A type of large elliptical galaxy with a bright nucleus surrounded by an extensive envelope. D galaxies are often radio galaxies. The term comes from a classification system devised by W. W. Morgan. See also: cD galaxy.

A region of the Earth’s ionosphere at heights between about 50 and 90 kilometres 30 and 55 miles. The lower D layer, between heights of 50 and 70 kilometres, is also known as the C layer.

A close pair of strong spectral lines in the yellow region of the spectrum of sodium. The name comes from the identifying letters given by Joseph von Fraunhofer to prominent absorption lines in the solar spectrum. The wavelengths are 589.0 and 589.6 nanometres. See also: Fraunhofer lines.

A reddish type of asteroid, rare in the main belt, but found increasingly at greater distances from the Sun.

Ida.

Asteroid 1864, diameter 3.2 km, discovered by T. Gehrels in 1971. Its orbit crosses that of the Earth.

A form of Cassegrain telescope in which the profile of the primary mirror is ellipsoidal rather than the more usual paraboloid. A spherical mirror is used for the secondary. The resulting field of view is considerably smaller than that of a conventional Cassegrain telescope of the same size.

Asteroid 5335, discovered in 1991. It is in an unusual, highly elliptical orbit which ranges between 1.6 and 22 astronomical units from the Sun.

prismatic astrolabe.

A scale devised by the French astronomer André Danjon 1890-1967 to describe the relative darkness of the Moon during a total lunar eclipse. The scale runs from 0 zero, for a very dark eclipse, to 4, representing an eclipse in which the Moon is a very bright copper colour or orange.

Asteroid 41, diameter 204 km, discovered by H. Goldschmidt in 1856.

Allowing the eye a period of adjustment in the dark during which it becomes more sensitive to faint sources of light as the pupil gets larger. The time taken varies with individuals, but is likely to be at least ten minutes, and improvements may be noticed for up to an hour. Dark adaption is essential for visual astronomical observations. It is instantly destroyed by any exposure to bright lights. For that reason, dim red lights are normally used by astronomers where some lighting is essential, such as for reading charts.

Matter postulated to exist in the universe but not yet detected. The evidence for dark matter stems primarily from observations of the velocities of galaxies within galaxy clusters. The dynamical behaviour of cluster galaxies strongly suggests that the masses of clusters are about ten times greater than the mass contributed by the luminous parts of galaxies. For individual galaxies it is possible to estimate the distribution of mass within them from the way the rotational velocity varies between the centre and the edge. Such measurements for giant spiral galaxies show that there is more matter in the galaxy than can be accounted for by glowing stars and gas. The presence of dark matter is also an important element in theories that seek to account for how galaxies formed in the early universe. There are two main categories of such theories, those calling for cold dark matter, and those requiring hot dark matter. The cold dark matter would take the form of exotic elementary particles that interact only weakly with radiation and with the baryons neutrons and protons of ordinary atoms. Such material could start to form into clumps early in the history of the universe following the Big Bang, when any fluctuations in the density of neutrons and protons would be smoothed out by their interaction with the high density of radiation present. These structures could survive to some extent on a relatively small scale, creating the framework for the formation of galaxies. Clusters and superclusters of galaxies would then be built up through the action of gravity. The alternative hot dark matter theory postulates dark matter particles with large random velocities at the era in the universe when matter starts to dominate over radiation. Neutrinos would be a possible candidate if they had a small but finite mass. In this scenario, the largest-scale structures form first and then fragment into clusters and galaxies, which is in direct contrast with the prediction of the cold dark matter theory. Computer simulations suggest that the cold dark matter does not produce as much large-scale structure as is actually observed, while hot dark matter appears to produce too many voids and stringy structures. It is not yet possible to determine whether either of these theories is correct. Other evidence for dark matter comes from comparing the mass contributed by galaxies with that needed theoretically for a closed universe. The observed matter is only around 2 per cent of that required cosmologically. There are numerous candidates for the dark matter, massive galactic haloes, brown dwarfs, very low mass stars, neutrinos and WIMPs being just a few. See also: critical density, missing mass.

absorption nebula.

An establishment for the collection, evaluation, storage and dissemination of scientific information. See also: Centre de Données Astronomiques.