A dwarf nova.

A photometric system introduced in the 1950s by H. L. Johnson and W. W. Morgan. It is based on the measurement of stellar magnitudes in three wide spectral bands called U ultraviolet, B blue and V visual, which are centred on wavelengths 350, 430 and 550 nanometres, respectively. Colour indices formed by computing U-B and B-V may be used to deduce some of the physical properties of individual stars or groups of stars. To extend the usefulness, more bands were added by Johnson in 1965 extending into the infrared. These are called R, I, J, H, K, L, M and N, ranging from 0.7 to 10.2 micrometres.

Abbreviation for unidentified flying object.

small Astronomy Satellite.

Abbreviation for United Kingdom Infrared Telescope.

Abbreviation for United Kingdom Schmidt Telescope.

The study of electromagnetic radiation from astronomical sources in the wavelength band 10-320 nanometres. Ultraviolet UV radiation is strongly absorbed by the Earth’s atmosphere, so all observations have to be carried out from satellites. The earliest observations were made during brief rocket flights in the 1940s and 1950s. The first satellite to make systematic ultraviolet observations was the first Orbiting Solar Observatory OSO-1 in 1962. The highly successful International Ultraviolet Explorer IUE was launched in 1978 and continued to operate until 1996. The ultraviolet is often subdivided into the extreme UV EUV, 10-100 nm, the far UV FUV, 100-200 nm and the near UV NUV, 200-320 nm. The most extreme UV, at the transition to X-radiation in the approximate waveband 6-60 nm, is also known as the XUV. At these wavelengths, the techniques of X-ray astronomy are required, but the rest of the UV band can be observed and analysed by methods similar to those used in the visible part of the spectrum. The main difficulty is the limited range of transparent materials and reflective coatings suitable for use in the UV. Glass, for example, is strongly absorbent, and quartz or fluorite have to be used. In a UV telescope carried in the Astro-1 observatory on board the Space Shuttle, the reflection problem was tackled by the use of the rare metal iridium, which is effective down to wavelengths of 40 nm. Ultraviolet astronomy is important because many of the spectral lines most valuable for analysis, of both atoms and molecules, lie in this waveband. Hotter stars, with surface temperatures in excess of 10,000 K, emit most of their energy in the UV. Even for cooler stars such as the Sun, UV studies are needed for energetic phenomena. The interstellar medium is another important object of study for ultraviolet astronomy though, at wavelengths below 91.2 nm, almost all the UV radiation is absorbed by hydrogen, the most widely distributed element in the universe, making the detection of distant sources difficult at such short wavelengths.

A European Space Agency mission, launched on 6 October 1990, to study the interplanetary medium and the solar wind at different solar latitudes. It provided the first opportunity for measurements to be made over the poles of the Sun. Its trajectory used the gravity assist technique to take it out of the plane of the solar system. After an encounter with Jupiter in February 1992, the spacecraft swung back towards the Sun to pass over the solar south pole in 1994 and the north pole in 1995 and again in 2000-01 it explored both the northern and southern solar polar regions, which gave many unexpected results. In particular the southern magnetic pole was found to be much more dynamic and without any fixed clear location. It is, of course, wrong to say that the Sun has no magnetic south pole. The Sun is not a magnetic monopole, the pole is merely more diffusely located than the north pole. Ulysses approached aphelion in 2003/2004 and made further distant observations of Jupiter. On May 1, 1996, the spacecraft unexpectedly crossed the ion tail of Comet Hyakutake C/1996 B2, revealing the tail to be at least 3.8 AU in length. Jones, et al., 2000 Ulysses’ mission has been extended until at least March 2008, enabling it to continue operating while flying over the Sun’s poles for the third time in 2007 and 2008. At some point, the craft’s RTG will lose too much wattage. Not enough power will be on hand to operate science instruments, while keeping the hydrazine fuel from freezing.

An area of total shadow, such as the zone on the surface of the Earth from which totality is observed during a solar eclipse.

The dark central region of a sunspot, where the magnetic field is vertical and typically has a strength a few thousand times that at the surface of the Earth. The temperature is about 3,500 K, compared with 6,000 K for the surrounding photosphere. See also: penumbra.

A satellite of Uranus, discovered by W. Lassell in 1851. Images from the Voyager 2 encounter in 1986 show that Umbriel is much darker than the other four major satellites of Uranus. It appears that the surface has been covered by dark material relatively recently in astronomical terms. It is also covered with craters; one of them, 110 kilometres in diameter, is very bright, in marked contrast to the rest of the surface.

A term for dune-like features on Mars.

Asteroid 92, diameter 194 km, discovered in 1867 by C. H. F. Peters.

English name for the constellation Monoceros.

Any phenomenon in the sky for which the observer does not have a ready rational explanation. The term is often used in connection with hypothetical manifestations of unnatural objects from space.