A direct measurement of the expansion of W 3 ( OH ) is made by comparing Very Large Array images taken \sim 10 yr apart .
The expansion is anisotropic with a typical speed of 3 to 5 km s ^ { -1 } , indicating a dynamical age of only 2300 yr .
These observations are inconsistent with either the freely expanding shell model or a simple bow shock model .
The most favored model is a slowly expanding shell-like H ii region , with either a fast rarefied flow or another less massive diffuse ionized region moving towards the observer .
There is also a rapidly evolving source near the projected center of emission , perhaps related to the central star .