INFORMATIONS

In astronomy, a blue giant is a hot star with a luminosity class III (giant) or II (luminous giant). In the Hertzsprung-Russell Diagram these stars are above and to the right of the main sequence.

The term applies to a variety of stars at different stages of development, all evolved stars that move from the main sequence, but have a little more in common, so blue giant simply refers to stars in a particular region of the Hertzsprung-Russell Diagram. rather than a type of star. They are much rarer than red giants because they only develop from more massive, less common stars and because they are short-lived in the blue giant stage.

The name blue giant is sometimes misapplied to other luminous, high-mass stars such as main sequence stars simply because they are large and hot.

PROPERTIES

Blue giant is not a strictly defined term and is applied to a wide variety of different types of stars. What they have in common is: a moderate increase in size and luminosity compared to main sequence stars of the same mass or temperature and are hot enough to be called blue which means spectral class O, B and sometimes early A . They have temperatures from about 10,000 K upwards, zero-age main sequence masses (ZAMS) greater than about twice the Sun (M☉) and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (R☉) compared to red giants that are up to 100 R☉

EVOLUTION

In the simplest case, a hot luminous star begins to expand as its hydrogen core is depleted and first becomes a blue subgiant and then a blue giant becoming cooler and more luminous. Intermediate-mass stars will continue to expand and cool until they become red giants. Massive stars also continue to expand as the burning of the hydrogen shell progresses but they do so with approximately constant luminosity and move horizontally on the Hertzsprung-Russell Diagram. This way they can quickly pass through the blue giant, luminous blue giant, blue supergiant and yellow supergiant classes until they become red supergiants. The luminosity class for such stars is determined from spectral lines that are sensitive to the star's surface gravity with more expanded and luminous stars receiving classifications I (supergiants) while slightly less expanded and more luminous stars receiving luminosity II or III. [5] Because they are massive, short-lived stars, many blue giants are found in O-B Associations, which are large collections of vaguely delimited young stars.