Molecular observations at high and low redshifts with the Odin satellite

The papers in this thesis have the aim of expanding our knowledge of different processes related to star formation in the early Universe and at present times. A complex chemistry occurs in star forming clouds and by observing molecules, information about the necessary physical and chemical conditions for star formation can be obtained. Our main focus is to observe the important water molecule using the Odin satellite since the strongest transition at 557 GHz is not observable using ground-based telescopes.The first two papers describe a spectral line survey in a previous unobserved spectral range towards the well-known Orion KL star forming nebula, covering a 42 GHz wide frequency band. We found 280 spectral lines from 38 species, and additionally 64 lines which remain unidentified. Water is very abundant in the powerful molecular outflows, while less abundant in the extended quiescent gas cloud. In addition to water, we observed H_2^18O and, for the first time in the interstellar medium, also H_2^17O. Remarkable similarities are found between the outflow emission observed from the water isotopologues and those of SO_2 and SO. The third paper covers observations towards the star forming region within the molecular cloud S140. Here we focused the observations on three species: H_2O, NH_3, and CO including the isotopologues ^13CO and H_2^18O. The observations were performed at five positions to reveal emission changes moving across the star forming center towards the edge of the cloud. The water abundance is found to be about 100-1000 times lower in the outflows as compared to Orion KL, but is similar in the extended cloud.The emissions of water and ammonia in the extended cloud show a striking similarity.Paper IV describes our searches for weak signals emitted from molecules before the first stars were born during the cosmic Dark Ages. Star formation during this epoch differs substantially from star formation at present times, due to the extremely low molecular abundances. No observational evidence exists today from this epoch, and the search is extremely difficult. A detection would, however, be of utmost importance and would for the first time give direct evidence about the beginning of structure and star formation in the Universe