  Radiative and mechanical feedback of massive stars regulates star formation
and galaxy evolution. Positive feedback triggers the creation of new stars by
collecting dense shells of gas, while negative feedback disrupts star formation
by shredding molecular clouds. Although key to understanding star formation,
their relative importance is unknown. Here, we report velocity-resolved
observations from the SOFIA (Stratospheric Observatory for Infrared Astronomy)
legacy program FEEDBACK of the massive star-forming region RCW 120 in the [CII]
1.9-THz fine-structure line, revealing a gas shell expanding at 15 km/s.
Complementary APEX (Atacama Pathfinder Experiment) CO J=3-2 345-GHz
observations exhibit a ring structure of molecular gas, fragmented into clumps
that are actively forming stars. Our observations demonstrate that triggered
star formation can occur on much shorter time scales than hitherto thought
(<0.15 million years), suggesting that positive feedback operates on short time
periods.
