  The young star cluster W3 (age 300-500 Myr) in NGC 7252 is the most luminous
star cluster known to date. Dynamical mass estimates result in 8 +/- 2 * 10^7
M_sun. With an effective radius of about 18 pc and a velocity dispersion of 45
km/s this object is rather one of the recently discovered ultra-compact dwarf
(UCD) galaxies than a star cluster. In our model we propose a formation
scenario for W3. Observations of interacting galaxies reveal regions of strong
star formation forming dozens up to hundreds of star clusters in confined
regions of several hundred parsec in diameter. The total mass of new stars in
these regions can reach 10^7 or even 10^8 solar masses. By means of numerical
simulations we have shown that the star clusters in these regions merge on
short time-scales forming objects like UCD galaxies or W3.
