asymptotic-giant branch (AGB)

After a low-mass star finishes burning helium in its core, it does not quietly fade. Instead it swells up a second time, even larger and more luminous than its first red-giant phase, into a cool, bloated, restless giant. This final flourish is the asymptotic-giant branch, or AGB — a star's last grand chapter before it sheds its outer layers and dies. The strange name comes from how these stars approach (asymptotically draw near) the original red-giant track on the Hertzsprung-Russell diagram from a different direction.

Inside an AGB star the core has gone quiet — a dead ball of carbon and oxygen, too cool to fuse further in a low-mass star — but two shells around it now take turns burning: an inner helium shell and an outer hydrogen shell, layered like an onion. This double-shell furnace is unstable: every so often the helium shell ignites in a brief, powerful flare called a thermal pulse. AGB stars become some of the largest stars known, hundreds of times the Sun's radius, and they pulsate and shudder, driving enormous winds that strip away their outer material at tremendous rates. The whole AGB phase lasts only about a million years.

The AGB is one of the universe's great chemical factories. The thermal pulses dredge freshly made carbon and slowly built heavy elements up to the surface, and the powerful winds blow this enriched material out into space, eventually forming a planetary nebula and leaving a white dwarf behind. Much of the carbon in your body and the universe's supply of elements like strontium and barium were forged in AGB stars. A point to keep straight: the 'asymptotic' label is a quirk of the diagram, not a description of the star itself — these are simply a star's final, most productive giant years.