A red giant is a luminous, cool, evolved star that has left the main sequence after using the hydrogen fuel in its core. Compared with main-sequence stars of similar mass, red giants have much larger radii and lower surface temperatures, which give them a characteristic orange to deep red hue (red). The class covers a range of masses — roughly from about one-half to ten times the mass of our Sun — and includes both single stars and components of binary systems.

Key characteristics

Red giants display a number of defining features that follow from their internal structure and energy generation mechanisms:

  • Extended envelope: The outer layers expand dramatically, often making the star tens to hundreds of times wider than when it was on the main sequence.
  • Cooler surface temperature: Photospheric temperatures drop (typically 3,000–5,000 K), producing the orange-red colors observers see.
  • Shell burning: After core hydrogen is exhausted, hydrogen fusion continues in a shell surrounding an inert helium core; later stages may ignite helium in the core.
  • High luminosity: Although cooler, the much larger surface area makes red giants far more luminous than the Sun.

Formation and evolution

Stars become red giants as a normal phase of stellar evolution. When hydrogen in the core is depleted the core contracts and heats while the envelope expands. If the core reaches the conditions for helium fusion, the star moves into the horizontal-branch or red-clump phase; more massive giants follow more complex paths, possibly undergoing thermal pulses on the asymptotic giant branch. These processes change surface composition through convective mixing and can drive strong stellar winds that return material to the interstellar medium.

Examples and observation

Some bright red giants are familiar to naked-eye observers. Well-known examples include Aldebaran, Arcturus, Betelgeuse, and the long-period variable Mira. Their relative proximity and brightness make them important targets for studying stellar atmospheres, pulsations, and mass loss. Professional astronomers and amateur observers both monitor these stars for variability and surface changes.

Relevance to the Solar System

Our Sun is currently a main-sequence star but is expected to expand into a red giant in several billion years. Many scientists model that its radius will increase substantially: models suggest the expanding envelope could reach the orbits of the inner planets. As a result, Mercury, Venus, and possibly Earth may be engulfed or experience dramatic changes in surface conditions, though precise outcomes depend on mass loss rates and tidal interactions.

Notable facts and distinctions

Red giants should be distinguished from red supergiants, which are evolved massive stars with far greater masses and larger radii. Red giants are important contributors to galactic chemical evolution because their winds and eventual planetary nebulae return enriched material to space. They also provide observational tests for stellar evolution theories through measured luminosities, radii, pulsation periods, and surface abundances.