Red Giants
A red giant is a low- or intermediate-mass star (roughly 0.5 to 8 solar masses) that has exhausted core hydrogen and is fusing hydrogen in a shell around an inert helium core; its envelope has expanded to tens or hundreds of solar radii and cooled to a surface temperature of about 3,000–5,000 K, giving it a red-orange colour and a luminosity of tens to thousands of times the Sun's.
Curious
On the main sequence, a star fuses hydrogen in its core. When that hydrogen is gone, the core can no longer produce energy and contracts and heats, while hydrogen in a surrounding shell begins to fuse. The extra energy output pushes the outer layers outward. Although the star becomes much more luminous, its surface spreads over such a large area that each patch is cooler and redder, hence “red giant”.
The Sun is expected to become a red giant in about five billion years, growing large enough to swallow Mercury and probably Venus; whether Earth is engulfed depends on details of mass loss and tidal effects that are still debated. Familiar red giants in the night sky include Arcturus and Aldebaran. On the Hertzsprung–Russell diagram they sit in the upper right.
Exploring
| Stage | Core | Energy source | Approx. duration for 1 M☉ |
|---|---|---|---|
| Main sequence | Hydrogen burning | Core hydrogen fusion | ~10 Gyr |
| Red giant branch | Inert helium | Hydrogen shell fusion | ~1 Gyr |
| Horizontal branch | Helium burning | Core helium + shell hydrogen | ~100 Myr |
Deep Dive
Why the envelope expands. After core hydrogen exhaustion, the helium core contracts nearly isothermally until electron degeneracy supports it (for stars below about 2 solar masses). The hydrogen shell around it burns at high temperature, producing more energy than the main-sequence core did, and the envelope responds by expanding and becoming convective. As the star climbs the red giant branch, its luminosity is set largely by the mass of its helium core, an observed core-mass–luminosity relation that makes the red giant branch tip a usable distance indicator.
The helium flash. In stars below about 2 solar masses, the core is degenerate when it reaches roughly 108 K, and helium ignites explosively in a thermonuclear runaway because degenerate pressure does not respond to temperature. The energy goes into lifting the degeneracy rather than emerging directly, so the flash is not seen from outside; the star then settles onto the horizontal branch (or red clump) burning helium to carbon and oxygen via the triple-alpha process (see Stellar Nucleosynthesis).
Mass loss and the Sun's fate. Red giants lose mass through stellar winds, with rates uncertain to a factor of a few. Published models of the Sun give a maximum red-giant-branch radius of order 170–250 solar radii, the exact value depending on the mass-loss law assumed, which is why Earth's fate cannot be stated with certainty. Asteroseismology of red giants with Kepler and TESS now measures their masses, radii and core states directly, testing these models (see Stellar Structure).
Concept Relationship Map
Requires: main sequence, stellar structure
Enables: white dwarfs, planetary nebulae
Commonly confused with: red supergiants (far more massive; end as supernovae)
Sources
- Iben, I. (1967). Annual Review of Astronomy and Astrophysics, 5, 571–626.
- Kippenhahn, R., Weigert, A., Weiss, A. (2012). Stellar Structure and Evolution (2nd ed.). Springer.
- Schröder, K.-P., Smith, R.C. (2008). “Distant future of the Sun and Earth revisited.” Monthly Notices of the Royal Astronomical Society, 386, 155–163.