How Cold Is Space?

How cold is space exactly?

Average temperature is 2.7 Kelvin, or about -454°F (-270°C). This is the temperature of the cosmic microwave background (CMB), the relic radiation left over from when the universe was about 380,000 years old. The CMB has been steadily cooling as the universe expands, dropping from thousands of degrees in the early universe to its current near-zero level. The CMB fills all of space and sets the baseline temperature of empty regions of the cosmos. Without it, space would be even colder.

What's the coldest place in space?

The Boomerang Nebula, at about -458°F. The nebula is a region where gas is rapidly expanding outward from a dying star, with the expansion cooling the gas to about 1 Kelvin (-458°F), below the cosmic microwave background temperature. The Boomerang Nebula is the coldest natural place ever measured in the universe. Other regions of space can also reach below the CMB temperature through various cooling processes. The fact that natural processes can produce temperatures below the CMB is an interesting wrinkle in our understanding of cosmic temperatures.

How hot can space get?

Extremely hot in places. Near stars and in the hot gas around galaxies, temperatures can reach millions of degrees. The Sun's corona is about 1.8 million°F. Hot intergalactic gas in galaxy clusters can be even hotter, tens of millions of degrees. These hot regions are typically very tenuous, with low density even though high temperature, so an object in such regions wouldn't actually feel hot in the way you'd expect on Earth. Temperature in low-density space depends more on individual particle energies than ambient warmth.

Why is space so cold despite stars?

Because stars are tiny compared to the vast distances between them. Stars produce enormous amounts of heat, but the space between them is mostly empty. Heat doesn't transfer easily through near-vacuum, so most of space stays cold even in galaxies full of stars. Earth is warm because the Sun's relatively close. An object in deep space far from any star would slowly radiate away its heat into the surrounding cold and eventually approach the CMB temperature, cooling indefinitely if isolated from heat sources.