These objects have already reached the red supergiant stage but eventually evolve back into a blue supergiant phase ( Walmswell et al. In this stage the star has eventually developed a convective envelope and is rising up the Hayashi line in the HRD. However, another scenario is possible for objects with intermediate masses (3 < M < 9 M ⊙) and it is called the blue loop. Under these conditions CNO abundances remain unchanged ( Stothers & Chin 1976 1991). On the other hand, less massive A-F supergiants (M < 10 M ⊙) have also initiated the He-core burning without visiting the red giant branch, they develop a fully convective intermediate zone, the envelope is able to establish thermal equilibrium and the stars stay in the blue supergiant phase. In this phase, the A-F supergiants are able to resume thermal and radiative equilibrium through convection in the outer layers and shows an altered CNO abundance due to the first dredge-up (1DUP) event. These objects have already left the main sequence and begun the ignition of He in the blue supergiant region, but thermal instabilities cause in them a rapid expansion towards a red supergiant phase. Stellar evolutionary models constructed for solar metallicities predict that massive supergiants (M ≥10 M ⊙) are in the phase of helium core burning ( Schaller et al. When H is exhausted in massive stars the post-He core burning phase can be aﬀected in several ways. The chemical abundances of the light elements CNO have been crucial to discriminate their evolutionary states ( Lyubimkov et al. Some of these massive objects have been classified as supergiant stars with masses between 5 to 20 M ⊙, and A-and-F spectral types, which are moderately evolved. These objects are visually luminous in galaxies and thus are suitable candidates for studies of stellar and chemical evolution ( Luck et al. The existence of massive young objects in the galactic plane is not surprising, since it is an area of star formation. These objects in their rapid evolution undergo changes through the process of nucleosynthesis and return their chemical elements to the interstellar medium by stellar winds and supernova events. The process of chemical evolution in the Galaxy can be understood from the study of its massive stars.

CN CNO equilibrium abundance dredge up