A century ago, it was unclear whether the stars in the sky were clustered in groups, or widely spread in the universe. Without accurate stellar distances, it was impossible to obtain a reliable... Show moreA century ago, it was unclear whether the stars in the sky were clustered in groups, or widely spread in the universe. Without accurate stellar distances, it was impossible to obtain a reliable spatial stellar distribution to know which stars are part of our Galaxy and those that were beyond. Recently, the Gaia mission provided accurate position and velocity measurements for a billion stars in the Milky Way. However, these measurements are limited by dust that absorbs and scatter the optical light, particularly forward the Galactic plane. In contrast, radio waves are not affected, and therefore, can easily penetrate the Galactic plane providing complementary data. Radio campaigns are currently measuring the positions and velocities of bright stellar sources in the Galactic plane using VLBI. This thesis demonstrates how accurate are these astrometric measurements for young massive and evolved stars. These results are (1) compared with simulations of unobserved Galactic areas to determine the structural parameters of the Galaxy, and (2) cross-matched between optical and infrared surveys to characterize different stellar populations. Finally, a study of a particular binary system was carried out that demonstrates the stellar information can be obtained by having accurate astrometry at different frequencies and epochs. Show less
