Red Giant BranchEdit
The Red Giant Branch (RGB) is a prominent phase in the life cycle of many stars, marking a transition from the stable hydrogen-burning on the main sequence to later stages of evolution. It occurs in stars with initial masses roughly between about 0.8 and 8 solar masses, as they exhaust hydrogen in their cores and move off the main sequence. During this phase, the star expands dramatically and develops a cool, luminous outer envelope that gives it a characteristic red appearance in observations such as color-magnitude diagrams Color-magnitude diagram.
In the RGB, the energy generated in the radiative core is no longer produced by hydrogen fusion there. Instead, hydrogen continues to fuse in a thin shell around an inert helium core. The shell-burning shell causes the core to contract and the envelope to expand, driving a large increase in radius and luminosity. The surface temperature falls, so the star becomes cooler and redder even as its total energy output climbs. For many low-mass stars, the helium core becomes electron-degenerate and grows until helium ignition occurs in a thermonuclear flash, a dramatic event that propels the star onto the next phase of evolution. For somewhat more massive stars, helium ignition can occur more quietly as the core is non-degenerate. In either case, the RGB marks the stage before helium burning commences in the core and the star moves toward the horizontal branch or the clump in the Hertzsprung–Russell diagram Stellar evolution.
Structure and Evolution on the Red Giant Branch
Internal structure
RGB stars possess an inert helium core that is encased by a rapidly burning hydrogen shell. The shell’s fusion adds material to the core, increasing its mass and gravitational pull. The surrounding convective envelope extends outward, and its deepening convection can transport material from the interior to the surface in a process known as the first dredge-up. The combination of shell burning, core contraction, and envelope expansion drives the distinctive luminosity and radius changes that define the RGB. The thermal and mechanical properties of the core, including whether it is degenerate, play a crucial role in the subsequent ignition of helium Degenerate matter and Helium ignition (which can manifest as a helium flash in very low-mass stars) Helium flash.
Evolutionary path
The ascent up the RGB follows the exhaustion of hydrogen in the core on the main sequence. As hydrogen fuses in a shell, the core grows in mass and becomes denser, while the envelope inflates. The star becomes increasingly luminous as its surface cools and reddens. The depth of the convective envelope during the first dredge-up changes the surface abundances of light elements (notably C and N) and influences subsequent evolution. The end of the RGB is reached when helium begins to burn in the core—via a helium flash for degenerate cores, or more quiescently for non-degenerate cores—moving the star away from the RGB to the horizontal branch or red clump, depending on metallicity and mass Helium ignition.
Mass loss on the RGB
RGB stars lose mass through winds driven by radiation pressure, pulsations, and convection in the outer layers. While the exact rate of mass loss on the RGB remains an area of active study, it has important consequences for later stages of evolution, including the structure of the resulting white dwarf and the behavior of the star on the subsequent asymptotic giant branch. Empirical prescriptions, such as the approximate Reimers formulation, are used to model this mass loss in stellar evolution calculations, but uncertainties in the physics of the winds and in the dependence on metallicity and luminosity persist Stellar wind Mass loss.
Observational markers and variability
RGB stars populate distinctive sequences in color-magnitude diagrams, with their location primarily set by luminosity and metallicity. The RGB also exhibits features such as the RGB bump, a local increase in star counts at a particular luminosity caused by changes in the stellar structure during the first dredge-up. The brightest portion of the RGB—the tip of the red giant branch (TRGB)—serves as a useful standard candle for estimating distances to nearby galaxies, because its luminosity is comparatively bright and relatively insensitive to age for old stellar populations. Observational studies of the RGB draw on spectroscopy, asteroseismology, and multi-band photometry to infer metallicity, convection properties, and internal structure Asteroseismology Color-magnitude diagram.
Observational properties and implications
The RGB is most easily identified in stellar populations with well-defined ages, such as star clusters and the bulges of galaxies. The effective temperature of RGB stars typically lies in the range of a few thousand kelvin, producing red optical colors. Metallicity affects both the opacity of the outer layers and the temperature at a given luminosity, so metal-rich RGB stars are usually cooler and redder at a fixed brightness than their metal-poor counterparts. Spectroscopic analyses of RGB stars provide insights into surface abundances altered by the first dredge-up, including changes in carbon-to-n nitrogen-to-oxygen ratios that reflect internal mixing processes. The study of RGB stars in globular clusters and nearby galaxies has been central to understanding stellar populations, chemical evolution, and distance scales Stellar nucleosynthesis.
The RGB also plays a crucial role in enriching the interstellar medium. Mass loss from RGB stars contributes gas and processed elements to the surrounding medium, affecting subsequent generations of star formation and the chemical evolution of galaxies. The progression toward the asymptotic giant branch (AGB) follows RGB ascent and eventually leads to more substantial mass loss and thermal-pulse behavior, with the star’s outer layers expelled and a white dwarf remnant remaining for many low- to intermediate-mass stars Asymptotic giant branch Stellar wind.
Debates and uncertainties
The precise rate of mass loss on the RGB remains uncertain, with different observational tracers and models yielding varying results. This uncertainty has implications for the inferred ages of stellar populations and the subsequent evolution along the AGB. Ongoing work in infrared surveys and asteroseismology aims to pin down these rates more reliably Stellar wind.
The details of mixing in RGB envelopes, including the depth and efficiency of the dredge-up and any extra mixing processes beyond standard convection (such as thermohaline mixing or rotation-induced mixing), are active areas of study. These processes affect surface abundances and can influence interpretations of spectroscopy in old stellar populations First dredge-up Convection.
The precise luminosity of the TRGB as a distance indicator depends on metallicity and age distributions. While TRGB distances are powerful for nearby galaxies, calibrations and zero-points continue to be refined, particularly in the era of precision cosmology. Cross-calibration with other distance indicators, such as Cepheid variables and standard candles in the cosmic distance ladder, remains an important topic Color-magnitude diagram Cepheid variable.
Helium ignition—whether it occurs via a flash in low-mass, degenerate cores or more gradually in higher-mass stars—has ramifications for the early evolution after the RGB and for the morphology of the horizontal branch. The transition between these regimes depends on mass, metallicity, and the details of core degeneracy, and is a subject of theoretical modeling and observational tests Helium flash.