This was a very interesting talk. Two facts which surprised me are that most stars are binary and that Dr Bate considers that most stars have planets.

Stars are formed mainly in clusters, but can be formed in groups of 1 - 10. A cluster forms when a cloud of molecular hydrogen collapses under its own gravity. There are inevitably slightly denser regions within the cloud, and these clump together to form protostars. The stars heat up and warm the gas, the gas pressure rises and the gas is blown away. The Orion nebula is at the gas stage while the Pleiades are at the "gas blown away" stage.

Each individual forming star consists of a roughly round cloud, which starts to spin and forms into a disc under centrifugal force. A binary can be formed when one forming star moves into the disc of another one. In a crowded cluster, and with the cloud/disc stage lasting hundreds of thousands of years, this is common. Another way for a binary to form is for two discs to condense so close together that they create a tide in each other, but not so close that they coalesce. This is less common.

Planets condense out of the dust and gas in the disc, but there are problems with this - it is easy to model the creation of metre-size objects, and also the formation of bigger objects from kilometre-size ones. But there are no models for going from metre size to kilometre size. Gas giants are thought to form in the outer reaches of a system where it is cold enough for gas and ices to remain - further in, they would evaporate from the heat. But how do we get the "hot Jupiters" observed circling stars other than the Sun?

- Kathy Hall