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A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature.^[1] Typically, it will have a radius of between 10 and 100 solar radii and luminosity between 10 and 1,000 times the Sun's. Stars still more luminous than giants are referred to as supergiants and hypergiants.^[2][3] A hot, luminous main sequence star may also be referred to as a giant.^[4] Apart from this, because of their large radii and luminosities, giant stars will lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung-Russell diagram, and will correspond to luminosity classes II or III in the Yerkes classification.^[5]

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.^[5] A star whose initial mass is less than approximately 0.4 solar masses will not become a giant star. This is because such stars have their interior thoroughly mixed by convection and therefore continue fusing hydrogen until it is exhausted throughout the star, at which point they become white dwarfs, composed chiefly of helium. This exhaustion, however, is predicted to take significantly longer than the lifetime of the Universe up to now.^[6]

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.^{[7], § 5.9.}

If the star's mass, when on the main sequence, was below approximately 0.5 solar masses, it is thought that it will never attain the central temperatures necessary to fuse helium.^{[8], p. 169.} It will therefore remain a hydrogen-fusing red giant until it eventually becomes a helium white dwarf.^{[7], § 4.1, 6.1.} Otherwise, when the core temperature reaches approximately 10⁸ K, helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process.^{[7],§ 5.9, chapter 6.} The energy generated by helium fusion causes the core to expand. This causes the pressure in the surrounding hydrogen-burning shell to decrease, which reduces its energy-generation rate. The luminosity of the star decreases, its outer envelope contracts again, and the star leaves the red giant branch.^[9] 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.^{[7], chapter 6.} If the star is not heavier than approximately 8 solar masses, it will eventually exhaust the helium at its core and begin to fuse helium in a shell around the core. It will then increase in luminosity again as, now an AGB star, it ascends the asymptotic giant branch of the Hertzsprung-Russell diagram. After the star sheds most of its mass, its core will remain as a carbon-oxygen white dwarf.^{[7], § 7.1–7.4.}

For main-sequence stars with masses great enough to eventually fuse carbon (approximately 8 solar masses)^{[7], p. 189}, this picture must be modified in many ways. These stars do not increase greatly in luminosity after leaving the main sequence, but they will become redder. They may become red supergiants, or mass loss may cause them to become blue supergiants.^{[10], pp. 33–35;  [2]} Eventually, they will become white dwarfs composed of oxygen and neon, or will undergo a core-collapse supernova to form neutron stars, or black holes.^{[7], § 7.4.4–7.8.}

Well-known giant stars of various colors include

• Alcyone (η Tauri), a blue-white (B-type) giant,^[11] the brightest star in the Pleiades.^[12]
• Thuban (α Draconis), a white (A-type) giant.^[13]
• σ Octantis, a yellow-white (F-type) giant.^[14]
• α Aurigae Aa, a yellow (G-type) giant, one of the stars making up Capella.^[15]
• Pollux (β Geminorum), an orange (K-type) giant.^[16]
• Mira (ο Ceti), a red (M-type) giant.^[17]

• Blue giant
• Hypergiant
• Red giant
• Supergiant