Horizontal branch
Stellar stage after red-giant branch, fusing helium in core.
The horizontal branch (HB) is a phase in the life of a star with a mass similar to the Sun’s, occurring right after the red-giant branch. During this stage, the star gets its energy from two sources: helium fusion in the core (through the triple-alpha process) and hydrogen fusion in a surrounding shell (via the CNO cycle). When core helium fusion ignites at the tip of the red-giant branch, the star’s structure changes dramatically: its luminosity drops, its envelope contracts somewhat, and its surface temperature rises. These stars were first spotted in early photographic surveys of globular clusters, which measured brightnesses across many stars, and they were conspicuously missing from any open cluster examined at the time. The name “horizontal branch” comes from the fact that, in low-metallicity groups like globular clusters, these stars form a roughly horizontal line on a Hertzsprung–Russell diagram. Because all stars in a single globular cluster are about the same distance from Earth, their apparent magnitudes directly reflect their absolute magnitudes, so features tied to absolute magnitude show up clearly on a cluster’s H-R diagram without being blurred by distance uncertainties.
After a star runs out of hydrogen in its core, it leaves the main sequence, starts fusing hydrogen in a shell around a helium core, and becomes a giant on the red-giant branch. In stars up to about 2.3 times the Sun’s mass, the helium core becomes degenerate—it doesn’t produce energy—and keeps growing and heating up as the hydrogen shell adds more helium. If the star is more than roughly half the Sun’s mass, the core eventually gets hot enough to fuse helium into carbon via the triple-alpha process. This fusion ignites across the entire core, causing a quick temperature spike and a surge in the fusion rate. Within seconds, the core sheds its degeneracy and expands rapidly, in an event called the helium flash. Stars with non-degenerate cores start helium fusion more smoothly, without a flash. The energy from this event is absorbed by the overlying plasma, so it’s not visible from the outside. The star then settles into a new equilibrium, shifting from the red-giant branch to the horizontal branch on the H-R diagram.
Stars with initial masses between about 2.3 and 8 times the Sun’s mass have larger helium cores that never become degenerate. Instead, their cores hit the Schönb
- field
- Stellar astrophysics
- known_for
- Stage of stellar evolution following the red-giant branch, powered by core helium fusion and shell hydrogen fusion
Lore & Background
Horizontal branch stars were discovered with the first deep photographic photometric studies of globular clusters. They were notable for being absent from all open clusters that had been studied up to that time. The horizontal branch is so named because in low-metallicity star collections like globular clusters, HB stars lie along a roughly horizontal line in a Hertzsprung–Russell diagram. Because the stars of one globular cluster are all at essentially the same distance from us, their apparent magnitudes all have the same relationship to their absolute magnitudes, and thus absolute-magnitude-related properties are plainly visible on an H-R diagram confined to stars of that cluster, undiffused by distance and thence magnitude uncertainties.
After exhausting their core hydrogen, stars leave the main sequence and become giants on the red-giant branch. In stars with masses up to 2.3 times the mass of the Sun, the helium core becomes degenerate. When the core reaches the temperature necessary for helium fusion, the initiation of fusion begins across the core region, causing a rapid increase in fusion rate and a helium flash. The star then changes to a new equilibrium state, switching from the red-giant branch onto the horizontal branch. Stars initially between about 2.3 M☉ and 8 M☉ have larger helium cores that do not become degenerate; they contract and heat up, triggering helium fusion smoothly without a flash. Stars more massive than about 8 M☉ also ignite their core helium smoothly and go on to burn heavier elements as a red supergiant.
Stars remain on the horizontal branch for around 100 million years, becoming slowly more luminous. When their core helium is exhausted, they progress to helium shell burning on the asymptotic giant branch (AGB), where they become cooler and much more luminous. The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000 K. Theories to explain these stars include binary interactions and 'late thermal pulses' after the planetary nebular phase.
Reader's Guide
The horizontal branch is a critical phase in the evolution of low-mass stars, bridging the red-giant branch and the asymptotic giant branch. Its study provides insight into stellar structure, nuclear fusion processes, and the properties of globular clusters. The branch's morphology—the distribution of stars along it—varies among clusters and is influenced by metallicity, age, rotation, and helium content. This variation, known as the 'Second Parameter Problem,' remains a long-standing puzzle in stellar astrophysics, as clusters with identical metallicity can have very different horizontal branch morphologies, as seen in the pair NGC 288 and NGC 362. The horizontal branch also hosts RR Lyrae variable stars, which pulsate with periods up to 1.2 days and create a prominent gap in color-magnitude diagrams due to the difficulty of obtaining accurate averaged magnitudes for these variables. The red clump, a related class of younger, more metal-rich stars, represents one extreme of horizontal-branch morphology, with all stars at the red end. Understanding the horizontal branch thus informs models of stellar evolution, the age and composition of stellar populations, and the dynamics of globular clusters.
Did You Know?
- Horizontal branch stars are powered by helium fusion in the core via the triple-alpha process and by hydrogen fusion via the CNO cycle in a shell.
- The horizontal branch is so named because in low-metallicity star collections like globular clusters, HB stars lie along a roughly horizontal line in a Hertzsprung–Russell diagram.
- The hottest horizontal-branch stars, called extreme horizontal branch, have temperatures of 20,000–30,000 K.
- The RR Lyrae gap in globular cluster color-magnitude diagrams arises because variable stars are often omitted from graphic presentations due to the difficulty of obtaining their true averaged magnitudes.
More in Stars And Stellar Phenomena 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record