ultraviolet starsHot stars that have considerable emission at ultraviolet and XUV wavelengths, i.e. between about 12–320 nm; these include early-type O and B stars, Wolf-Rayet stars, subdwarf O and B stars, hot white dwarfs, and the central stars of most planetary nebulae. For an O star with a surface temperature of about 30 000 K, over 80% of its radiation lies in the ultraviolet. In comparison the Sun, with a surface temperature of about 5800 K, emits most of its radiation in the visible region of the spectrum (380–750 nm). Space observations have allowed the UV energy distribution of such stars to be measured, providing a more precise determination of their total radiated power. In addition emission and absorption line spectra have given information on the structure and element abundances in the stellar atmospheres, and have shown for both luminous and subluminous hot stars the presence of strong stellar winds. In many cases such winds involve a rate of mass loss that will substantially affect the evolution of the star. Extensive regions of ionized interstellar gas – the H II regions – surround many such hot luminous stars. See also ultraviolet astronomy.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006