Everything about Giant Star totally explained
A
giant star is a
star with substantially larger
radius and
luminosity than a
main sequence star of the same
surface temperature. Typically, giant stars have radii between 10 and 100
solar radii and luminosities between 10 and 1,000 times that of the
Sun. Stars still more luminous than giants are referred to as
supergiants and
hypergiants. A hot, luminous
main sequence star may also be referred to as a giant. Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class
V in the
Yerkes spectral classification) on the
Hertzsprung-Russell diagram and correspond to luminosity classes
II or
III.
Formation
A star becomes a giant star after all the
hydrogen available for
fusion at its core has been depleted and, as a result, it has left the
main sequence.
If a star is more massive than this lower limit, then when it consumes all of the
hydrogen in its core available for
fusion, the core will begin to contract. Hydrogen now fuses to
helium in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools. During this portion of its
evolution, labeled the
subgiant branch on the
Hertzsprung-Russell diagram, the
luminosity of the star remains approximately constant and its
surface temperature decreases. Eventually the star will start to ascend the
red giant branch on the
Hertzsprung-Russell diagram. At this point the
surface temperature of the star, now typically a
red giant, will remain approximately constant as its
luminosity and radius increase drastically. The core will continue to contract, raising its temperature.
, § 5.9.
If the star's mass, when on the main sequence, was below approximately 0.5
solar masses, it's thought that it'll never attain the central temperatures necessary to fuse
helium.
, p. 169. It will therefore remain a hydrogen-fusing red giant until it eventually becomes a helium white dwarf. Its subsequent evolution will depend on its mass. If not very massive, it may be found in the
horizontal branch on the Hertzsprung-Russell diagram, or its position in the diagram may move in loops.
, pp. 33–35; the brightest star in the
Pleiades.
Thuban (α Draconis), a white (A-type) giant.
σ Octantis, a yellow-white (F-type) giant.
α Aurigae Aa, a yellow (G-type) giant, one of the stars making up Capella.
Pollux (β Geminorum), an orange (K-type) giant.
Mira (ο Ceti), a red (M-type) giant.Further Information
Get more info on 'Giant Star'.
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