Some of the most luminous stars have sporadic, violent mass-loss events whose causes are not understood. These evolved hot stars are called luminous blue variables (LBVs), and their instability may shape the appearance of the upper HR diagram. LBV eruptions are interestingly reminiscent of geysers or even volcanos. They have received considerable observational attention since 1980, but theoretical work to explain the instability has been scarce. In a typical LBV eruption, the star's photosphere expands and the apparent temperature decreases to near 8000 K. During these normal eruptions the bolometric luminosity remains constant, as typified by S Doradus, AG Carinae, and R 127. A few LBVs, specifically Eta Carinae, P Cygni, V12 in NGC 2403, and SN 1961 V, have giant eruptions in which the total luminosity actually increases by more than one or two magnitudes. The star may expel as much as a solar mass or more with a total luminous output rivaling a supernova. The classical LBVs have luminosities greater than M(Bol) congruent-to -9.6 mag, suggesting initial mass greater than 50 M.. These stars have very likely not been red supergiants as there are no evolved cool stars of comparable luminosity. Their instability may prevent their evolution to the red supergiant region. There is also a group of less luminous LBVs (M(Bol) congruent-to -8 to -9 mag) with low temperatures, smaller amplitudes, and lower mass-loss rates. These stars have probably been red supergiants and have shed a lot of mass prior to their current unstable state. Although the physical cause of the LBV instability is not yet understood, the most likely mechanisms involve radiation pressure (the opacity-modified Eddington limit) or dynamical instabilities in the outer layers as the star evolves off the main sequence. In this review, we summarize the physical characteristics and behavior of LBVs and discuss their brief but critical role in massive star evolution, and possible mechanisms for their remarkable instability.
