Massive, intrinsically bright stars which display different scales of light and color variability, ranging from rapid micro-variations to rare outbreaks of catastrophic mass loss.
They represent a very short-lived (perhaps as little as 40,000 years) strongly mass-losing phase in the evolution of massive stars, during which they undergo deep erosion of the outer layers before they enter the Wolf-Rayet phase.
They include P Cygni stars, S Doradus stars, and Hubble-Sandage variables. They move dramatically across the H-R diagram in a repeated blue-red-blue pattern, losing mass through constant winds and occasional outbursts.
Text by Chris Clowes
Initially published on 03 December 2001.