Giant star
Giant stars are luminous, expanded post-main-sequence stars.
A giant star is a star with a substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. Giants lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III in the Yerkes spectral classification.
- field
- Astronomy
- known_for
- Stars with radii up to a few hundred times the Sun and luminosities over 10 times that of the Sun
- luminosity_classes
- II and III
- coined_by
- Ejnar Hertzsprung
Lore & Background
A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and leaves the main sequence. For stars above about 0.25 solar masses, the core contracts and heats up, hydrogen starts to fuse in a shell, and the outer layers expand and cool, forming a subgiant. The inert helium core grows until it reaches the Schönberg–Chandrasekhar limit, collapses, and may become degenerate, causing the star to move onto the red-giant branch where it stably burns hydrogen in a shell for a substantial fraction of its life. In stars above about 0.4 solar masses, the core eventually reaches 10^8 K and begins fusing helium to carbon and oxygen via the triple-alpha process. After core helium is exhausted, stars up to about 8 solar masses develop a degenerate carbon–oxygen core and begin helium shell burning, entering the asymptotic giant branch (AGB) with increased size and luminosity. High-mass stars above about 12 solar masses become blue giants, then blue supergiants, and later red supergiants, eventually ending as supernovae. Stars below about 0.25 solar masses never become giants.
Reader's Guide
Giant stars represent a critical phase in stellar evolution, marking the transition from main-sequence hydrogen burning to later stages. They are among the most visible stars in the night sky due to their high luminosity, and they serve as key laboratories for understanding nucleosynthesis, convection, and mass loss. The red-giant branch, horizontal branch, and asymptotic giant branch each correspond to distinct evolutionary stages with different internal structures and fusion processes. Giants also produce heavy elements through dredge-up events, enriching the interstellar medium. The classification of giants, including subgiants (class IV), bright giants (class II), and red giants, helps astronomers map stellar populations and ages. Despite their name, any main-sequence star is properly called a dwarf, regardless of size or luminosity. The study of giants continues to refine models of stellar life cycles and the chemical evolution of galaxies.
Did You Know?
- Giant stars have radii up to a few hundred times the Sun and luminosities over 10 times that of the Sun.
- A star becomes a giant after all hydrogen available for fusion at its core has been depleted.
- Stars below about 0.25 solar masses never become giant stars.
Frequently Asked Questions
What is a giant star in astronomy?
A giant star is a post-main-sequence star that has swelled to a much larger radius and higher luminosity than a main-sequence star at the same surface temperature. On the Hertzsprung–Russell diagram these stars plot above the main-sequence band.
Which Yerkes luminosity classes correspond to giant stars?
Giant stars fall under luminosity classes II and III in the Yerkes spectral classification. Class II designates bright giants, while class III covers the more numerous ordinary giants.
How large and luminous can a giant star become?
Giant stars can reach radii of several hundred times the Sun's, with luminosities exceeding ten times solar output. They represent the expanded, luminous phase a star enters after exhausting its core hydrogen and leaving the main sequence.
More in Astronomy 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record