In their "quiescent" state they are typically B-type stars, occasionally slightly hotter, with unusual emission lines. They are found in a region of the Hertzsprung–Russell diagram known as the "S Doradus instability strip". They are the least luminous have a temperature around 10,000 K and a luminosity about 250,000 times the Sun. The most luminous have a temperature around 25,000 K and a luminosity over a million times the Sun. These are some of the most luminous of all stars.