horizontal branch

After the drama of the helium flash, a low-mass star reaches a new, calmer plateau. It has shrunk back from its bloated red-giant size and now burns helium steadily in its core. On the maps astronomers draw of star populations, these stars line up along a nearly flat, horizontal stripe — hence the name, the horizontal branch. It is, in a sense, a star's second adulthood: a stable, energy-producing phase, briefer than the main sequence but built on the same idea of steady core fusion.

Here the star has a two-storey furnace. In the core, helium fuses into carbon and oxygen through the triple-alpha process; around that core, a shell of hydrogen keeps fusing too. This dual burning gives a stable structure that lasts roughly 50 to 100 million years for a Sun-like star — far shorter than its 10-billion-year main sequence, because helium yields much less energy than hydrogen and the star is now luminous. The branch is 'horizontal' on a Hertzsprung-Russell diagram because the stars on it have similar luminosities but spread out across a range of surface temperatures, from cool and reddish to hot and blue, depending on how much outer envelope each has kept.

The horizontal branch is a powerful tool for studying old stellar populations, especially in globular clusters, where its exact shape and color encode the cluster's age, chemical composition, and history of mass loss. A famous slice of it contains the RR Lyrae variable stars, which pulse with a regular beat and serve as standard candles for measuring distances within our galaxy. A point of confusion: the horizontal branch is the low-mass analog of the gentle, smooth helium ignition that heavier stars reach without a flash — same core fusion, different doorway in.