for all your effort! I would give you +5, honestly! $\endgroup$ – HotSaucey Jun 3 '16 at 10:57
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    $\begingroup$ +1. Just to make it clear for readers of this answer, when Pela refers to "metals" he isn't just talking about metals such as iron. To an astronomer, carbon, nitrogen, oxygen and neon are also metals. The only elements that aren't metals to an astronomer are hydrogen and helium (and perhaps lithium and beryllium). Paradoxically, lithium and beryllium are metals to a chemist, while carbon, nitrogen, oxygen and neon are not. The presence of any metals drastically changes the behaviors of interstellar gases and stars. The details on which metals are present isn't as important. $\endgroup$ – David Hammen Jun 3 '16 at 14:38
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    $\begingroup$ Excellent answer. One more thing to add is that there are many different kinds of temperature, and the answer here is just referring to the kinetic temperature. In the ISM, two other important temperatures are the excitation temperature and the ionization temperature. Ordinarily these are the same as the kinetic temperature, but in the ISM they can be substantially different. Here is a good overview of the variety of temperatures: ay201b.wordpress.com/2013/03/07/… $\endgroup$ – J. O'Brien Antognini Jun 3 '16 at 20:08

The title of the question asks about interstellar space, but the body asks about the interstellar medium. These are two very different questions. The temperature of the interstellar medium varies widely, from a few kelvins to over ten million kelvins. By all accounts, the vast majority of the interstellar medium is at least "warm", where "warm" means several thousand kelvins.

I mean you can't just stick a thermometer in space, right?

You can if you have Star Trek or Star Wars technology. Assuming an old-style bulb thermometer released in a place far removed from a star, the temperature of that thermometer would drop rather quickly, eventually stabilizing at about 2.7 kelvin.

With regard to a macroscopic object such as an old-style thermometer or a human in a spacesuit, there's a big difference between the temperature of interstellar space and the temperature of interstellar medium. Even if the local interstellar medium is in the millions of kelvins, that macroscopic object will still cool to about 2.7 kelvin because there's no substance to that hot interstellar medium. The density of the interstellar medium is so very, very low that radiation losses completely dominate over conduction from the medium. The interstellar medium can be very hot precisely because it is a gas (gases are a bit weird), and because it is extremely tenuous (extremely tenuous gases are beyond weird).

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  • $\begingroup$ +1 Good point with the lower limit of 2.7 K. Of course you're right that no ordinary thermometer will be able to absorb energy as fast as it will lose it. I corrected my answer. $\endgroup$ – pela Jun 1 '16 at 22:21

Just one further complication. It is possible to set up "refrigerators" in interstellar space. These are situations which are effectively the opposite of masers - the energy levels of the material involved (in this case, formaldehyde) can end up behaving as if they are cooler than the surroundings. As a result, you can see formaldehyde in absorption against the cosmic microwave background.

Just another example of the fact that, at the low densities of interstellar space, you have to look at the details of how individual atoms and molecules are behaving, because they are only poorly linked by collisions to the surroundings. And that makes for some neat effects.

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This is a historically important issue and I think it is worth adding a bit about this history to the excellent responses given above. The story illustrates the physical meaning of "temperature of space". In 1940, McKellar (PASP, vol 52. p187) identified some strange interstellar lines, previously seen by Adams in 1939 in the spectrum of a star, as lines due to the rotation of CN and CH molecules. These lines were at the time unique.

Their relative intensities could only be understood if the rotation (ie. spin) was due to the collisions of the molecules with photons at a temperature of 2.7K. A year later he revised this to 2.3K. For obvious reasons he referred to this as the "spin temperature": the temperature derived from spinning molecules. No other source suggested itself, and it was not until 1966, after the discovery of the cosmic background radiation, that McKellar's interpretation was linked with the cosmic background radiation at 2.725K. McKellar had found a "thermometer in space".

Ironically, Hoyle in 1950 criticized Gamow's 1949 view of a hot big bang by saying that the Gamow theory would provide a higher temperature to space than allowed by McKellar's analysis.

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The cosmic background of neutrinos is at a temperature of ~1.95K, below that of the cosmic background photons at 2.7K. There is no inconsistency here because those neutrinos were once in equilibrium with the photons just before the photons got heated by the annihilating electrons (~1 second after big bang). The loss of electrons caused the neutrinos to decouple from the photons at that point and are no longer in equilibrium.

So the "temperature of space" depends on whether you cite the photon or neutrino temperature, and what you measure depends on what kind of thermometer you use. The curvature of space time can also be associated with a temperature, but that's another story.

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