Editing Giant star (section)

==Subclasses==
There are a wide range of giant-class stars and several subdivisions are commonly used to identify smaller groups of stars.

===Subgiants===
{{Main|Subgiant}}
Subgiants are an entirely separate spectroscopic luminosity class (IV) from giants, but share many features with them. Although some subgiants are simply over-luminous main-sequence stars due to chemical variation or age, others are a distinct evolutionary track towards true giants.

Examples:
* [[Gamma Geminorum]] (γ Gem), an A-type subgiant;
* [[Eta Bootis]] (η Boo), a G-type subgiant.
* [[Delta Scorpii]] (δ Sco), a B-type subgiant.

===Bright giants===
'''Bright giants''' are [[star]]s of luminosity class '''II''' in the [[Yerkes spectral classification]].  These are stars which straddle the boundary between ordinary giants and [[supergiant]]s, based on the appearance of their spectra.<ref name=abt>{{cite journal|bibcode=1957ApJ...126..503A|title=Line Broadening in High-Luminosity Stars. I. Bright Giants|journal=Astrophysical Journal|volume=126|pages=503|last1=Abt|first1=Helmut A.|year=1957|doi=10.1086/146423|doi-access=free}}</ref> The bright giant luminosity class was first defined in 1943.<ref name=dick>{{cite book|title=Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape|author=Steven J. Dick|page=176|publisher=Springer|isbn=9783030103804|year=2019|url=https://books.google.com/books?id=UymODwAAQBAJ}}</ref>

Well known stars which are classified as bright giants include:
* [[Canopus]]
* [[Albireo]]
* [[Theta Scorpii]]
* [[Beta Draconis]]
* [[Alpha Herculis]]
* [[Gamma Canis Majoris]]

===Red giants===
{{Main|Red giant}}
Within any giant luminosity class, the cooler stars of spectral class K, M, S, and C, (and sometimes some G-type stars<ref name=aaa503_2_521/>) are called red giants. Red giants include stars in a number of distinct evolutionary phases of their lives: a main [[red giant|red-giant branch]] (RGB); a red [[horizontal branch]] or [[red clump]]; the [[asymptotic giant branch]] (AGB), although AGB stars are often large enough and luminous enough to get classified as supergiants; and sometimes other large cool stars such as immediate [[post-AGB star]]s.  The RGB stars are by far the most common type of giant star due to their moderate mass, relatively long stable lives, and luminosity. They are the most obvious grouping of stars after the main sequence on most HR diagrams, although white dwarfs are more numerous but far less luminous.

Examples:
* [[Pollux (star)|Pollux]], a K-type giant.
* [[Epsilon Ophiuchi]], a G-type red giant.<ref name=aaa503_2_521>{{citation | display-authors=1 | last1=Mazumdar | first1=A. | last2=Mérand | first2=A. | last3=Demarque | first3=P. | last4=Kervella | first4=P. | last5=Barban | first5=C. | last6=Baudin | first6=F. | last7=Coudé du Foresto | first7=V. | last8=Farrington | first8=C. | last9=Goldfinger | first9=P. J. | title=Asteroseismology and interferometry of the red giant star ɛ Ophiuchi | journal=Astronomy and Astrophysics | volume=503 | issue=2 | pages=521–531 |date=August 2009 | doi=10.1051/0004-6361/200912351 | bibcode=2009A&A...503..521M |arxiv = 0906.3386 | s2cid=15699426 }}</ref>
* [[Arcturus]] (α Boötis), a K-type giant.
* [[Aldebaran]], a K-type giant
* [[R Doradus]], a M-type giant.
* [[Mira]] (ο Ceti), an M-type giant and prototype Mira variable.
* [[Chi Cygni]], a S-type giant.
* [[TX Piscium]], a C-type giant ([[carbon star]]).

===Yellow giants===
Giant stars with intermediate temperatures (spectral class G, F, and at least some A) are called yellow giants. They are far less numerous than red giants, partly because they only form from stars with somewhat higher masses, and partly because they spend less time in that phase of their lives. However, they include a number of important classes of variable stars. High-luminosity yellow stars are generally unstable, leading to the [[instability strip]] on the HR diagram where the majority of stars are pulsating variables. The instability strip reaches from the main sequence up to hypergiant luminosities, but at the luminosities of giants there are several classes of pulsating variable stars:
*[[RR Lyrae variable]]s, pulsating horizontal-branch class A (sometimes F) stars with periods less than a day and amplitudes of a magnitude of less;
*[[W Virginis variable]]s, more-luminous pulsating variables also known as type II Cepheids, with periods of 10–20 days;
*[[Classical Cepheid variable|Type I Cepheid variables]], more luminous still and mostly supergiants, with even longer periods;
*[[Delta Scuti variable]]s, includes subgiant and main-sequence stars.

Yellow giants may be moderate-mass stars evolving for the first time towards the red-giant branch, or they may be more evolved stars on the horizontal branch. Evolution towards the red-giant branch for the first time is very rapid, whereas stars can spend much longer on the horizontal branch. Horizontal-branch stars, with more heavy elements and lower mass, are more unstable.

Examples:
* [[Sigma Octantis]] (σ Octantis), an F-type giant and a Delta Scuti variable;
* [[Capella (star)|Capella Aa]] (α Aurigae Aa), a G-type giant.
* [[Beta Corvi]] (β Corvi), a G-type bright giant.

==={{anchor|Blue giants}}Blue (and sometimes white) giants===
{{Main|Blue giant}}
The hottest giants, of spectral classes O, B, and sometimes early A, are called [[blue giant]]s. Sometimes A- and late-B-type stars may be referred to as white giants due to higher B-Vs (see <ref>{{Cite web |title=Star B-V color index to apparent RGB color |url=https://stackoverflow.com/questions/21977786/star-b-v-color-index-to-apparent-rgb-color |access-date=2025-02-14 |website=Stack Overflow |language=en}}</ref> for K to B-V to RGB).

The blue giants are a very heterogeneous grouping, ranging from high-mass, high-luminosity stars just leaving the main sequence to low-mass, [[horizontal-branch |horizontal-branch stars]]. Higher-mass stars leave the main sequence to become blue giants, then bright blue giants, and then blue supergiants, before expanding into red supergiants, although at the very highest masses the giant stage is so brief and narrow that it can hardly be distinguished from a blue supergiant.

Lower-mass, core-helium-burning stars evolve from red giants along the horizontal branch and then back again to the [[asymptotic giant branch]], and depending on mass and [[metallicity]] they can become blue giants. It is thought that some [[post-AGB star]]s experiencing a late [[thermal pulse]] can become peculiar{{clarify|date=October 2016}} blue giants.

Examples:
* [[Meissa]] (λ Orionis A), an O-type giant.
* [[Alcyone (star)|Alcyone]] (η Tauri), a B-type giant, the brightest star in the [[Pleiades]];
* [[Thuban]] (α Draconis), an A-type giant.