I was recently reading an article on space.com about : What's the Temperature of Outer Space.

They said :

Some parts of space are hot! Gas between stars, as well as the solar wind, both seem to be what we call "empty space," yet they can be more than a thousand degrees, even millions of degrees.

But here is my question - "How do we define temperature in outer space?"

We perceive temperature as the average translational kinetic energy associated with the disordered motion of atoms and moleculules, so how can the temperature be so high at some places, if there isn't any (or very, very few) molecules out there in vacuum?

  • $\begingroup$ Temperature is what a thermometer reads. Whatever it is that temperature is a measure of, it is reflected by the reading on a thermometer. The average translational kinetic energy you mention, results from that definition retrospectively. I hope that clears some things up. $\endgroup$ – George Nov 30 '17 at 10:18
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    $\begingroup$ @George that is incorrect. Temperature is related to log(entropy); a thermometer is a tool which measures temperature correctly only when in a proper environment, such as earth's atmosphere $\endgroup$ – Carl Witthoft Nov 30 '17 at 13:43
  • $\begingroup$ Temperature is a physical quantity expressing the subjective perceptions of hot and cold. That's the definition. All other relations, including the one with entropy you mention stem from that definition and these relations are defined retrospectively. A quick search of the definition of temperature will convince you of this. $\endgroup$ – George Nov 30 '17 at 14:40
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    $\begingroup$ @George: Sorry, wikipedia is not an authoritative source on this. Thermodynamics is. And subjective perceptions for sure don't have a solid, reproducible definition. $\endgroup$ – AtmosphericPrisonEscape Nov 30 '17 at 22:24
  • $\begingroup$ Wikipedia is never an authoritative source. All I'm saying is that the definition is subjective perception, but the physical meaning of temperature is indeed the measure of the average kinetic energy of particles due to their thermal movement. My point is that there is a distinction between the definition and what it represents. At least that is what we were taught in thermodynamics back in the day. $\endgroup$ – George Dec 1 '17 at 6:58

First of all, the medium they are mentioning is far from empty even though it is much less dense than what we do know on Earth (see this question for more details).

Then, the problem with temperature in astrophysics is that the medium you study is in general far from thermodynamic equilibrium (thermodynamic equilibrium means that there are no flows of enery or matter within the system and with the outside of the system).

Most commonly, however, we can use another flavor of equilibrium to estimate temperature. This is the kinetic equilibrium: since most collisions are elastic (meaning that the energy is conserved), particles velocities will follow a Maxwell-Boltzmann distribution. Depending on the ionization fraction (how much is your medium ionized?) this temperature will either the kinetic temperature of electrons or the kinetic temperature of hydrogen.

Then, if you want to understand these high temperatures, you have to take into account the photoionization rate and the photoelectric heating (a process in which you eject electron from the matter of the studied medium due to UV absorption). The thermal state of the medium then depends on the equilibrium between the energy absorption by the matter and the re-emission of this energy in thermal radiation. In this framework, the energy that can be absorbed comes from the interstellar radiation field (ISRF). In the solar neighborhood, it is dominated by six different types of radiation: galactic synchrotron emission of relativistic electrons (electrons accelerated to tremendous speed in the galaxy), the cosmic microwave background, the infrared emission of the matter heated by the light emitted by stars, emission from ionized plasma, the light emitted by stars itself, and lastly X-rays from hot plasma.



In the absense of any particles, we evaluate temperature by the radiation wavelengths seen. In "empty space" only the cosmic background radiation exists, so we mark it pretty cold. see, for example, this note.

However, if you look at a chunk of space containing even a few particles, then the temperature is defined based on the mean kinetic energy of the particles, or more precisely, the amount of entropy present. See temperature as intensive variable


You are correct in your definition of heat, in that it does need a medium. The "heat" in space, however, is radiant. It does need a medium in order to be called heat, but the energy is there. Scientists measure the radiant energy in space to see how hot it would be if it were in a medium. The space is not physically hot, but the energy there means that if you were to travel there, the energy would he transferred into you, and you would become hot.


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