Luminous Blue Variables/S Dor stars

The observed H-R diagram has an upper luminosity limit of which the contour line is temperature dependent (Humphreys & Davidson 1987). Some of the most massive and luminous (106Lo) stars near that line (P Cyg, AG Car, HR Car, η Car, ...) sporadically show dramatic mass-ejections (seen as ‘eruptions’) followed by periods of quiescence. Such stars are called hypergiants, some of them are Luminous Blue Variables (LBVs), though LBVs, do not necessarily need to be blue, since the phenomenon is not restricted to early-type stars (de Jager & van Genderen 1989). The above-mentioned LBVs, together with 164GSco = HD 160529 (Fig. 2.1) and WRA751 are notorious galactic LBVs. de Koter (1993) estimates the number of LBVs in our galaxy at no more than 60, but the number of LBVs that possibly can be considered for observation, obviously, is much less. In the LMC, the well-known LBVs (also called S Dor stars) are S Dor (Fig. 2.2), R71 (Figs. 2.3, 2.4) and R127 (Fig. 2.5), with R66, R81 (Fig. 2.6) and R110 as additional candidates. Finally there are the ‘ Hubble-Sandage variables’, discovered by Hubble & Sandage (1953) in M31 and M33, which are identifiable with the S Dor variables, that complete the group that is commonly designated as LBVs. During outburst these stars are - apart from supernovae - the visually brightest stars in the universe, and thus potentially belong to the most powerful extragalactic distance-indicators (Wolf 1989). Today, only a few dozen LBVs are known.