stellar diameter

stellar diameter

The 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 :

L = 4πR 2σT eff 4
where σ is Stefan's constant.

References in periodicals archive ?
2-day orbit, it comes within a stellar diameter of this large star's surface.
Michelson, who calculated a stellar diameter with the aid of the interferometer he had invented in 1880.
Among the topics are global-scale simulations of stellar convection and their observational predictions, answering fundamental questions about cataclysmic variables and classical novae, the long baseline interferometric observations of asteroids: the physical characterization of binary systems, heterodyne interferometry with a supercontinuum local oscillator, extremely precise measurements of stellar diameters and binary stars with coherent integration, and probing dust formation around evolved stars with near-infrared interferometry.
Other dynamic events that these systems can measure include grazing occultations, accurate astrometry of fast moving Near Earth Objects, double star parameters and stellar diameters from lunar occultations, refinement of star catalogue proper motions and linkage to the ICRF from long series of observations, Baily's Bead timing, and checking the digital elevation models derived from recent lunar missions.
Bond's measurements show that the two stars making up this binary system are so close together -- just one or two stellar diameters apart -- that they appear as one.