In astronomy and astrophysics, a blue giant is a hot star with a luminosity class of II (bright giant) or III (giant).
The word applies to various stars in varying phases of development; all evolved stars that have moved from the central sequence but have little else in common, so blue giant refers to stars in a distinct region of the HR diagram rather than a particular type of star. They are rarer than red giants because they only develop from less common and more massive stars and because they have brief lives in the blue giant stage.
A blue giant is not a defined term, and it is applied to a wide diversity of distinct types of stars.
What they have in common is:
- A relatively moderate increase in luminosity and size compared to main-sequence stars of the same temperature or mass and are hot enough to be called blue, meaning spectral class B, O, and sometimes A.
- They have recorded temperatures from around 10,000 K upwards, zero-age main sequence (ZAMS) masses larger than about twice the Sun (M☉), and total magnitudes around 0 or brighter.
- These stars are just 5–10 times the radius of the Sun (R☉), compared to red giants, which around 100 R☉.
The coolest and least luminous stars referred to as blue giants are on the horizontal branch, intermediate-mass stars that have moved through a red giant phase and are now consuming helium in their cores. Depending on the chemical composition and mass, these stars slowly move bluewards until they down the helium in their cores, and then they return redwards to the AGB (asymptotic giant branch).
Stars located in the giant blue area of the HR diagram can be in very many stages of their lives, but all are evolved stars that have primarily exhausted their main hydrogen supplies.
A luminous, hot star starts to unfold in the most uncomplicated case as its core hydrogen is exhausted and first becomes a blue subgiant, then a blue giant, becoming both colder and more brilliant. Intermediate-mass stars will continue to cool and expand until they become red giants. Large stars also continue to grow as hydrogen shell burning advances, but they do so at about constant radiance and move horizontally across the HR diagram. In this way, they can immediately pass through bright blue giant, blue giant, blue supergiant, and yellow supergiant classes until they eventually become red supergiants. The luminosity class for such stars is defined from spectral lines that are susceptible to the star’s surface gravity, with more luminous and expanded stars being given I (supergiant) classifications. In contrast, somewhat less developed and more luminous stars are given luminosity II or III. Because they are huge stars with short lives, many blue giants are located in O-B associations, which are extensive collections of loosely connected young stars.
There are other extremely evolved hot stars not normally referred to as blue giants:
- Wolf–Rayet stars, highly bright and characterized by their intense temperatures and prominent nitrogen and helium emission lines;
- post-AGB stars shaping planetary nebulae, related to Wolf–Rayet stars but less massive and smaller; blue stragglers, rare luminous blue stars perceived apparently on the main sequence in batches where main-sequence stars of their glow should have evolved into supergiants or giants;
- And the true blue supergiants, the most extensive stars developed beyond blue giants and recognized by the effects of more significant expansion on their spectra.
Now You Know