A hypothetical element, the existence of which was postulated in the nineteenth century to account for unidentified emission lines in the spectra of some luminous nebulae. It is now known that these lines are attributable to known elements, but they are not usually observable under laboratory conditions. For that reason they are described as forbidden lines by physicists.

Abbreviation for near-Earth object.

A major planet of the solar system, normally the eighth in order from the Sun. Between 1979 and 1999 the eccentric orbit of Pluto brings it temporarily closer than Neptune. It is one of the four gas giant planets, having a small rocky core surrounded by an icy mantle of frozen water, methane and ammonia. Its diameter is almost four times the Earth’s. The outer atmosphere is mainly molecular hydrogen with 15-20 per cent helium by mass and some methane. Neptune was discovered by J. G. Galle of the Berlin Observatory on 23 September 1846 following predictions made independently by John Couch Adams in England and Urbain J. J. Leverrier in France. Their calculations were based on discrepancies between the observed and predicted orbits of Uranus since its discovery in 1781, which were attributed to the gravitational perturbations of an unknown planet. Viewed from Earth, Neptune is a seventh or eighth magnitude object and so not visible to the naked eye. With high magnification and larger telescopes, it is seen as a faintly bluish disc, the colour coming from methane in the upper atmosphere. Surface features are not detectable by ground-based optical observation, though bright spots are observed in the infrared. Close-up images were obtained by Voyager 2 during its flyby of Neptune in August 1989. Observations with the Hubble Space Telescope HST, capable of resolving atmospheric detail, began in 1994. In many ways, such as size and structure, Neptune is similar to Uranus. But by contrast with Uranus, Neptune has distinctive and varying cloud features in a highly dynamic atmosphere. The most prominent feature found by Voyager 2 was termed the Great Dark Spot, and appeared to be somewhat similar in nature to Jupiter’s Great Red Spot. Located about 20° south of the equator, it rotated anticlockwise in a period of about 16 days. Bright cirrus-like clouds had formed over this and other small dark spots. However, it had completely disappeared when observations were made with the HST in 1994. Meanwhile, another dark spot, not seen by Voyager, had formed in the northern hemisphere. It too was accompanied by bright clouds. Subsequent observations with the HST revealed that the pattern of clouds was changing, though the underlying banded structure of the atmosphere remained stable. There are two main cloud layers in Neptune’s upper atmosphere. The highest consists of crystals of methane ice, and this lies over a lower opaque blanket of cloud that may contain frozen ammonia or hydrogen sulphide. There is also a high-altitude haze of hydrocarbons produced by the action of sunlight on methane. Regular radio bursts detected by Voyager 2 revealed that Neptune has a magnetic field and is surrounded by a magnetosphere. The bursts occurred at intervals of 16.11 hours, apparently the rotation period of the planetary core. The atmospheric features rotate at different rates, also moving in latitude. Wind speeds up to 2,200 km/hour were measured. The magnetic axis is tilted at 47° to the rotation axis and it is thought that the asymmetric field may originate in the mantle rather than the core. Based on the total radiated energy, the average temperature is 59 K. It is not understood why Neptune radiates 2.7 times more energy than it receives from the Sun. Observations made from the ground during occultations by Neptune had suggested the presence of incomplete ring arcs. Voyager 2 detected four tenuous rings, one of which is clumpy in a way that can account for the occultation observations. The mission also discovered six new moons around Neptune, bringing the total number known, with Triton and Nereid, to eight. One of moons found by Voyager, Proteus, is more than twice the size of Nereid with a diameter of about 400 kilometres.

The small outermost moon of Neptune, discovered by Gerard Kuiper in 1949. The best Voyager 2 image obtained was from a distance of 4.7 million kilometres 2.9 million miles, not sufficiently close to reveal surface detail but good enough to obtain a more precise diameter of 170 kilometres 105 miles.

English name for the constellation Reticulum.

Un-ionized atomic hydrogen gas. It is an important component of the interstellar medium, accounting for perhaps half its mass, even though its density - typically 50 atoms per cubic centimetre - is very low. The temperature of neutral hydrogen ranges between 25 and 250 K, which is too cold for it to emit visible radiation. However, its radio emission at a wavelength of 21 centimetres has made it possible to map the distribution of neutral hydrogen in the spiral arms of our own Galaxy and other nearby galaxies.

The attempt to detect neutrinos from cosmic sources, especially the Sun. Neutrinos are elementary particles with no electric charge and almost no mass, and they interact only very weakly with other matter. They travel essentially at the velocity of light and are produced in vast quantities by the nuclear reactions that take place in the centres of stars and in supernova explosions. Because they hardly interact with matter at all, neutrinos are very difficult to detect. The longest-running experiment to search for solar neutrinos, at Homestake Mine, South Dakota, was designed to use the fact that occasional neutrinos interact with a chlorine atom, converting it to a radioactive isotope of the gas argon. The detector consisted of a tank containing 400,000 litres of the cleaning fluid, carbon tetracholide. Such experiments need to be underground to avoid confusion from events due to cosmic rays. Theoretical considerations suggested that one interaction should be detected per day by this set-up. In practice, only one-third that number have been seen. This discrepancy is known as the neutrino problem. In another form of neutrino detector shown to work successfully, detectors in a large tank of water pick up Cerenkov radiation generated by the interaction of electrons with solar neutrinos. Detectors of this type, the Kamiokande experiment in Japan and a similar detector in Ohio, made the first observation of neutrinos from a supernova - those from SN1987A. In 1996 Kamiokande was superseded by a larger version, Super-Kamiokande. A European collaboration GALLEX and a Russian experiment have been designed to make use of the interaction of neutrinos with gallium. As results become available from new, more sensitive experiments, it is hoped to determine whether the current theories of solar physics are faulty or whether unknown physical processes are the cause of the neutrino problem.

A star with a mass between about 1.5 and 3 solar masses that has collapsed under gravity to such an extent that it consists almost entirely of neutrons. Neutron stars are only about 10 kilometres across and have a density of 10 to the power of 17 kg/m3. They are formed in supernova explosions and observed as pulsars. Once nuclear fuel is exhausted in a star, the core starts to cool and the internal pressure falls, leading to contraction. This is a sudden and catastrophic event for stars of more than 1.8 solar masses, which implode until the pressure between neutrons balances the inward pull of gravity. In the resulting supernova, much of the original mass of the star is blown off into space. A stellar remnant of three solar masses or more will collapse into a black hole rather than a neutron star.

A catalogue of non-stellar objects compiled by J. L. E. Dreyer of Armagh Observatory and published in 1888. It listed 7,840 objects. A further 1,529 were listed in a supplement that appeared seven years later, called the Index Catalogue IC. The Second Index Catalogue of 1908 extended the supplementary list to 5,386 objects. The NGC and IC numbers are widely used to identify non-stellar astronomical objects. The NGC got its title because the project was seen as a development of John Herschel’s General Catalogue of Nebulae, which had been published in 1864.

A telescope incorporating design features that take advantage of the most recent technology. Such features include computer control, a lightweight primary mirror, an altazimuth mounting, automatic or remote operation, and special attention to the thermal environment.

The Moon’s phase when it is at the same celestial longitude as the Sun and thus totally unilluminated as seen from Earth. See also: phase.

The system of date determination currently in use, introduced on 14 September 1752 in Britain and its American colonies when they adopted the Gregorian calendar. The eleven days 3-13 September 1752 were eliminated and the day on which the count of years changes was moved from 25 March to 1 January. Dates according to the Julian calendar in use previously are known as Old Style Dates.

A 3.5-metre 138-inch reflecting telescope of the European Southern Observatory, located at the La Silla Observatory in Chile. Regular observations with the telescope started in 1990. The name reflects a number of innovative features incorporated in the design. The relatively thin mirror is kept to the required shape by means of an active optics system which analyses the shape of a stellar image and controls the mirror supports about once per second. The altazimuth mount and special enclosure are designed for maximum stability and pointing accuracy, and minimum disturbance of images from air turbulence. The telescope can be operated remotely from ESO’s headquarters in Germany via satellite link.

A simple type of reflecting telescope, designed by Isaac Newton 1642-1727, who demonstrated it to the Royal Society in London in 1671. The primary mirror is a paraboloid or spherical for small apertures, and the secondary is a flat mirror positioned in the converging reflected beam at an angle of 45° to the optical axis to form an image just outside the main tube. The design is widely used for small amateur instruments, but is not suitable for large telescopes.

A project to launch a successor to the Hubble Space Telescope. It is being proposed that such a telescope should be primarily for infrared astronomy and placed in orbit around the Sun, rather than around the Earth. Such an orbit would have the advantage of minimizing stray light and temperature changes. The suggestion is that the telescope would be a 6- or 8-metre reflector, its mirror possibly constructed in segments.