Left panel: map of the surface brightness of "the Antennae" in emission from ionized hydrogen. The brightest zones are the zones occupied by clusters of massive stars. Right panel: map of the expansion velocity of the bubbles detected in the Antennae, pushed outwards by the stellar winds and supernova explosions of the stars in the cluster. The colours give the values of the expansion velocity at each point on the map. The largest bubble is 1,500 light years in size. Credit: IAC.