Blue supergiants are supergiant stars (class I) of spectral type O.
They are extremely hot and bright, with surface temperatures of between 20,000 - 50,000 degrees Celsius.
The best known example is Rigel, the brightest star in the constellation of Orion.
It has a mass of around 20 times that of the Sun and gives out more light than 60,000 suns added together.
Despite their rarity and their short lives blue supergiant stars are heavily represented among the stars visible to the naked eye; their inherent brightness trumps their scarcity.
Blue supergiants represent a slower burning phase in the death of a massive star.
Due to core nuclear reactions being slightly slower, the star contracts and since very similar energy is coming from a much smaller area (photosphere) then the star's surface becomes much hotter.