Blue giant

Blue giants are stars with a spectral type of O or B (meaning noticeably blue in appearance) and a luminosity class of III (giant). The blue giant is a massive star that has exhausted the hydrogen fuel in its core and left the main sequence. Blue giants have a surface temperature of around 30 000 K and are 10 000 times as bright as the Sun. As they grow older they expand and cool, becoming red giants. Blue giants are also extremely luminous, reaching absolute magnitudes of -5, -6 and sometimes higher. Because there surface temperature is so high (20,000 K or more) a large fraction of their energy output is in the ultraviolet range, making it invisible to our eyes.