Expansion of W3(OH)

A direct measurement of the expansion of W3(OH) is made by comparing Very L arge Array images taken similar to 10 yr apart. The expansion is anisotropi c with a typical speed of 3-5 km s(-1), indicating a dynamical age of only 2300 yr. These observations are inconsistent with either the freely expandi ng shell model or a simple bow shock model. The most favored model is a slo wly expanding shell-like H II region, with either a fast rarefied how or an other less massive, diffuse, ionized region moving toward the observer. The re is also a rapidly evolving source near the projected center of emission, perhaps related to the central star.