This paper argues that star forming environments should be classified into finer divisions than the traditional isolated and clustered modes .
Using the observed set of galactic open clusters and theoretical considerations regarding cluster formation , we estimate the fraction of star formation that takes place within clusters .
We find that less than \sim 10 \% of the stellar population originates from star forming regions destined to become open clusters , confirming earlier estimates .
The smallest clusters included in the observational surveys ( having at least N \sim 100 members ) roughly coincide with the smallest stellar systems that are expected to evolve as clusters in a dynamical sense .
We show that stellar systems with too few members N < N _ { \star } have dynamical relaxation times that are shorter than their formation times ( \sim 1 - 2 Myr ) , where the critical number of stars N _ { \star } \approx 100 .
Our results suggest that star formation can be characterized by ( at least ) three principal modes : [ I ] isolated singles and binaries , [ II ] groups ( N < N _ { \star } ) , and [ III ] clusters ( N > N _ { \star } ) .
Many – if not most – stars form through the intermediate mode in stellar groups with 10 < N < 100 .
Such groups evolve and disperse much more rapidly than do open clusters ; groups also have a low probability of containing massive stars and are unaffected by supernovae and intense ultraviolet radiation fields .
Because of their short lifetimes and small stellar membership , groups have relatively little effect on the star formation process ( on average ) compared to larger open clusters .