The solar system is located in a spiral galaxy known as the Milky Way. The galaxy formed from an ill defined globe of hydrogen and helium gas approximately 12 to 14 billion years ago. This proto galaxy gradually shrank under its own gravitational attraction. As it did so, smaller concentrations of gas condensed within it. These were the basis of globular clusters of stars, each orbiting the centre of what was to become the galaxy. Each in turn broke onto hundreds of thousands of smaller knots of gas, which evolved into stars.
The bulk of the proto galaxy collapsed into the centre without forming part of a globular cluster. As the gas collapsed, it whirled faster round the centre, as water whirls down a plughole. Most of the matter formed a huge flattened globe – the galaxy’s central bulge. Most of this gas broke up into stars, but some was spun out into a great rotating disc, 100,000 light years across. The rotation of the disc slowed the condensing of gas into stars, which continues today.