6 - The superassociations and supergiant shells

Summary

The concentrations of luminous, blue stars in the Magellanic Clouds have attracted much attention. Shapley (1956) noted that the large gaseous nebulae in the LMC are frequently associated with groups of stars but also that some of the larger star groups are free of conspicuous nebulosity. As these stellar aggregations were too large to be called clusters or associations in the sense used in our Galaxy he called them ‘Constellations’. He estimated their diameters to be between 250 and 600 pc and their content of blue supergiants, with a red magnitude brighter than 14.0, to be between 14 and 32. A few red supergiant stars were seen in each of them. In the region of 30 Doradus, Shapley (1955) identified a number of red stars by comparing blue (B) and infrared (I) plates and concluded that the very red stars were of spectral class M0 or later. Only two of the 21 most luminous of these stars are in the vicinity of the core of 30 Doradus.

In the SMC Shapley found only one object rich enough to be called a constellation in the sense used for the LMC. It is the aggregate comprising NGC 456, NGC460 and NGC 465 in the Wing area.

Shapley's designation is still used to identify the five most conspicuous stellar aggregates in the LMC. Improved techniques have extended and redefined them and led to the identification of more such formations.