A Class O star is the brightest type of star, sitting on the line between main sequence and giant. Traditionally, they are coloured blue. They are also the rarest type of main-sequence star, making up only 0.00001% of luminous stars. A typical class O star has a mass of 50 solar masses, a radius of 10 solar radii, a luminosity of 100,000 solar luminosities, a surface temperature of 40000 K and a lifespan of 10 million years.