When a star's core runs dry, its outer layers balloon outward
Once a Sun-like star uses up the hydrogen in its core, the core shrinks while the layers above swell to tens or hundreds of times the Sun's width. Astronomers call this the mirror principle. The result is a red giant: cooler at the surface, yet thousands of times brighter.
Red giants grow from main-sequence stars weighing roughly 0.3 to 8 solar masses. When core hydrogen runs low, the core's nuclear output falters, it contracts and heats, and eventually a surrounding shell gets hot enough to fuse hydrogen. The fresh energy from that shell inflates the envelope. Surfaces on these stars sit around 3,000 to 5,000 kelvin, compared with nearly 6,000 for the Sun, so they glow yellow-orange to red. Their sheer size, up to about 200 solar radii, pushes luminosity to nearly three thousand times the Sun's. Arcturus, 36 light-years away, is one; Gacrux, at 88 light-years, is the nearest M-class giant.
Artists usually draw them with crisp edges, but their envelopes are so thin that no sharp surface exists; the star fades gradually into a corona. Instead of the Sun's countless small granules, a red giant's surface carries a handful of huge convection cells, and their shifting causes the brightness changes common in these stars.
What happens next depends on mass. In stars under about two solar masses, the helium core becomes degenerate and heats to roughly 100 million kelvin, at which point the whole core ignites almost at once in a helium flash; heavier stars start burning helium gently. After core helium is spent, helium and hydrogen burn in nested shells around a carbon-oxygen core, the asymptotic giant branch. Churning called dredge-up can then haul carbon to the surface, creating carbon stars. For a star of one solar mass the whole giant phase lasts about a billion years, most of it on the first ascent.
Stars below eight solar masses never ignite that carbon-oxygen core. They cast off their outer layers as a planetary nebula, leaving a white dwarf. Being a giant need not doom a planetary system, either: one study suggests a Sun-mass star could host a habitable zone lasting billions of years at 2 astronomical units during its giant stage.
Source: Red giant
