association.

A type of very young star in an early phase of evolution where contraction is still taking place. The prototype, T Tauri, is an irregular variable within a dark dust cloud in the constellation Taurus. All T Tauri stars vary irregularly. Their absorption line spectra show that their surface temperatures are in the range 3,500-7,000 K. They are found in dense interstellar clouds, usually alongside young, main-sequence O and B stars, but the T Tauri stars are far more luminous than main-sequence stars of the same temperature. Strong emission lines also feature in the spectrum; these come from a low-density envelope of gas around the stars. Large numbers of T Tauri stars have been discovered, notably in the Rho Ophiuchi r Oph dust cloud, through the strong infrared radiation they emit. Loose groupings of T Tauri stars are known as T associations. Strong bipolar outflows twin-lobed jets stream out from T Tauri stars at speeds of several hundred kilometres per second. Where the outflow compresses and heats the interstellar gas, the resulting luminous nebulae are observed as Herbig-Haro objects.

A type of asteroid characterized by a fairly low albedo.

English name for the constellation Mensa.

A form of iron-nickel alloy found in iron meteorites. It contains up to 7.5 per cent of nickel by weight. See also: Widmanstätten figures, octahedrite.

Abbreviation for International Atomic Time. The order of the initials is that of the term in French.

The extended part of a comet that grows from the head when the comet is in the vicinity of the Sun. See also: ion tail, dust tail.

A large region of ionized hydrogen, 900 light years across, in the Large Magellanic Cloud.

A star of the same type as the Sun, lying at a distance of 11.7 light years. It is the seventeenth-nearest star known, but, at magnitude 3.5, one of only a handful of nearby stars visible to the naked eye.

An annual meteor shower, of relatively low activity, the twin radiants of which lie in the constellation Taurus. The peak occurs around 3 November. The meteoroid stream responsible is associated with Comet Encke.

A conspicuous zodiacal constellation, supposedly representing the head and forequarters of a bull. It was listed by Ptolemy c. AD 140 and is possibly one of the most ancient of constellations. The brightest star is the first magnitude Aldebaran, which appears to belong the Hyades cluster, though it is in fact in the foreground. In total, there are fourteen stars brighter than fourth magnitude. The Pleiades cluster and the Crab Nebula also lie within the boundaries of Taurus.

The radio source associated with the Crab Nebula.

The Apollo 17 landing site on the Moon, located on the south-east border of Mare Serenitatis, in the region of the crater Littrow. The valley is completely surrounded by mountains, some more than 2,000 metres 6,500 feet high. The centre of the lava-flooded valley was chosen for the manned landing to allow exploration of both the North and South massifs enclosing the area.

The location in Germany of the Karl Schwarzschild Observatory.

One of the brighter stars in the Pleiades.