One of the two stars of the Plough in Ursa Major with Merak called the Pointers. It is a giant K star of magnitude 1.8 with a fifth magnitude companion that orbits it in 44 years. Dubhe, literally the bear, is a shortened version of an Arabic name meaning the back of the greater bear.

A large planetary nebula, a quarter of a degree in diameter, in the constellation Vulpecula. It was discovered by Charles Messier in 1764. The name is descriptive of its hourglass shape.

Now part of the School of Cosmic Physics of the Dublin Institute for Advanced Studies, Eire. It was founded in 1783 as the university astronomical and meteorological observatory and is located at Castleknock, County Dublin. The only remaining instrument at the historical site is a 30-centimetre 12-inch refractor dating from 1868, which is maintained for demonstrations to the public.

small particles of matter, typically of order 10-100 nanometres in diameter, which co-exist with atoms and molecules of gas in interstellar space. The dust grains are thought to consist mainly of silicates and/or carbon in the form of graphite. They form in the extended atmospheres of red giant stars. Dark clouds of dust are evident when they obscure the light from stars and luminous gas clouds, as happens in the plane of the Milky Way. Though tenuous, such clouds are very effective at absorbing visible light, but radiation at millimetre and longer wavelengths can pass through the dust clouds unimpeded. The presence of dust is also revealed by the emission of infrared radiation, generated when the grains are warmed by the absorption of visible and ultraviolet radiation. The temperature of dust is typically in the range 30-500 K. Dust grains are thought to play an important role in the formation of interstellar molecules, acting as host surfaces on which gas atoms can combine. The molecules so formed can then leave the surfaces of the grains. Dust clouds are also an important constituent of star-forming regions. The dust appears to shield interstellar molecules from the destructive effect of high-energy radiation and provides protostars with an efficient means of radiating away surplus energy.

A dark streak of obscuring dust observed against the bright background of the Milky Way, or in other galaxies. See also: dust grains.

One of the two types of tail comets develop as they approach the Sun. The dust tail is composed of particles about one micrometre in size that shine by reflected sunlight. Dust tails can be as much as ten million kilometres long. They curve away from the Sun under the influence of solar radiation pressure.

Delta Scuti star.

A small elliptical or spheroidal galaxy containing between a few hundred thousand and a few million stars - far fewer than a typical galaxy. Dwarf galaxies have low luminosity and may be so loose as to resemble large open clusters of stars. Six dwarf spheroidal galaxies are known that are satellites of the Milky Way Galaxy. Their diameters range between 2 and 7 per cent of the size of the Milky Way. Dwarf ellipticals are referred to as dE galaxies.

A type of cataclysmic variable star that exhibits sudden increases in brightness at intervals ranging from several days to a year. The dwarf novae are binary systems consisting of a main-sequence star and a white dwarf. Mass transfer takes place from the main-sequence star to the white dwarf via an accretion disc that builds up around it. Outbursts take place when hot regions form on the accretion disc. Dwarf novae are also known as U Geminorum stars. Several subtypes have been identified. SS Cygni stars increase in brightness by between 2 and 6 magnitudes in an outburst lasting several days. SU Ursae Majoris stars have occasional supermaxima, two magnitudes brighter and five times longer than normal. Z Camelopardalis stars sometimes undergo periods when the eruptions are suspended for weeks or years.

The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines a "dwarf planet" as a celestial body that, within the Solar System.

(a) is in orbit around the Sun;
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape;
(c) has not cleared the neighbourhood around its orbit; and
(d) is not a satellite

The term "dwarf planet" applies only to objects in the Solar System. and is quite distinct from "planet" and "small solar system body".

The IAU's position on dwarf planets within our solar system was formally adopted at the 2006 IAU general assembly. It differs from the definition of "planet" in that a dwarf planet has not cleared its orbital neighbourhood. Before the adoption of the 2006 resolution, there was no formally specified scientific definition of "dwarf planet".

Under the IAU's definition, our solar system is currently considered to have three dwarf planets: Ceres, Pluto and Eris. However, many more objects could be added to the list once it has been determined whether criterion (b) is fulfilled.

A star that is of normal size for its mass. The term is often used to describe any star on the main sequence in the Hertzsprung-Russell diagram, that is, any star producing energy by converting hydrogen to helium in its core. The name arises from the natural comparison with giant and supergiant stars, which have greatly increased in size as a consequence of the evolutionary process. However, the largest main-sequence stars may be approaching the size of some red giants, so the distinction can become blurred. See also: white dwarf, stellar evolution.

A nearby barred spiral galaxy discovered at the Dwingeloo Observatory in 1994 through its radio emission. It had been overlooked in previous optical images because it lies in the plane of the Milky Way, where it is obscured by dust and gas. Its estimated distance is 10 million light years.

A radio astronomy observatory in the Netherlands, established in 1956, and the administrative headquarters of the Netherlands Foundation for Research in Astronomy NFRA. It also hosts the Joint Institute for VLBI in Europe. The 25-metre 82-foot Dwingeloo radio telescope is owned and operated by the NFRA. See also: VLBI, Westerbork Observatory.

Formally defined as the ascending node of the Earth’s mean orbit on the Earth’s equator, which is equivalent to the intersection of the ecliptic with the celestial equator where the Sun’s declination changes from south to north. See also: equinox, catalogue equinox.

An imaginary object introduced to assist in the defining of mean solar time. It is in effect a point that moves uniformly around the sky along the ecliptic, coinciding with the actual position of the Sun at its perihelion passage time.