Giant star

A giant star is substantially larger and more luminous than a main-sequence star of similar surface temperature. Many giants form when stars exhaust hydrogen in their cores and expand.

Stellar evolution: The changes in a star’s structure and properties over its lifetime. Giant stars are a phase in the life cycles of stars.
Hertzsprung–Russell diagram: A plot that relates stars’ luminosities to their surface temperatures or spectral types. The giant branch separates stars that share temperatures but differ greatly in luminosity.
Aldebaran: An orange giant star that is the brightest star in the constellation Taurus. It is a nearby, well-studied example of a red giant.
Red giant: A cool, luminous giant star with an expanded outer envelope. Red giants are the familiar cool, swollen subset of giant stars.
Hydrogen shell burning: The fusion of hydrogen in a layer surrounding a star’s helium-rich core. Shell fusion supplies energy as the core contracts and the outer layers expand.
Stellar luminosity: The total energy a star emits per unit time across all wavelengths. High luminosity, rather than size alone, is central to the giant classification.
Arcturus: An orange giant star and the brightest star in the constellation Boötes. Its color and luminosity illustrate the properties of evolved giant stars.
Blue giant: A hot, luminous star that lies above the main sequence and is larger than a main-sequence star of similar temperature. Blue giants show that the giant classification is not limited to cool red stars.
Helium flash: A rapid onset of helium fusion in the degenerate core of a low-mass red giant. It marks the transition from red-giant ascent to stable core helium burning in low-mass stars.
Effective temperature: The temperature of a blackbody that would emit the same energy per unit area as an object. Comparing giants at similar effective temperatures exposes their much larger radiating surfaces.

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