We have determined the location of the line-opacity modified Eddington limit for stars in the LMC using the most recent atmosphere models combined with a precise mapping to the HR Diagram through up-to-date stellar evolution calculations .
While we find , in agreement with previous studies , that the shape of the modified Eddington limit qualitatively corresponds to the Humphreys-Davidson ( HD ) limit defined by the most luminous supergiants , the modified limit is actually a full magnitude higher than the upper luminosity limit observed for LMC stars .
The observed limit is consistent with atmosphere models in which the maximum value of the ratio of the radiation force outwards to the gravitational force inwards , Y _ { max } , is 0.9 , i.e. , the photospheres of stars at the observed luminosity limit are bound .
As massive stars evolve , they move to higher , and therefore less stable values of Y _ { max } , so mass loss , either sporadic or continuous , may halt their natural redward evolution as they approach the Y _ { max } = 0.9 limit .
We assess the metallicity dependence of this limit .
If the limit does determine the most luminous stars , and the value of Y _ { max } corresponding to the luminosity limit in the LMC is universal , then the brightest supergiants the SMC should be only marginally brighter ( 0.3 mag ) than those of the LMC , in agreement with observations .
Moreover , the brightest supergiants in M31 should be 0.75 mag fainter than those in the LMC .