| 1. Star formation is initiated when interstellar clouds of gas and dust are compressed to the critical point where gravitational collapse of a portion of the cloud is initiated. The cloud fragments and the individual pieces condense to form a cluster of stars. |
| 2. The hottest stars form first since they are most massive. They modify their environment by gobbling up available mass (thus suppressing the formation of additional massive stars), emission of matter in stellar winds, emission of intense UV radiation that ionizes and erodes nearby portions of the cloud, and generation of shock waves from these processes that may compress surrounding portions of the cloud and trigger a secondary wave of star formation. |
| 3. The clusters of stars evolve. The more massive ones evolve faster and become Type II supernovae. The supernova explosions both create and distribute new heavy elements in the explosion, and distribute elements processed in the star before the explosion to the interstellar medium; some of the processed mass is removed from the cycle into neutron stars and black holes left behind in the core. The less massive stars evolve more slowly, but they eventually enrich the interstellar medium too by becoming red giants and emitting their envelopes in planetary nebulae. Part of their mass is removed from the cycle in the white dwarf cores left behind by the planetary nebulae. |
| 4. The shock waves associated with the preceding step, and perturbations from other sources such as spiral density waves (see the later discussion of spiral galaxies in Chapter 23), initiate a new round of star formation, with the raw material for these stars now enriched in heavy elements produced and distributed by the preceding generation of stars. |
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