This thesis is a compendium of three published articles (plus an additional unpublished work) that represents the fruits of research focused on the massive Cygnus OB2 star-forming region. It is one of the nearest, most massive and youngest Galactic OB association and it provides an excellent target to study massive star properties, formation and evolution in a very active environment. Our main goal is to increase our current knowledge of the Cygnus OB2 association and its massive star population. With this aim, we have carried out four different (but related) studies where the main physical properties of the Cygnus OB2 massive population have been characterized.
Firstly, we present a membership study of Cygnus OB2 and its surroundings for completing the currently known population of early-type stars in the region. From new spectroscopic observations, we have increased the number of OB stars known in the area in spite of the existing limitations for completing the whole census. We also investigated the age and extinction distribution of the region, and whether there exists a correlation between the observed age spread and Galactic longitude.
Since there exist firm indications that many massive stars of Cygnus OB2 have exploded as supernovae during the history of the region, we include a chemical analysis of a sample of OB stars in Cygnus OB2 for checking possible
inhomogeneities across the whole association and whether there also exists a correlation of chemical composition with Galactic longitude that could be caused by self-enrichment processes.
A persistent problem for massive star studies has been always the inaccuracy of their distances. Fortunately, the second data release of the Gaia satellite has provided high-quality astrometry for a large number of Galactic massive stars, including those in Cygnus OB2. Thus, a study of the spatial substructure of the association is also included in this work, where we have quantified the line-of-sight substructure within the association by using an inference approach and creating a parametrized model that reproduces the observed parallax distribution.
Finally, we present the most complete spectroscopic census of O stars done so far in Cygnus OB2. We have updated the binary fraction currently known in the association, and taking into account the results derived in all previous studies, we performed the quantitative spectroscopic analysis of all the isolated stars. We obtained the distribution of rotational velocities and derived physical and spectroscopic parameters, providing new temperature and gravity scales for O-type stars. To conclude, we interpret the evolutionary status of the region
using the Hertzsprung-Russell diagram and its spectroscopic version.