Efficiency of Star Formation

- Only in high efficiency SFRs do we produce stellar densities to make bound clusters (roughly 30%).

- Bound open clusters only appear to form in clouds too small to produce O stars ( <10⁴ M_solar) Above this limit, we generally only see unbound OB associations, which dissipate over time.

- Similarly, we can get "over the hump" of O star temps at very large cloud masses ( ≈10⁸ M_solar) where the mass is large enough to withstand the high temperatures without dissipating - in other words, very high virial velocities.