These clouds start to shrink under their own gravity. As the cloud gets smaller, it breaks into clumps. Each clump eventually becomes so hot and dense that nuclear reactions begin. When the temperature reaches 10 million degrees Celsius, the clump becomes a new star.

After their birth, most young stars lie at the centre of a flat disc of gas and dust. Most of this material is eventually blown away by the star’s radiation. Before this happens, planets may form around the central star.

Star-forming dark cloud imaged by ISOInfrared observatories are able to detect heat coming from invisible stars that are forming inside such clouds. One of the most powerful of these is ESA's Herschel space observatory, launched in May 2009. Herschel will spend at least three years studying the dusty clouds where stars – large and small - are born.

Clouds of gas are common in our galaxy and in other galaxies like ours. These clouds are called nebulae. A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. The majority of the gas in nebulae consists of molecules of hydrogen and helium--but most nebulae also contain atoms of other elements, as well as some surprisingly complex organic molecules. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova. The source of the organic molecules is still a mystery.