From the December 2005 issue

If space is empty, and if temperature measures how fast particles move, then how can space have a temperature?

By | Published: December 1, 2005 | Last updated on May 18, 2023
Space is not empty. It contains very tenuous gases, plasmas, and various types of radiation, like light, infrared, ultraviolet, X rays, etc. As you note, we’re measuring the average speed of randomly moving particles when we measure temperature.

The solar wind, which consists of electrically charged gases (ions and electrons) flowing away from the Sun at 850,000 mph (1.3 million kilometers per hour) on average, permeates interplanetary space. Near Earth, each cubic centimeter of space — a volume equivalent to that of a sugar cube — contains between 100 and 10,000 of these particles.

In interstellar space, between the stars of our Milky Way Galaxy, we’d find about one particle in every cubic centimeter. In intergalactic space, between the galaxies of our universe, we would need to expand our sampling volume by a million times — to a cubic meter, or 35 cubic feet — in order to catch anywhere between 1 and 100 ions, electrons, and neutral atoms.
Although the densities of these gases are at least 1,000 million million (1015) times thinner than air, they are not zero. The temperatures of these gases range from only a few degrees above absolute zero (the temperature of cold interstellar clouds) to over a hundred million degrees (the temperature of some intergalactic plasma).

But, even if a certain volume of space were completely empty of particles, it would still contain several forms of radiation, such as diffuse starlight, and the residual microwave radiation from the Big Bang.

Imagine a thermometer floating in space. As rarefied particles collide with this thermometer, the energy they transfer to it will increase its temperature bit by bit. The thermometer also will lose some of this energy by emitting infrared radiation (heat). The thermometer’s temperature is a balance between energy gained by particle collisions and radiation versus energy lost by giving off heat.

For a large, solid object embedded in a very tenuous, but hot, gas, this balance point is usually much colder than the gas temperature as indicated by its particle motions. Placed in intergalactic space, such an object would typically stabilize at a temperature less than 10 kelvins (–436° F [–260° C]). This is why we think of space as being frigid.

If we placed our thermometer somewhere that was completely devoid of all matter – that is, a true vacuum – its temperature would come to equilibrium with energy absorbed from radio waves, light, and the other radiation permeating space, such as the cosmic microwave background.

In deep intergalactic space, our thermometer would register a temperature of just 2.7 K (-454.8° F [–270.5° C]).

In deep interstellar space, the light of distant stars would raise its temperature between 4 and 10 kelvins more, depending on the details of the environment.

Placed near Earth and exposed to sunlight, the thermometer would reach a temperature close to Earth’s average surface temperature: 288 K (59° F [15° C]). — JOHN BALLY, UNIVERSITY OF COLORADO AT BOULDER