Understanding the processes by which stars and planets form in the cold and dark interiors of molecular clouds continues to be one of the most fascinating challenges of modern astrophysics. What fraction of clouds forms stars and on which timescales? How does the formation of high mass stars differ from that of their low-mass counterparts? What is the impact of young stars on their surroundings? How does the gas and dust evolve and coagulate in disks to form planets? Because of high dust obscuration, star birth is best studied at infrared and submillimeter wavelengths. New observational facilities now have the sensitivity to probe the youngest protostars which are still accreting much of their mass, as well as the spatial resolution to resolve planet-forming zones in circumstellar disks.
The interstellar medium also contains much more diffuse atomic and ionized gas which is not in the process of forming stars. Similarly, the space between galaxies is filled with a hot and diffuse intergalactic medium. Supernovae, winds and photons from massive stars or entire galaxies maintain a complex temperature, density and elemental abundance structure, from which eventually new galaxies may form. See http://ism.strw.leidenuniv.nl/.
In Leiden, studies on star and planet formation range from the formation of low-mass stars in local galactic clouds to that of high-mass ultra-young star clusters in nearby galaxies. Specific focus areas include the structure of planet-forming disks, the early stages of growth of planets from submicron-sized dust grains to centimeter-sized dust bunnies, the chemical evolution of the material from cloud cores to disks, the interaction of ultraviolet photons and shocks from young stars with the surrounding cloud, the maser phenomenon, and the atomic and molecular gas in starburst galaxies, in low-metallicity galaxies such as the LMC, near the centers of active galaxies and in cluster cooling flows. Special effort is put into developing modeling and radiative transfer tools to analyze dust continuum and atomic and molecular line observations.
Studies of the diffuse medium include the role of microscopic processes involving large molecules and small dust grains in the shaping of the macroscopic structures of the interstellar medium, the structure of the Galactic magnetic field through radio polarization data and quasar absorption line observations of the interstellar medium in high-redshift galaxies.