  Massive stars born in star clusters terminate star cluster formation by
ionizing the surrounding gas. This process is considered to be prevalent in
young star clusters containing massive stars. The Orion Nebula is an excellent
example associated with a forming star cluster including several massive stars
(the Orion Nebula Cluster; ONC) and a 2-pc size H{\sc ii} region (ionized
bubble) opening toward the observer; however, the other side is still covered
with dense molecular gas. Recent astrometric data acquired by the Gaia
satellite revealed the stellar kinematics in this region. By comparing this
data with star cluster formation simulation results, we demonstrate that
massive stars born in the ONC center were ejected via three-body encounters.
Further, orbit analysis indicates that $\theta^2$ Ori A, the second massive
star in this region, was ejected from the ONC center toward the observer and is
now returning to the cluster center. Such ejected massive stars can form a hole
in the dense molecular cloud and contribute to the formation of the 2-pc
bubble. Our results demonstrate that the dynamics of massive stars are
essential for the formation of star clusters and H{\sc ii} regions that are not
always centered by massive stars.
