In astronomy, a blue giant is a star with a spectral type of O or B and class of III (giant). In the standard Hertzsprung-Russell diagram, blue giants are found in the upper left corner, thanks to their high luminosity and early spectral type. Lunar astronomy: the large crater is Daedalus, photographed by the crew of Apollo 11 as they circled the Moon in 1969. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude (CM) diagram) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ...

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. In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ...

Most stars of this type are found in O-B associations, large collections of loosely bounded 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 supernova. Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ...

Blue giant is a misused term, as giant implies an advanced evolutionary state. Stars such as Rigel are blue supergiants but ordinary hydrogen fusing O to B stars such as Regulus are dwarf stars. 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 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 transitionary 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. Rigel (pronounced ) (β Orionis) is the brightest star in the constellation Orion and the seventh brightest star in the sky, with visual magnitude 0. ... 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°C. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii up to about 25 solar radii. ... Regulus (α Leo / α Leonis / Alpha Leonis) is the brightest star in the constellation Leo and one of the brightest stars in the nighttime sky. ... Bellatrix (γ Ori / γ Orionis / Gamma Orionis) is the third brightest star in the constellation Orion and the twenty-second brightest star in the nighttime sky. ... Arcturus (α Boo / α Boötis / Alpha Boötis) is the brightest star in the constellation Boötes, and the third brightest star in the night sky, with a visual magnitude of −0. ...

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. Spica (α Vir / α Virginis / Alpha Virginis) is the brightest star in the constellation Virgo, and one of the brightest stars in the nighttime sky. ... Pleiades refers to: Pleiades (star cluster) an open cluster of stars in the constellation Taurus. ...

See also

• Blue supergiant