Astronomy Colloquium

Using the VLBA, we are measuring distances to massive star forming regions by trigonometic parallax.  We routinely achieve parallax accuracy of ~10 micro-arcseconds, which corresponds to 10% distance accuracy at 10 kpc!  The first results from the Bar and Spiral Structure Legacy (BeSSeL)
Survey will be presented.  These measurements directly address the nature of the spiral structure, size, rotation speed, and mass of the Milky Way.