Star formation occurs due to the collapse of dense parts of molecular clouds. To study star formation in detail it is important to also study the interstellar medium (ISM) and molecular clouds as precursors to the star formation. Once a star has formed it has a strong impact on the evolution of the surrounding medium. The UV radiation ionises the surrounding gas and creates a hot HII region. The heated gas starts to expand into the surrounding cold molecular cloud, creating a shock-wave. This shock-wave compresses the cold gas leading to pillar like structures. In the tip of those structures the formation of new stars is possible. Another important task to understand star formation in more detail is to study this process under extreme conditions. One example is the region close to the milky-way galactic centre black hole. Here tidal forces, which would disrupt a typical molecular cloud due to the deep potential, prevent star formation in the standard way. Still there are observations of a disk of stars on a sub-parsec scale near the black hole. The large scale impact of star formation on the ISM is due to supernovas. This process can heat and stir the ISM, leading to its multiphasic structure. It also enriches the ISM with elements of higher order. The so called local bubble, a region of low density gas inside which our solar system currently resides, is a interesting source to study the effect of supernova feedback. The bubble is believed to have formed by supernovas blowing out a large, peanut shaped region in the ISM. At the edges of this region, clusters of stars can form. The supernova feedback of those stars is directed towards the low density bubble (the so called champagne-effect) and enriches the bubble with heavy elements.
Stars are known to form in massive clouds of molecular hydrogen (GMCs), which are highly structured and turbulent. Understanding the origin and evolution of these clouds is a key problem in astrophysics. Here at the LMU and MPE we perform simulations of galaxies and colliding flows to better understand the nature of GMCs and their formation. Simulations of the interstellar medium (ISM) in galaxies show the formation of GMCs by the agglomeration of smaller clouds, as gas passes through the spiral arms (left). These calculations highlight how the properties of the clouds are related to their formation: the spacing of the clouds along the arm is proportional to the strength of the spiral potential, whilst the cloud rotations and internal velocity dispersions are regulated by cloud-cloud interactions and collisions. Without stellar feedback, these clouds last several 10's, or even 100 or more Myrs. We are currently performing calculations which include stellar feedback. In this case, the lifetimes of the GMCs are typically much shorter, the lifetimes and also the morphologies of the clouds governed by the star formation efficiency adopted in the calculations.