It is usual among astronomers to date a star's age from the zero-age main sequence onward, since the time it has spent contracting out of the interstellar medium to the main sequence is only a small fraction of its life span, typically a few tenths of a percent. Some stellar models indicate that protostars with masses of less than about 0.1M. never become hot enough at their centers to fuse hydrogen. They pass the lower end of the main sequence and continue contracting toward extremely high densities. There is little doubt among astronomers that rotation is also a crucial factor during the collapse of interstellar clouds and the contraction of the resulting fragments through the protostar stage into main-sequence stars. Figure 17.12 is an attempt to depict the formation of an O association on one end of a giant molecular cloud.
Star formation advances across the cloud by the formation of new O stars.
This in turn initiates new fragmentation and collapse, forming more stars. When the massive O stars reach the end of their lives, they also undergo a supernova outburst that adds to the compression of the cloud and furthers star formation. Finding H II regions in dark-cloud complexes definitely demonstrates that stars are forming, as the example in Figure 17.13 shows. We do not definitely know whether all stars that will originate in a giant molecular cloud form simultaneously. The formation of individual stars may spread over a period as long as 10 million years.