  Recent studies of the nearest star-forming clouds of the Galaxy at
submillimeter wavelengths with the Herschel Space Observatory have provided us
with unprecedented images of the initial conditions and early phases of the
star formation process. The Herschel images reveal an intricate network of
filamentary structure in every interstellar cloud. These filaments all exhibit
remarkably similar widths - about a tenth of a parsec - but only the densest
ones contain prestellar cores, the seeds of future stars. The Herschel results
favor a scenario in which interstellar filaments and prestellar cores represent
two key steps in the star formation process: first turbulence stirs up the gas,
giving rise to a universal web-like structure in the interstellar medium, then
gravity takes over and controls the further fragmentation of filaments into
prestellar cores and ultimately protostars. This scenario provides new insight
into the inefficiency of star formation, the origin of stellar masses, and the
global rate of star formation in galaxies. Despite an apparent complexity,
global star formation may be governed by relatively simple universal laws from
filament to galactic scales.
