Star size

The size of a star refers to its radius — the distance from its center to its surface. Star size is usually measured in solar radii (R☉), where 1 solar radius equals approximately 696,000 kilometers. Depending on their type and life stage, stars can be much smaller or larger than the Sun.

Stars don’t remain the same size throughout their lives. Over time, they can expand or contract based on their mass and where they are in their evolutionary path. For example, stars like the Sun swell into red giants in their later stages, growing to dozens or even hundreds of times their original size during the main sequence.

Astronomers estimate star size in several ways. When a star’s distance, luminosity, and temperature are known, its radius can be calculated using the Stefan–Boltzmann law. For some nearby or large stars, the radius can be measured more directly through interferometry — a method that combines light from multiple telescopes to detect extremely small angular sizes.

In the Stellar Catalog, star sizes are listed in solar radii (R☉). For very small stars, such as red dwarfs or brown dwarfs, the size is sometimes given in Jupiter radii (R♃) — where 1 Jupiter radius is about 10% of the Sun’s radius.