We determine the rotation velocity of the spiral pattern of the Galaxy, by direct observation of the birthplaces of open clusters of stars in the galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, like the existence of a given number of spiral arms, or the presence of a bar in the central regions. The results first confirm that a dominant fraction of the open clusters are formed in spiral arms, a question which has been a subject of controversy. We find that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves, and show that the corotation radius is close to the solar galactic orbit. This proximity has many interesting consequences, like the preservation of life on the Earth, and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the galactic disk.
Milk Way; spiral structure; spiral arms
