This thesis aims to study the earliest heavily obscured phases of massive star formation in starburst (SB) galaxies through a multi-line analysis of the HC3N rotational transitions from the ground and vibrationally excited states (HC3N*). Due to its unique spectroscopic properties, we also discuss the potential of HC3N* as a tool to discern whether the obscured nuclei of galaxies are being heated by a SB or by an accreting super massive black hole (i.e. an active galactic nucleus; AGN). Star formation is one of the most relevant physical processes in the formation and evolution of a galaxy. In particular, the energy released back (feedback) to their surrounding interstellar medium (ISM) from massive stars profoundly impacts the galaxy’s evolution. In a SB galaxy, most star formation takes place in the nucleus and in the form of compact massive star clusters known as superstar clusters (SSCs), which represent one of the most extreme models of star formation. Another major source of energy that can profoundly impact the ISM of a galaxy is an AGN. Both massive star formation and the AGN can reduce the star formation efficiency of a galaxy and even halt future star formation through feedback mechanisms. Hence, to study the physical conditions of the gas driving the formation of SSCs and shed light on the formation of massive stars, which is still not well understood, we need to observe the earliest phases of massive star formation before significant feedback intervenes...