30 Doradus is the most luminous HII region in the Local Group of Galaxies and is outstanding in many ways. Its structure is fascinatingly complex, with a large number of bright arcs and apparently dark areas in between, and it is also eloquently called the Tarantula Nebula. Its precise boundary has been difficult to establish. In the Henize (1956) catalogue a large nebulosity, N135, ∼ 2 deg, encompasses 30 Doradus, SGS LMC 2 and SGS LMC 3, and would, seen at a greater distance, be identified as a single supergiant HII region (Fig. 10.1). Its central portion, N157≡DEM263 (Davies et al. 1976, DEM), contains in its NW part N157A, 30 Doradus ≡NGC 2070, and N157B ≡NGC 2060. Henize estimated the diameter of N157A to be ∼ 30′ often values as low as 15′ may be seen for 30 Doradus.

30 Doradus provides a most spectacular O association for which detailed information is now available (Fig. 10.2). Its brightest object, R136 ≡HD 38268, now known to be a cluster of clusters in itself, was for a while considered to be a supermassive star by many astronomers. Several of the brighter members in the immediate surroundings of the dense cluster core belong to the rare O3If*/WN-A class (Melnick 1985, Walborn 1986) and are among the most massive stars known.