Composite image of the nebula RCW 120. The newly formed O star is moving at ~4 km/s toward the South, sweeping up a dense shell of gas with a bow shock at the bottom and broken open at the top. The ring-shaped clouds around the nebula were detected by the Spitzer Space Telescope. SOFIA measured the glowing gas shown in red and blue to study the nebula’s expansion speed and determine its age. The blue and red gas represent, respectively, emission at velocities between -23 and -16 km/s, expanding in the direction toward Earth, and -1 and +3 km/s, expanding away from Earth. The expansion of this shell has triggered the birth of stellar neighbors at breakneck speeds – and reveals that the nebula is younger than previously believed.
