of stars. They have in common a moderate increase in size and luminosity compared to main-sequence stars of the same mass or temperature, and are hot enough 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 Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. They have in common 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. Their temperatures exceed around 10,000 K, and they have zero age main sequence (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 300 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. There are no strict upper limits for giant stars, but early O types become increasingly difficult to classify separately from main sequence and supergiant stars, have almost identical sizes and temperatures to the main-sequence stars from which they develop, and very short lifetimes. A good example is Plaskett's star, a close binary consisting of two O type giants both over 50 M☉, temperatures over 30,000 K, and more than…

Watch an animation of the stars in the Omega Centauri cluster as they rearrange according to luminosity and temperature, forming a Hertzsprung-Russell (H–R) diagram. To estimate just how much the luminosity and temperature of a star change as it ages, we must resort to calculations. Theorists compute a series of models for a star, with each successive model representing a later point in time. Stars may change for a variety of reasons. Protostars, for example, change in size because they are contracting, and their temperature and luminosity change as they do so. After nuclear fusion begins in the star’s core (see Stars from Adolescence to Old Age), main-sequence stars change because they are using up their nuclear fuel. Given a model that represents a star at one stage of its evolution, we can calculate what it will be like at a slightly later time. At each step, the model predicts the luminosity and size of the star, and from these values, we can figure out its surface temperature. A series of points on an H–R diagram, calculated in this way, allows us to follow the life changes of a star and hence is called its evolutionary track.

DURING THE MAIN-SEQUENCE PHASE Stars change relatively little as they age during their main-sequence phase … off the main sequence? b c. expands and heats. d e 2. Where do main-sequence stars end up as they evolve … Stars 499 61.3 Main-Sequence Lifetime of a Star SOI 61.4 Changes During the Main-Sequence Phase 502 Giant