Astronomy Colloquium

In the past five years several trends have synergistically enhanced the importance of the most massive stars for astrophysics and cosmology. The GRB and first-stars connections remain valid, but now we also have the recognition of a class of Supernova Impostors, a revolution in our understanding of the Eddington Limit, recent theoretical models of ultra-high-energy supernovae, and other unexpected developments. Collectively, stars above 100 Msun have become essential both for cosmology and for astrophysical processes in general. A large fraction of everything known about such objects is based on Eta Carinae, which is far easier to observe than any other object in the class. Eta's bizarre phenomena have repeatedly exposed gaps and errors in existing theory. Observations of this object have undergone another revolution in the past decade, while, independently, it shows signs of changing to a different physical state. Here I shall review the current significance of the most massive stars, of supernova impostors, and recent surprises concerning Eta Carinae.