Largest stars by radius: overview and notable candidates

How radii are estimated

Two principal observational methods determine a star's radius: direct angular-diameter measurements (often with optical/infrared interferometry) combined with distance, and indirect calculations using luminosity and effective temperature. Both approaches depend on accurate distances and accounting for extended atmospheres, pulsations and circumstellar dust that can bias apparent size.

Notable candidates

• Red supergiants — Many of the largest candidates are extreme red supergiants found in our Galaxy and nearby galaxies. These include well-known names often cited among the biggest, such as Mu Cephei, Betelgeuse and Antares, and more extreme, highly uncertain examples like VY Canis Majoris, UY Scuti and several members of massive clusters.
• Cluster supergiants — Stars in young massive clusters (for example, members of Stephenson 2 and similar associations) have produced some of the highest radius estimates, but those values change with refined distance measurements.
• Extragalactic examples — A few stars in the Large and Small Magellanic Clouds have been proposed as very large, but different metallicity and dust environments complicate comparisons with Milky Way objects.

Because estimates vary, many summaries present lists of largest stars as collections of candidates rather than a definitive ranking. Variables such as pulsation, mass loss and obscuring dust shells make a single value for "radius" ambiguous in evolved supergiants and hypergiants.

Distinguishing radius from mass and luminosity is important: some very luminous stars (for example luminous blue variables) are not the physically largest in radius. Conversely, the largest radii are typically found in cool, extended red supergiants whose outer layers are tenuous and unstable.

In short, discussion of the largest stars combines observational astronomy, stellar evolution theory and caution: numbers change with improved data, and lists should be treated as snapshots rather than final rankings.