  Results from gamma-ray observations by the H.E.S.S. telescope array in the
direction of the young stellar cluster Westerlund 2 are presented. Stereoscopic
imaging of Cherenkov light emission of gamma-ray induced showers in the
atmosphere is used to study the celestial region around the massive Wolf-Rayet
(WR) binary WR 20a. Spectral and positional analysis is performed using
standard event reconstruction techniques and parameter cuts. The detection of a
new gamma-ray source is reported from H.E.S.S. observations in 2006. HESS
J1023-575 is found to be coincident with the young stellar cluster Westerlund 2
in the well-known HII complex RCW 49. The source is detected with a statistical
significance of more than 9 sigma, and shows extension beyond a point-like
object within the H.E.S.S. point-spread function. The differential gamma-ray
spectrum of the emission region is measured over approximately two orders of
magnitude in flux. The spatial coincidence between HESS J1023-575 and the young
open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires
a look into a variety of potential models to account for the observed
very-high-energy (VHE) gamma-ray emission. Considered emission scenarios
include emission from the colliding wind zone of WR 20a, collective stellar
winds from the extraordinary ensemble of hot and massive stars in the stellar
cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble
itself, and supersonic winds breaking out into the interstellar medium (ISM).
The observed source extension argues against a single star origin of the
observed VHE emission.
