Considering that the range of stellar masses is quite narrow, stars vary to a surprising extent in size. Near the small end of the scale are white dwarfs with only about one hundredth the radius of the Sun. More compact still, neutron stars may measure only about 24 kilometers (15 miles) across. In contrast, the largest red supergiants may have a radius of about 1,000 solar radii, so that if they were put in place of the Sun they would engulf the orbit of Mars and possibly even that of Jupiter.

The sizes of stars may be deduced theoretically or, in the case of a few relatively nearby supergiants, such as Betelgeuse and Antares, measured by the technique of speckle interferometry. Stellar radii can also be obtained, and with greater precision, by analyzing the light curves of eclipsing binaries, provided that the lengths of their orbits are known.