Giant star

image.'' A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence., § 5.9. If the star's mass, when on the main sequence, was below approximately , it will never reach the central temperatures necessary to fuse helium., p. 169. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf., chapter 6. When the core helium is exhausted, a star with up to about has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the earlier collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up, and causes a dramatic increase in size and luminosity. This is the asymptotic giant branch (AGB) analogous to the red-giant branch but more luminous, with a hydrogen-burning shell contributing most of the energy. Stars only remain on the AGB for around a million years, becoming increasingly unstable until they exhaust their fuel, go through a planetary nebula phase, and then become a carbon–oxygen white dwarf. They largely follow the tracks of lighter stars through RGB, HB, and AGB phases, but are massive enough to initiate core carbon burning and even some neon burning. They form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova, or they may leave behind an oxygen–neon white dwarf. O class main sequence stars are already highly luminous. The giant phase for such stars is a brief phase of slightly increased size and luminosity before developing a supergiant spectral luminosity class. Type O giants may be more than a hundred thousand times as luminous as the sun, brighter than many supergiants. Classification is complex and difficult with small differences between luminosity classes and a continuous range of intermediate forms. The most massive stars develop giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to the surface and high luminosity which produces a powerful stellar wind and causes the star's atmosphere to expand. Low-mass stars A star whose initial mass is less than approximately will not become a giant star at all. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of years, much longer than the current age of the Universe. They steadily become hotter and more luminous throughout this time. Eventually they do develop a radiative core, subsequently exhausting hydrogen in the core and burning hydrogen in a shell surrounding the core. (Stars with a mass in excess of may expand at this point, but will never become very large.) Shortly thereafter, the star's supply of hydrogen will be completely exhausted and it is expected to become a helium white dwarf, although the universe is too young for any such star to exist yet, so no star with that history has ever been observed. ==Subclasses==

There are a wide range of giant-class stars and several subdivisions are commonly used to identify smaller groups of stars. Subgiants 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 stars of luminosity class II in the Yerkes spectral classification. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra. The bright giant luminosity class was first defined in 1943. Well known stars which are classified as bright giants include: • Canopus • Albireo • Theta Scorpii • Beta Draconis • Alpha Herculis • Gamma Canis Majoris Red giants Within any giant luminosity class, the cooler stars of spectral class K, M, S, and C, (and sometimes some G-type stars • 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 variables, pulsating horizontal-branch class A (sometimes F) stars with periods less than a day and amplitudes of a magnitude of less; • W Virginis variables, more-luminous pulsating variables also known as type II Cepheids, with periods of 10–20 days; • Type I Cepheid variables, more luminous still and mostly supergiants, with even longer periods; • Delta Scuti variables, 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 Aa (α Aurigae Aa), a G-type giant. • Beta Corvi (β Corvi), a G-type bright giant. Blue (and sometimes white) giants The hottest giants, of spectral classes O, B, and sometimes early A, are called blue giants. Sometimes A- and late-B-type stars may be referred to as white giants due to higher B-Vs (see 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 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 stars experiencing a late thermal pulse can become peculiar blue giants. Examples: • Meissa (λ Orionis A), an O-type giant. • Alcyone (η Tauri), a B-type giant, the brightest star in the Pleiades; • Thuban (α Draconis), an A-type giant. == See also ==