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 (LB Vs), 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 LB Vs, specifically Eta Carinae, P Cygni, V12 in NGC 2403, and SN 1961V, 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 LB Vs have luminosities greater than M Bol --9.6 mag, suggesting initial mass greater than 50 M 0 . 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 LB Vs (M Bol --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 LB Vs and discuss their brief but critical role in massive star evolution, and possible mechanisms for their remarkable instability.

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A luminous optical transient (OT) that appeared in NGC 300 in early 2008 had a maximum brightness, M V −12 to −13, intermediate between classical novae and supernovae. We present ground-based photometric and spectroscopic monitoring and adaptive-optics imaging of the OT, as well as pre-and postoutburst spacebased imaging with the Hubble Space Telescope (HST) and Spitzer. The optical spectrum at maximum showed an F-type supergiant photosphere with superposed emission lines of hydrogen, Ca ii, and [Ca ii], similar to the spectra of low-luminosity Type IIn "supernova impostors" like SN 2008S, as well as cool hypergiants like IRC +10420. The emission lines have a complex, double structure, indicating a bipolar outflow with velocities of ∼75 km s −1 . The luminous energy released in the eruption was ∼10 47 erg, most of it emitted in the first two months. By registering new HST images with deep archival frames, we have precisely located the OT site, and find no detectable optical progenitor brighter than broadband V magnitude 28.5. However, archival Spitzer images reveal a bright, nonvariable mid-infrared (mid-IR) preoutburst source. We conclude that the NGC 300 OT was a heavily dust-enshrouded luminous star, of ∼10-15 M , which experienced an eruption that cleared the surrounding dust and initiated a bipolar wind. The progenitor was likely an OH/IR source which had begun to evolve on a blue loop toward higher temperatures, but the precise cause of the outburst remains uncertain.