Blue giant

In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II (bright giant). In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence.

The term applies to a variety of stars in different phases of development, all evolved stars that have moved from the main sequence but have little else in common, so blue giant simply refers to stars in a particular region of the HR diagram rather than a specific type of star. They are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives in the blue giant stage.

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

Blue giant is not a strictly defined term and it 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, meaning spectral class O, B, and sometimes early A. They have temperatures from around 10,000 K upwards, ZAMS masses 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 which are up to 100 R_☉.

The coolest and least luminous stars referred to as blue giants are on the horizontal branch, intermediate-mass stars that have passed through a red giant phase and are now burning helium in their cores. Depending on mass and chemical composition these stars gradually move bluewards until they exhaust the helium in their cores and then they return redwards to the asymptotic giant branch (AGB). The RR Lyrae variable stars, usually with spectral types of A, lie across the middle of the horizontal branch. Horizontal-branch stars hotter than the RR Lyrae gap are generally considered to be blue giants, and sometimes the RR Lyrae stars themselves are called blue giants despite some of them being F class. The hottest stars, blue horizontal branch (BHB) stars, are called extreme horizontal branch (EHB) stars and can be hotter than main-sequence stars of the same luminosity. In these cases they are called blue subdwarf (sdB) stars rather than blue giants, named for their position to the left of the main sequence on the HR diagram rather than for their increased luminosity and temperature compared to when they were themselves main-sequence stars.

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