Cameron Moran, of Queenstown, asks :-

Camryn Mabey, of Riverdale School, asked:-

How were stars formed? How did the sun get fire?

Gerry Gilmore, a New Zealand astronomer now based at Cambridge University in England and who uses enormous telescopes in Hawaii, Chile and Australia to examine such questions, responded.

Star formation starts with the ingredients: mostly hydrogen and helium (created in the Big Bang) but mixed with other chemical elements - especially carbon, oxygen and nitrogen (created in earlier generations of stars) in a large cloud, called a molecular cloud. The name relates to the important though trace ingredients, molecules (over 100 types are known, but CO and water are the most common), which allow the cloud gasses to cool, and collapse into dense 'cores'. These cores collapse over one to a few million years into dense clouds about the size of an eventual solar system.

The very centre contains the most dense region, where infalling matter collapsing under its own weight forms an extremely dense and hot gas cloud, growing and becoming hotter much faster than it can cool. The centre of this cloud eventually gets so dense and hot, being compressed by its own mass - above one million degrees - that the atomic nuclei are forced together, initiating nuclear fusion. This nuclear ',burning' is really a melting process, where four hydrogen atoms 'melt' together to form a new atom of helium. The helium atom weighs a little less than did the four hydrogen atoms, with the `lost' mass being converted to heat-energy in an amount given by E = mass times the speed of light times the speed of light. This heat generates enough pressure to sustain the weight of the gas cloud above it, leaving a stable, dense, hot gas cloud which is a star.

The residual gas, dust and molecules left over tend to form a disk, spinning rapidly around the new `proto'-star. Inside that disk lumps grow, accrete mass, and planets, comets, asteroids, etc form, until, after a few million years, the heat from the new star boils away what is left, and one has a star and solar system.