Tara_Li wrote:Why wouldn't a large starbirth nebula, such as the Orion Complex (but perhaps a bit larger) work for that purpose? At what density regime does the Interstellar Medium quit being gravity dominated, and become a fluid? We see turbulance and bowshock in very low density nebulae, after all.
One indication that a star forming nebula has insufficient mass to act as a local center of gravity is that after the star formation period ends, the bright new stars are often seen to have proper motions quite different from each other (within the overall motion of the now-dissipated cloud of gas and dust) and they subsequently wander away. The Sun was born in such a region almost 5 billion years ago, but scientists have so far been unable to identify with certainty any of its birth companions.
Of course, clusters
can form and remain gravitationally bound, but judging from the location of the clusters that we can see, this doesn't happen within the more crowded regions of a galaxy and therefore can't produce the eddy that sparked this conversation.
Rob