= Stellar radius
{title2=$R_*$}
A <stellar radius> is the <radius> assigned to a <star> using a stated surface convention. A gaseous <star> has no sharp solid boundary: in a model, a commonly used reference is a layer near outward <optical depth> of order one. Different spectral bands can sample different layers, particularly in very extended stellar atmospheres.
For an approximately spherical <star>, a measured angular diameter $\theta$ and distance $d$ give $R_\star\simeq d\theta/2$ for small $\theta$ in radians. Alternatively, <luminosity> and <effective temperature> give $L=4\pi R_\star^2\sigma T_{\rm eff}^4$ through the <Stefan–Boltzmann law>. <Eclipsing binaries> offer another route through orbital geometry. This measurement matters directly for <exoplanet transit photometry>, whose <exoplanet transit depth> primarily supplies the planet-to-star <radius> ratio.
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