B. Whitmore: The life and death of star clusters

Stars generally form in clusters and in a starburst, there can be super-star-clusters (SSCs) that make the ones still forming in our Milky Way and also 30 Doradus in the LMC look rather pathetic.

The speaker talks about the clusters in the Antennae and by age dating them one can trace the different encounters of the two galaxies, during each of which SF is triggered. Maybe surprisingly, the kinematics over a cluster region seems to be smoothly rotating with small velocity dispersions. So one does not need high velocity "smashing in" of stuff, the increase of pressure seems to be enough.

Looking at different regions, it can be seen that around a massive older cluster, newer ones are found, the formation of which was triggered by the wind from the first one. This depends very much on density: if the wind has "free way" on one side, not much will happen there, but if it runs into some dense material, this gets compressed and can form new star clusters. This is what I meant yesterday by "positive feedback".

A major point that I should have mentioned is that what makes clusters so interesting to study, is that one can be sure that all the stars inside formed at the same time. So it is possible to just treat the as single objects and determine ages and other properties for each of them. And of course they are much brighter than a single star and can be studied at larger distances.

The IMF of SC is a power-law over many scales which means that low mass clusters are much more frequent than big ones. The speaker argues that there is no special "burst-mode" of star formation, but that the really big SSCs are just the tip of the whole IMF and it is only statistics that make this tip populated only in regions where you have a lot of star formation.