Stellar radius

Stellar radius is the distance from a star’s center to its visible photosphere. It describes the star’s size and can vary with the measurement method and the star’s changing state.

Stefan–Boltzmann law: A law relating a blackbody’s emitted power per unit area to the fourth power of its temperature. Together with luminosity and effective temperature, it yields a star’s radius.
Photosphere: The visible outer layer of a star from which most of its observed light escapes. This boundary defines the radius used for ordinary stellar measurements.
Hertzsprung–Russell diagram: A plot of stars’ luminosities against their surface temperatures or spectral types. Radius helps explain why stars occupy different regions of the diagram.
Stellar diameter: The distance across a star through its center, equal to twice its radius for a sphere. Observations often measure angular diameter, while catalogs report radius.
Effective temperature: The temperature of a blackbody emitting the same total flux as a star or other radiating object. It is the temperature term used with luminosity to infer stellar radius.
Optical depth: A measure of how strongly a medium absorbs or scatters radiation along a path. A photospheric radius is often assigned near optical depth one.
Stellar evolution: The changes in a star’s structure and observable properties over its lifetime. Expansion and contraction make radius a direct tracer of evolutionary change.
Effective radius: A characteristic radius assigned to an object whose boundary or shape is not sharply defined. Extended stellar atmospheres can make the adopted radius depend on a stated convention.
Stellar luminosity: The total energy a star radiates per unit time across all wavelengths. Its value, paired with effective temperature, constrains the star’s size.
Solar radius: A standard unit of length equal to the Sun’s radius. Stellar sizes are commonly reported relative to this reference.

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