5 - The youngest field population

Summary

Luminous stars in the Magellanic Clouds are very suitable for the study of evolutionary processes because their distances and, hence, their luminosities can be determined with a reasonably good precision, permitting a direct comparison with theoretical tracks in the HR diagram. The evolutionary paths of these massive stars depend critically upon the mass-loss rates at different stages of the stellar development. The mass lost may determine whether the stars evolve back to the blue or end their lives as red supergiants (e.g. Chiosi and Maeder 1986, Wood and Faulkner 1987). There are at least some stars in the LMC and the SMC showing that a post-red supergiant evolution occurs, resulting in anomalous He-burning A-type supergiants (Humphreys et al. 1991). Other stars, more massive, may return to the blue stage as WR stars (Sect. 5.2.3 or, like SN 1987A, as B supergiants (Sect. 9.4).

The youngest population in the Magellanic Clouds (age ≤ 100 Myr) contains the most recently formed stars and the remaining or returned gas and dust. Best studied among the stars are the OB stars and a minor number of supergiants and hypergiants of types B–G, suitable for abundance analyses. Massive stars are believed to be born primarily in clusters and in associations. There is, however, a fair number of massive stars so far from any cluster or OB association that they can hardly have moved from those centres of star formation to their present locations during their short lifetimes.