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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, a blue giant is a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 with a spectral type of O or B (thus being noticeably blue in appearance) and a luminosity class of III (giant
Giant star

A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same effective temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun....
). In the standard Hertzsprung-Russell diagram
Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude, luminosity, stellar classification, and effective temperature ofstars....
, 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 magnitude
Absolute magnitude

In astronomy, absolute magnitude measures a celestial object's intrinsic brightness. To derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer....
s of -5, -6 and even higher. Their surface temperature is high enough (20,000 K
Kelvin

The kelvin is a Units of measurement of temperature and is one of the seven SI base units. The Kelvin scale is a Thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero ....
 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 association
OB star

OB stars are hot, massive stars of spectral types Stellar classification#Class O or Stellar classification#Class B which form in loosely organized groups called OB associations....
s, large collections of loosely bound young stars.






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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
, a blue giant is a star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
 with a spectral type of O or B (thus being noticeably blue in appearance) and a luminosity class of III (giant
Giant star

A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same effective temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun....
). In the standard Hertzsprung-Russell diagram
Hertzsprung-Russell diagram

The Hertzsprung-Russell diagram shows the relationship between absolute magnitude, luminosity, stellar classification, and effective temperature ofstars....
, 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 magnitude
Absolute magnitude

In astronomy, absolute magnitude measures a celestial object's intrinsic brightness. To derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer....
s of -5, -6 and even higher. Their surface temperature is high enough (20,000 K
Kelvin

The kelvin is a Units of measurement of temperature and is one of the seven SI base units. The Kelvin scale is a Thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero ....
 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 association
OB star

OB stars are hot, massive stars of spectral types Stellar classification#Class O or Stellar classification#Class B which form in loosely organized groups called OB associations....
s, large collections of loosely bound young stars. Since they are so hot (but not very 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
Red giant

A red giant is a luminous giant star of low or intermediate mass that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower....
). 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 giant
Bright giant

The luminosity class II in the Spectral classification#Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between giant star and supergiants, and the classification is in general given to giant stars with exceptionally high luminosity, but which are not sufficiently bright or massive t...
s (class II), very much unlike red giant stars such as Arcturus
Arcturus

|- bgcolor="#FFFAFA"| note : || H and K emission vary.Arcturus is the brightest star in the constellation Bo?tes. With a visual magnitude of -0.05, it is also the list of brightest stars in the night sky, after Sirius and Canopus ....
 (K1III) which represent the final stage of stellar evolution
Stellar evolution

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only few millions of years to trillions of years , considerably more than the age of the universe....
 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
Supergiant

Supergiants are among the most massive stars. In the Hertzsprung-Russell diagram they occupy the top region of the diagram. In the spectral classification supergiants are class Ia or Ib ....
 (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
Spica

Spica is the brightest star in the constellation Virgo , and the list of brightest stars in the nighttime sky. It is 260 light years distant from Earth....
 or the Pleiades
Pleiades (star cluster)

File:Pleiades Lanoue.pngIn astronomy, the Pleiades are an open star cluster in the constellation of Taurus . It is among the nearest star clusters to Earth, and Randall Munroe's favorite astronomical object....
, their exceptional brightness making earlier astronomers believe they were elderly giants and the classification has simply stuck.

Blue giants should not be confused with blue supergiant
Blue supergiant

Blue supergiants are supergiant stars of spectral classification O or B.They are extremely hot and bright, with surface temperatures of 20,000-50,000 K....
s such as Rigel
Rigel

Rigel is the brightest star in the constellation Orion and the list of brightest stars in the sky, with visual magnitude 0.18. Although it has the Bayer designation "beta", it is almost always brighter than Alpha Orionis ....
 or ordinary hydrogen-fusing O to B stars such as Regulus
Regulus

Regulus is the brightest star in the constellation Leo and list of brightest stars in the nighttime sky, and lies approximately 77.5 light years from Earth....
.

See also

  • Giant star
    Giant star

    A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same effective temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun....
  • Blue supergiant
    Blue supergiant

    Blue supergiants are supergiant stars of spectral classification O or B.They are extremely hot and bright, with surface temperatures of 20,000-50,000 K....