The stuff between the stars

Look at the Milky Way on a dark night and you see a band of light with dark rifts running along it. The rifts are not gaps in the stars. They are clouds of dust lying in front, blocking the light of everything behind. The space between the stars is not empty. It holds gas, and mixed into the gas a little dust.

It is very thin all the same. A cubic centimetre of the air you are breathing, about the size of a sugar cube, holds about 25 million million million molecules. Spread the gas of the galaxy out evenly and the same cube of space would hold about one atom. Even the thickest clouds out there are a better vacuum than any pump on the Earth can make.

But space is so big that it adds up. This is a small dark cloud called Barnard 68, 410 light years away. It looks like a hole in the sky. It is a cloud of cold gas and dust with about twice the mass of the Sun, and it hides every one of the stars behind it.

Here is the same cloud again, with infrared light added in red. Infrared slips past dust that stops ordinary light, and the stars behind show through. About 3,700 were counted through the cloud, more than 1,000 of them visible only in infrared. They look red for the reason a sunset does: dust takes out the blue first.

Most of the stuff between the stars is hydrogen gas, and cold hydrogen gives out no light at all. It does give out one faint radio signal. Now and then the tiny particle going round a hydrogen atom flips over, and the atom lets go of a radio wave exactly 21 centimetres long. Radio telescopes tuned to it have mapped the gas across the whole galaxy, through all the dust.

All of this matters because it is what stars are made of. Where the gas is thickest and coldest it falls together into new stars, as in these pillars. When stars die they throw much of their gas back out, with new elements added. The same atoms go round again and again.