What Is a Red Giant? A Star Running Out of Fuel and Swelling Enormously

A star is a balance between gravity pulling inward and pressure from the energy released by fusion pushing outward. When hydrogen at the core is exhausted, fusion there stops and the core contracts under gravity, which heats it, and that heating ignites hydrogen fusion in a shell surrounding the core where fresh fuel remains. The shell burns far more vigorously than the core did, since it is hotter, and the increased energy output inflates the outer layers enormously. Those layers cool as they expand, which is why the star becomes red despite producing far more energy, since the surface temperature falls while the total output rises. The star therefore becomes larger, cooler at the surface, brighter overall and much less dense in its outer regions, which is the whole transformation.

The sequence after expansion depends on the star's mass and has several stages:

• •The contracting core eventually becomes hot enough to fuse helium into carbon, which happens abruptly in lower-mass stars
• •Helium fusion stabilises the star temporarily at a smaller size
• •When core helium runs out, the process repeats with shells burning around a carbon core and the star expands again, further than before
• •Stars around the mass of the sun cannot ignite carbon and end here
• •Pulsations and strong winds strip the outer layers, which drift away and are lit as a glowing shell
• •A hot dense core is left behind as a white dwarf, which cools indefinitely

The sun will follow this course and the timing is reasonably well constrained. It is roughly halfway through its hydrogen-burning life, with around five billion years remaining before the core is exhausted. It is already becoming gradually brighter, by about ten percent per billion years, and that slow increase will make Earth uninhabitable well before the dramatic events, with estimates placing the end of complex surface life at around a billion years from now as rising output drives away the oceans. When expansion begins the sun will grow to engulf the inner planets, with whether Earth survives depending on the balance between the sun losing mass, which widens orbits, and tidal interaction, which pulls the planet inward, and current work suggests Earth is likely to be engulfed. The remnant will be a white dwarf about the size of Earth.

Several of the brightest stars in the sky are in this phase and are easy to identify. The reddish stars conspicuous to the unaided eye are mostly giants or the more extreme supergiants, and the colour is directly visible without equipment once a person knows to look, with a bright red star in Orion and another in Scorpius being the standard examples. Their size is difficult to convey and the figures help, since placing one of them where the sun is would put its surface beyond the orbit of Mars. Several are variable, changing in brightness over months as they pulsate, which is observable by comparison with neighbouring stars. One of them dimmed conspicuously in 2019 and 2020, prompting speculation that it might be about to explode, and the cause was established as a cloud of dust ejected by the star itself passing across the line of sight.

These stars are the source of a substantial share of the chemical elements. The deep convective mixing during the giant phases brings material processed in the interior to the surface, and the strong winds then expel it into space, which distributes carbon, nitrogen and a range of heavier elements produced by slow neutron capture in the interior. Carbon in particular comes largely from stars of this kind rather than from supernovae, which means the carbon in living things was assembled in the cores of stars that ended quietly. Dust forms in the cool expanding outer layers, which is where a large share of interstellar dust originates, and that dust shields molecular clouds and is the material planets are eventually built from. The quiet end of a moderate star therefore contributes more to the composition of the next generation than its brightness suggests.

Core hydrogen exhaustion makes the core contract and heat, which ignites a shell burning far more vigorously and inflates the outer layers, so the star grows and brightens while its surface cools. The sun has around five billion years before this and will render Earth uninhabitable long before through gradual brightening. Carbon in living things came largely from stars ending this way.