Interferometry in the infrared provides unique insights into many types of astrophysical objects such as young stellar objects, stars in late evolutionary stages and active galactic nuclei. In particular, it is now possible to create aperture-synthesis images in the near-infrared. These images can be obtained with a spectacular resolution by combining (interfering) the light from the telescopes of interferometers such as ESO's Very Large Telescope Interferometer (VLTI). This method is able to reconstruct images with a resolution equivalent to the theoretical resolution of a telescope with a diameter of 200 m. Interferometry in the spectral ranges of 1-2 and 7-13 microns can simultaneously provide a high spectral and high spatial resolution, reaching an angular resolution of a few milli-arcseconds.
Stars mostly form as part of a whole cluster of stars. In this phase the young stars are surrounded by a gas/dust disc, which under favourable circumstanes provides the material for the formation of a planetary system. In these young clusters the stars are so densely packed that close fly-bys are a common event. The acting gravitational forces between the stars are so strong that they potentially disturb the discs and just form planetary systems. Computer programs are developed allowing to simulate this situation on high-performance computers. Thus it is possible to determine in how far the cluster environment influences star and planet formation.