  We report the first evidence of an on-going collision between two star
clusters in our Galaxy, namely IC 4665 and Collinder 350. These are open
clusters located at a distance of ~330 pc from the Sun and ~100 pc above the
Galactic plane, and they both have prograde motions with only a small
difference in their velocities (Collinder 350 moves ~5 km/s faster than IC
4665); as inferred from ESA/Gaia based catalogue. Interestingly, the two
clusters are physically separated by only 36 pc in space; a distance that is
smaller than the sum of their respective radii. Furthermore, the clusters
exhibit signatures of elongated stellar density distributions, and we also
detect an onset of an inter-cluster stellar bridge. Moreover, the orbit
analysis suggests that the younger cluster IC 4665 (age=53 Myr) must have
formed at a distance > 500 pc away from Collinder 350 (age=617 Myr). These
findings together imply that the two clusters do not represent merging of two
objects in a binary system, rather, what we are witnessing is an actual
collision between two independently formed star clusters. This collision
phenomenon provides a unique opportunity to explore new aspects of formation
and evolution theory of star clusters.
