On the Main-Sequence Band and the Hertzsprung GAP

Stars evolve rapidly away from the main-sequence as soon as an inert helium core develops. Evolutionary sequences have been computed for the masses 30.1 ⊙, 8.94 ⊙, 3.89 ⊙, 1.52 ⊙. These integrations enable a band to be determined, the lower edge of the band being given by the zero-ago main-sequence and the upper edge by the moment of formation of an inert core. Stars near the main-sequence should lie within this band (apart from aspect effects introduced by rapid rotation). A comparison is made with recent work by Blaauw on the Scorpio–Centaurus group. The stars of the nebular region within the group appear to have an age of about 12.5 × 10 6 years, whereas the best theoretical age estimate for the whole group ∼ 26 × 10 6 years. The age determined by Blaauw from dynamical considerations ∼20 × 10 6 years.