stellar diameterThe diameters of stars range from several hundreds of millions of kilometers for supergiants through about one million km for the Sun down to a few thousand km for white dwarfs and about ten km for neutron stars. For convenience the diameter or radius is usually expressed in terms of the Sun's radius, RO. For main-sequence stars, radii vary from 0.1 to over 10 RO. The size of a star changes drastically toward the end of its life as it progresses through the red giant stage to its final gravitational collapse. It can also change periodically (see pulsating variables).
In most cases stars are too distant for their size to be determined directly. The diameter of a large star at a known distance can however be found geometrically if its apparent diameter can be measured. Measuring techniques are based on the interference of light. The apparent diameters of bright supergiants and red giants were first measured in the 1920s (see Michelson stellar interferometer). Very much smaller apparent diameters can now be measured with optical interferometers. A telescope cannot resolve the disk of a star; the disk of supergiants can however be imaged and measured using speckle interferometry. Occultations of stars by the Moon provide a further interference method: the star does not disappear instantaneously but fades over a few seconds; the unobstructed light produces a characteristic interference pattern of intensity against time from which angular size can be determined.
The sizes of many eclipsing binaries have been found from the shape of their light curves. In these measurements the sizes are obtained in terms of stellar radius relative to orbital radius. If the binary is also a spectroscopic binary the absolute values of the radii can be found. For the majority of (smaller) stars diameters can be inferred only from Stefan's law, i.e. from known values of effective temperature, T eff, and luminosity, L :