Waves on the surface of the Orion molecular cloud

Orion nebula making waves The molecular cloud in the Orion nebula, at about 414 parsecs from Earth, gives us our closest view of massive-star formation. It has been predicted from star-formation models that the gases heated and ionized in the process will generate wave-like structures as they are blown over pre-existing molecular gas. These waves have now been observed, in a new series of radio maps of the Orion nebula. The waves are thought to result from Kelvin–Helmholtz instability, a phenomenon seen at the interface between fluids with different densities and velocities, and further observations of such periodic structures should provide insight into the mechanisms of massive-star formation and its effects on the surrounding region of the molecular cloud. It has long been suspected that the development of hydrodynamical instabilities can compress or fragment a molecular cloud (in which stars are born). One key signature of an instability would be a wave-like structure in the gas, although this has not yet been seen. Now, the presence of 'waves' is reported at the surface of the Orion cloud, near where massive stars are forming. The waves probably arise as gas that is heated and ionized by massive stars is blown over pre-existing molecular gas. Massive stars influence their parental molecular cloud, and it has long been suspected that the development of hydrodynamical instabilities can compress or fragment the cloud 1 , 2 . Identifying such instabilities has proved difficult. It has been suggested that elongated structures (such as the ‘pillars of creation’ 3 ) and other shapes arise because of instabilities 4 , 5 , but alternative explanations are available 6 , 7 . One key signature of an instability is a wave-like structure in the gas, which has hitherto not been seen. Here we report the presence of ‘waves’ at the surface of the Orion molecular cloud near where massive stars are forming. The waves seem to be a Kelvin–Helmholtz instability that arises during the expansion of the nebula as gas heated and ionized by massive stars is blown over pre-existing molecular gas.