A measure of the sunspot activity on the solar disc, taking into account spot groups as well as individual spots. It was devised by Rudolf Wolf of the Zürich Observatory, and is also known as the Zürich sunspot number. The value, R, is calculated from the formula R=k10g + f where g is the number of sunspot groups, f the total number of spots and k a weighting factor depending on the instruments used and the observer. The value of k is about 1 for telescopes of 100 mm aperture.

A small irregular galaxy belonging to the Local Group. It lies at a distance of 2.8 million light years.

A member of a class of rare, exceptionally hot stars with surface temperatures of 20,000-50,000 K. Their spectra show strong broad-band emission lines: in WC stars, carbon dominates, whereas in WN stars the dominant emission lines are of nitrogen. It is believed that there is a genuine difference in composition between the two subgroups. The emission lines are thought to originate in a rapidly expanding envelope through which the star is losing mass. Some are the central stars of planetary nebulae, but their evolutionary status is not fully understood. The name comes from two nineteenth-century French astronomers, Charles Wolf and Georges Rayet.

wormhole A hypothetical tunnel-like structure in the fabric of spacetime. Theorists have suggested that on the distance scale of the Planck length 10 to the power of -35 metres, spacetime might have a foam-like structure, riddled with wormholes. It might be possible for such a wormhole to pinch off and form a new universe.

A low-relief feature on the lunar surface which may extend for hundreds of kilometres across a mare. Wrinkle ridges appear to be a feature associated with volcanic activity and the flow of lava in the Moon’s distant past.

The study of X-radiation from astronomical sources. The X-ray waveband is usually considered to cover the wavelength range from about 10 to 0.01 nanometres, between the extreme ultraviolet XUV and gamma-rays. The corresponding energy range is 0.1-100 keV. No X-rays from space can penetrate the atmosphere to the ground, so all X-ray astronomy is carried out with instruments on rockets or satellites. X-rays from the Sun were detected during rocket flights in the 1950s. The first X-ray source beyond the solar system to be discovered was Scorpius X-1, found in 1962 by a group at American Science and Engineering, led by Ricardo Giacconi. By 1970 there were more than forty known X-ray sources detected during rocket-borne experiments. However, satellites were needed to conduct more extensive surveys. US military Vela satellites operating between 1969 and 1979 carried X-ray detectors. The first satellite dedicated to X-ray astronomy was Uhuru 1970, the first of the Small Astronomy Satellite series. In 1973, a telescope capable of producing X-ray images was used successfully to image the Sun during the Skylab mission. This X-ray telescope used an array of concentric, cylindrical mirrors to reflect the X-rays at grazing incidence and bring them to a focus, and detectors capable of recording the positions of arrival of the photons over a field of view. Such an imaging X-ray telescope was used for objects other than the Sun for the first time by the Einstein Observatory. In 1985, a different type of X-ray telescope, using the coded mask technique, was deployed in orbit on Spacelab 2. This is capable of forming images at higher energies and incorporates a diaphragm with a complex pattern of holes. Other important X-ray astronomy satellites have included Copernicus 1971, EXOSAT, Ginga 1987, ROSAT 1990 and BeppoSAX 1996. Thermal radiation in the X-ray band comes from sources at temperatures in excess of one million degrees. Much of the X-ray emission detected from astronomical sources is produced in non-thermal processes, such as the interaction between electrons and ions in plasmas which can produce both continuous radiation and X-ray spectral lines and nuclear reactions in interacting binary star systems. The largest class of bright X-ray sources consists of interacting binary stars in which one component is a degenerate star - a white dwarf, a neutron star or a black hole. There are two categories of such X-ray binary. In high-mass binaries, the companion is a star of 10 or 20 solar masses, and matter from its extended envelope flows directly on to the degenerate star. In low-mass binaries the two components are of similar mass, and mass transfer takes place via an accretion disc. As it gains gravitational energy, the material flowing between the stars reaches temperatures high enough for the emission of X-rays. Such binaries often show periodic variability attributable to the orbital period of the system, the rotation period of the degenerate star or the precession of the accretion disc. The X-ray luminosity ranges from 100 to 100,000 times the total luminosity of the Sun. Some systems, the X-ray bursters, show much more dramatic and random variations. The other main types of source of astronomical X-rays are the hot diffuse gas surrounding galaxies and present between the galaxies in clusters, supernova remnants and active galactic nuclei. In 1996, for the first time, X-rays were detected from several comets. See also: XMM, Yohkoh.

X-ray astronomy, X-ray burster.

A stellar X-ray source showing violent and random changes in their emission. X-ray bursters were discovered with the Dutch satellite ANS in 1976. The bursts may last for several days and may recur, but are not periodic. A rapid burster repeats at intervals no longer than 10 seconds. The numbers known are counted in tens; most are in the galactic plane though some are in globular clusters. The generally accepted model is that of an interacting binary system, similar to a nova, except that accretion takes place on to a neutron star rather than a white dwarf, and the material transferred is predominantly helium rather than hydrogen. The X-ray burst occurs when the accumulation of accreted material reaches the critical temperature and density to detonate a thermonuclear explosion. See also: X-ray astronomy.

XMM.

A stellar source of X-rays. See also: X-ray astronomy.

A project of the European Space Agency for an orbiting X-ray astronomy observatory to be launched in 1999 with a minimum lifetime of ten years. The large collecting area, and the unprecedented sensitivity and resolution of its detectors, will reduce the time required to make an observation by a factor of over 100 and will allow many more spectra to be obtained than has been possible previously. The satellite will carry three identical telescopes, each consisting of 58 nested precision reflectors.

Rossi X-ray Timing Explorer.

A term sometimes applied to the short wavelength end of the ultraviolet region of the electromagnetic spectrum in the range 6-60 nanometres, where it merges with the X-ray band. It overlaps the region also known as EUV. See also: ultraviolet astronomy.

An antenna for receiving radio waves, consisting of a small number of parallel dipoles. The commonest domestic television aerials are of this type. Arrays of Yagis are used in radio astronomy to construct cheap aperture synthesis telescopes for sky survey work. See also: radio telescope.

The effect of its rotation on the path of a small particle orbiting the Sun. Rotation causes a temperature variation, so thermal energy is re-radiated anisotropically. See also: Poynting-Robertson effect.