**Kris Davidson**

Minnesota Institute for Astrophysics, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA; kd@umn.edu; Tel.: +1-612-624-5711

Received: 20 December 2019; Accepted: 25 January 2020; Published: 5 February 2020

**Abstract:** Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV's) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, *the fundamental causes of giant eruptions and LBV events remain elusive*. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergen<sup>t</sup> radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star's photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature.

**Keywords:** Eddington Limit; eruption; supernova impostor; LBV; Luminous Blue Variable; stellar outflow; strange modes; iron opacity; bistability jump; inflation
