sentences:  It is classified as a giant star on the basis of its spectrum, but the hottest O stars develop these spectral features while still burning hydrogen in their cores, as a result of vigorous convection and powerful stellar winds.When first modelled, Pismis 24-1 was thought to be a single star with a mass of or more, higher than expected to be theoretically possible.This has been steadily reduced as the star was discovered to be double, then triple, and as newer models of stellar atmospheres have been developed.