S Doradus star
An S Doradus star is a massive, blue supergiant variable star, also known as a Hubble-Sandage variable or a luminous blue variable. S Doradus stars are the most luminous stars in the Galaxy and are easily identified in other nearby galaxies, including the Andromeda Galaxy and the Triangulum Galaxy. They are named after the prototype, S Doradus, in the Large Magellanic Cloud.
Only about 20 such stars, with the classification SDor, are listed in the General Catalogue of Variable Stars, making them among the rarest types of star known.
S Dor variables can be subject to five different types of instability: the very rare genuine eruptive episodes (the "SD-eruptions"), two different brightening phases caused by slow pulsations (the "SD-phases"): one on a time scale of years, the other on a time scale of decades at a more or less constant luminosity and two types of microvariations: one on a time scale of weeks, the other on a time scale of about 100 days.1
It is not the case that all S Dor variables are always spectacular. Indeed, it has been estimated that most of them will not be spectacular at all for at least 70% of their lifetime as an S Dor variable.
1. van Genderen, A. M. "S Doradus variables in the Galaxy and the Magellanic Clouds," Astronomy & Astrophysics, 366(2), 508-531, 2000.