Title: Stellar atmospheric instability in the upper part of the Hertzsprung-Russell diagram
Abstract: The upper limt of stellar luminosity in the Hertzsprung-Russell diagram is a line running approximately from (log T eff ; log (L/L ⊙ ) = (4.5; 6.3) via (4.0; 5.74) to (3.5; 5.7) (Humphreys and Davidson, 1979; Humpreys, 1983).