Star formation. Image 1 of 2. Artwork of a cloud of gas and dust coalescing during the formation of a star. Stars form when gas clouds contract under their own gravity. The gas increases in density and temperature as it contracts, eventually becoming hot enough for nuclear reactions to start. The initial ignition of a new star then blows away much of the surrounding material. Gas clouds rotate as they contract, which flattens the material into a disc. In the surrounding disc of gas and dust, planets and other bodies can form. For an image of this gas cloud at the ignition of a new star, see R590/101.