Colder than space.

Space is not quite as cold as cold can get. Wherever you look, it is filled with the faint glow of the oldest light, about 2.7 degrees above absolute zero. You might expect nothing in nature to be colder than that.

In 1995 two astronomers, Raghvendra Sahai and Lars-Åke Nyman, found something that is. Using a radio telescope in Chile, they looked at the Boomerang Nebula, a cloud of gas thrown off by a dying star in the constellation Centaurus, about 5,000 light-years away. The gas in its outflow is about one degree above absolute zero: the coldest natural place anyone has found.

The dying star is blowing its outer layers away in a wind of about half a million kilometres an hour. Gas that spreads out fast cools down, and this gas has spread out so fast that it has cooled below the glow of space itself.

The cold is also how they found it. Gas colder than the oldest light does not shine against it; it soaks a little of it up. Looking through the cloud at the glow behind, the astronomers saw it dimmed at the very wavelengths the cloud’s gas absorbs, and the dimming told them how cold the gas is. Later the ALMA telescopes, also in Chile, took a sharper look and confirmed the cold gas.

One degree above absolute zero, and still not zero. Even the coldest corner we know of stops short of the bottom of the scale.