  About 20% of all massive stars in the Milky Way have unusually high
velocities, the origin of which has puzzled astronomers for half a century. We
argue that these velocities originate from strong gravitational interactions
between single stars and binaries in the centers of star clusters. The ejecting
binary forms naturally during the collapse of a young ($\aplt 1$\,Myr) star
cluster. This model replicates the key characteristics of OB runaways in our
galaxy and it explains the $\apgt 100$\,\Msun\, runaway stars around young star
clusters, e.g. R136 and Westerlund~2. The high proportion and the distributions
in mass and velocity of runaways in the Milky Way is reproduced if the majority
of massive stars are born in dense and relatively low-mass (5000-10000 \Msun)
clusters.

Astrophysics of Galaxies