In astronomy, a blue giant is a star with a spectral type of O or B (thus being noticeably blue in appearance) and a luminosity class of III (giant). In the standard Hertzsprung-Russell diagram, blue giants are found in the upper left corner, due to their high luminosity and early spectral type.
Blue giants are extremely luminous, reaching absolute magnitudes of -5, -6 and even higher. Their surface temperature is high enough (20,000 K or more) that a sizable fraction of their energy output is in the ultraviolet range, thus invisible to our eyes.
Most stars of this type are found in O-B associations, large collections of loosely bound young stars. Since they are so hot and so dense, their expected life is very short (in the order of tens or hundreds of million years), and current theories predict that most of them will end their lives as supernovae.
Blue giant is a misused term, as giant usually implies an advanced evolutionary state in which the star fuses helium in its core, instead of hydrogen (see red giant). There are no "real" blue giants, stable stars of classification OxIII or BxIII, instead stars such as Bellatrix (B2III) are middle-aged massive stars which are on the process of becoming massive bright giants (class II), very much unlike red giant stars such as Arcturus (K1III) which represent the final stage of stellar evolution for lower mass stars and are stable as giants. These stars, the massive and middle-aged blue giants, represent a transitory phase where the star is either to become a bright giant (and eventually a planetary nebula and massive white dwarf) or a supergiant (and eventually a supernova or rare oxygen-neon white dwarf) and no star remains as this kind of blue giant for very long. The equivalent evolutionary stage for a solar mass star would be the subgiant stage (class IV), where hydrogen fusion is slowing and helium fusion is yet to begin.
Other blue giants are merely misclassified hydrogen fusing dwarf stars, such as Spica or the Pleiades, their exceptional brightness making earlier astronomers believe they were elderly giants and the classification has simply stuck.