Red Giants
Eventually, every star runs low on the hydrogen fuel in its core. What happens next is a dramatic reversal: the core shrinks while the star as a whole swells to many times its original size, cools at the surface, and reddens — becoming a red giant.
Core exhaustion and the onset of shell burning
As described in the Main Sequence Stars lesson, a star spends most of its life in stable hydrostatic equilibrium, fusing hydrogen into helium in its core. But that fuel supply is finite. Once the hydrogen in the core itself is used up, core fusion stops — and with it, the outward pressure that had been holding the star in equilibrium disappears from the centre.
With nothing left to resist gravity at the core, the now-inert helium core (mostly the "ash" left behind by hydrogen fusion) begins to contract under its own weight. As it contracts, exactly as in the original protostar collapse, gravitational potential energy converts to heat, and the core temperature climbs. This heats the shell of hydrogen-rich material immediately surrounding the core to the point where that material becomes hot enough to ignite hydrogen fusion — not at the centre any more, but in a shell wrapped around the now-inert core. This is known as shell burning.
- The contracting helium core continues to heat up even as shell burning proceeds around it, since there is still no fusion happening within the core itself to halt its contraction.
- Shell burning is actually more vigorous than the core fusion that preceded it, because the shell sits in a region of higher temperature and pressure than the old core did at a similar stage — so the star's total energy output, and hence its luminosity, increases.
- All this extra energy pours outward into the star's outer envelope, which responds by expanding dramatically.
Why the star swells and reddens
The huge increase in energy flowing outward from shell burning pushes the outer layers of the star outward, and the star's overall radius increases enormously — often to tens or even hundreds of times its main-sequence size. Because this expanded outer envelope is spread over such a vastly larger surface area, its surface temperature actually drops, even though the star's total energy output has increased.
A cooler surface radiates light with its black-body spectrum peak shifted toward longer wavelengths — toward the red end of the visible spectrum — which is exactly the relationship described by Wien's law earlier in this module. This is why the star's surface visibly reddens, and why this evolutionary stage is called the red giant phase.
It can seem contradictory that a red giant is simultaneously cooler at its surface than it was on the main sequence, yet far more luminous overall. The resolution is Stefan's law, $L = 4\pi r^2 \sigma T^4$: luminosity depends on both surface temperature and surface area. A red giant's surface temperature $T$ has fallen, but its radius $r$ has grown so much that the $r^2$ term completely overwhelms the drop in $T^4$, giving a large net increase in $L$. This is exactly why red giants sit in the upper-right region of the H-R diagram — cool (red) in surface temperature, yet highly luminous, which by Stefan's law can only mean the star has become enormous.
Helium ignition
The contracting helium core doesn't stop heating up just because hydrogen shell burning has started around it. Eventually, if the core becomes hot and dense enough, helium fusion itself ignites in the core — fusing helium nuclei together to form carbon and, in further reactions, oxygen. This marks a further stage in the star's evolution, providing (for a time) a fresh source of core energy generation and briefly stabilising the core's contraction, before the star continues to evolve as its remaining nuclear fuel is progressively exhausted.
For a low-to-intermediate mass star like the Sun, this red giant phase — including hydrogen shell burning and, later, helium core fusion — is a comparatively brief, dramatic, and highly active final stage of nuclear burning, lasting a small fraction of the star's total lifetime, in sharp contrast to the long, steady billions of years spent on the main sequence beforehand. What becomes of the star once this fuel too is exhausted is covered in the next lesson, White Dwarfs.
The defining paradox of the red giant phase is that the core contracts while the star as a whole expands. Loss of core fusion pressure lets the core shrink and heat further; that extra heat ignites shell burning, which increases total energy output and pushes the outer envelope outward and cooler. A star doesn't become a red giant by "running out of energy" — quite the opposite, it becomes a red giant because its energy output actually increases, just from a shell around a shrinking core rather than the core itself.
Key takeaways
- Once core hydrogen is exhausted, core fusion stops and the helium core contracts under gravity, heating up as it does so.
- This heats the surrounding hydrogen-rich shell enough to ignite shell burning, which increases the star's total energy output.
- The extra energy pushes the outer envelope outward, so the star's radius grows enormously while its surface temperature falls.
- A cooler surface radiates more toward the red end of the spectrum (Wien's law), explaining the star's reddened appearance.
- Despite being cooler, the star becomes far more luminous overall, because Stefan's law shows luminosity depends on radius squared as well as temperature to the fourth power.
- This combination of low surface temperature and high luminosity is why red giants sit in the upper-right of the H-R diagram.
- If the contracting core becomes hot enough, helium fusion ignites, converting helium into carbon and oxygen.
- The red giant phase is a relatively brief, dramatic stage compared to the long, stable main-sequence phase that preceded it.