An extremely hot, massive star at the very top of the Hertzsprung–Russell diagram, with a luminosity up to 10 million times or more that of the Sun; also known as an S Doradus star or a Hubble–Sandage variable. Such stars display variability in brightness and colour on various scales, ranging from rapid microvariations of 0.1–0.2 mag. to rare outbreaks of catastrophic mass loss on a timescale of centuries when the brightness can increase by 3 magnitudes or more. One famous example of the latter type is Eta Carinae. LBVs represent a very short-lived (perhaps as little as 40 000 years) phase in the evolution of young, massive stars, during which they undergo deep erosion of the outer layers before they enter the Wolf–Rayet phase. The spectra typically exhibit prominent emission lines of H, He, and iron, often with P-Cygni profiles. During quiescence the spectrum may be B-type, while at maximum brightness the star may appear as an A- or F-type supergiant. The luminosity and temperature are also highly variable, due to surrounding shells of ejected matter which absorb the star’s visible and bluer radiation and re-radiate it at longer wavelengths.