Subgiant

A star that has exhausted hydrogen in its core and is expanding toward the red-giant phase. Subgiants are often assigned luminosity class IV.

Main sequence: The longest-lived stage of a star’s life, when it fuses hydrogen in its core. Subgiants have left this hydrogen-fusing stage after exhausting core hydrogen.
Stellar core: The central region of a star, where temperature, pressure, and nuclear reactions are concentrated. Core hydrogen exhaustion is the defining evolutionary change behind the subgiant phase.
Asteroseismology: The study of a star’s internal structure through oscillations observed at its surface. Subgiant oscillations reveal whether hydrogen burning has ceased in the core.
Red clump: A concentration of core-helium-burning stars on the color–magnitude diagram, especially among lower-mass stars. Red-clump stars have begun core helium fusion, unlike ordinary subgiants.
Hydrogen burning: The fusion of hydrogen nuclei into helium, which supplies energy in stars. Its cessation in the core marks the transition toward the subgiant phase.
Stellar envelope: The outer layers of a star surrounding its core. The envelope expands as the star moves toward the red-giant phase.
Stellar age: The elapsed time since a star formed, inferred from its properties and evolutionary state. Subgiants can constrain ages more precisely than many unevolved stars.
Horizontal branch: A region of the color–magnitude diagram occupied by stars burning helium in their cores. Its stars are in a later, core-helium-burning phase than subgiants.
Hydrostatic equilibrium: The balance between inward gravity and outward pressure in a stable astronomical object. Changing pressure support as the core contracts helps drive subgiant expansion.
Luminosity class: A spectral classification describing a star’s luminosity and surface-gravity-related properties. Subgiants are commonly identified by luminosity class IV.

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