Spherical episodic ejection of material from a young star

The exact processes by which interstellar matter condenses to form young st ars are of great interest, in part because they bear on the formation of pl anets like our own from the material that fails to become part of the star. Theoretical models suggest that ejection of gas during early phases of ste llar evolution is a key mechanism for removing excess angular momentum, the reby allowing material to drift inwards towards the star through an accreti on disk(1,2). Such ejections also limit the mass that can be accumulated by the stellar core(1,2). To date, these ejections have been observed to be b ipolar and highly collimated, in agreement with theory. Here we report obse rvations at very high angular resolution of the proper motions of an arc of water-vapour masers near a very young, massive star in Cepheus. We rnd tha t the arc of masers can be fitted to a circle with an accuracy of one part in a thousand, and that the structure is expanding. Only a sphere will alwa ys produce a circle in projection, so our observations strongly suggest tha t the perfectly spherical ejection of material from this star took place ab out 33 years earlier. The spherical symmetry of the ejecta and its episodic nature are very surprising in the light of present theories.