A face-on spiral galaxy in the constellation Canes Venatici at a distance of 13 million light years. It was the first galaxy to be recognized as having spiral structure when it was observed by Lord Rosse in 1845. It is accompanied by a much smaller irregular galaxy, NGC 5195, which is in orbit around it.

A star in an advanced state of stellar evolution, composed of degenerate matter. A white dwarf is created when a star finally exhausts its possible sources of fuel for thermonuclear fusion. The star collapses under its own gravity, compressing the matter to a degenerate state in which atomic nuclei and electrons that have been completely stripped from atoms are all packed together. The process stops when a quantum mechanical effect, the exclusion principle, comes into play; the electrons cannot be compacted further and so exert a resistance called degeneracy pressure. S Chandrasekhar demonstrated theoretically that the upper mass limit for white dwarfs is 1.4 times the mass of the Sun - larger collapsing masses must become neutron stars or black holes. The first white dwarf to be recognized as such was 40 Eridani B, observed in 1910. It was shown to have a surface temperature of 17,000 K but a total luminosity so low that it could be explained only if the star were smaller than the Earth. Other well-known white dwarfs include van Maanen’s star and Sirius B. Sirius B, first seen in 1862, has a mass about the same as the Sun’s in a diameter only five times the Earth’s, but is 10,000 times fainter than Sirius A, which is a normal A star. A few hundred white dwarfs are known but they may represent as much as 10 per cent of the stellar population. Their low intrinsic luminosity makes them difficult to detect. Though called white dwarfs as a group, these degenerate stars actually cover a range of temperatures and colours, from the hottest, which are white and have surface temperatures as high as 100,000 K, to cool red objects at only 4,000 K. Since they have no internal source of energy, white dwarfs are in a long process of gradually cooling off, during which the temperature declines. Their ultimate fate is to become a black dwarf - a non-luminous dead star. The spectra of white dwarfs are bewilderingly complex, reflecting a range of temperature and composition. Typically, the spectrum contains very broad absorption lines, though some show no lines at all. The line-forming region is only a few hundred metres thick. Some white dwarfs show only hydrogen lines, presumably because the helium and heavier elements have sunk to the bottom of the atmosphere under the strong gravitational force. In other stars, helium or metals are seen and no hydrogen remains. A new classification scheme for white dwarfs was proposed in 1983 by E. M. Sion and collaborators. The designations consist of three capital letters, the first being D for degenerate. The second letter indicates the primary spectrum: A, H only; B, neutral He with no H or metals; C, continuous; O, ionized He with neutral He or H; Z, metal lines only with no H or He; Q, carbon present. The third letter is for secondary spectral characteristics: P, magnetic with polarized light; H, magnetic without polarized light; X, peculiar or unclassifiable; V, variable. The old system was based on the usual spectral class sequence O, B, A, F, G, K, M prefixed by D.

A hypothetical object, never observed, which mathematically has properties inverse to those of a black hole and would be a place where matter spontaneously appears.

A characteristic geometrical pattern revealed when certain types of metallic meteorite are cut, polished and etched with dilute acid. They arise from the crystalline intergrowth of two different forms of iron-nickel alloy, kamacite and taenite. See also: octahedrite.

An open cluster of about 200 stars in the constellation Scutum. Its name comes from its shape, which is fan-like in small instruments and resembles somewhat a flight of wild ducks.

A 4.2-metre 160-inch altazimuth reflecting telescope in the Isaac Newton Group at the Observatorio del Roque de los Muchachos, La Palma, Canary Islands. It is operated by the Royal Greenwich Observatory. Observing time is shared between the collaborating countries - the UK, Spain and the Netherlands. It is a general-purpose telescope, equipped with a large range of instruments, and came into operation in 1987.

A design of reflecting optical telescope yielding good images over a field of view of 5° or more. It is a modified version of the Paul-Baker system. The primary mirror contains a hole, the size of which is 60 per cent of the total diameter and in which lies the focal plane. The shapes of all three mirrors depart substantially from the nearest paraboloid or sphere. The design’s advantages are that the telescope is much more compact than a Schmidt camera, there are no ghost images of the type caused by internal reflections in the corrector lens of a Schmidt camera, and it should be possible to build a telescope to this design larger than any existing Schmidt camera.

A change in the appearance of a sunspot as it is carried close to the Sun’s limb by rotation, in which the penumbra nearest the limb appears wider than that on the other side of the spot. This is because the sunspot is in a depression. The phenomenon was first observed by the Scottish astronomer, Alexander Wilson 1714-86, in 1769.

Acronym for electrically neutral weakly interacting massive particles. These have not yet been observed in particle accelerators; if they do exist, they are important in cosmology because they would contribute to the missing mass.

Asteroid 747, diameter 204 km, discovered in 1913 by J. Metcalf.

English name for the constellation Pegasus.

solstice.

A 3.5-metre 138-inch telescope at Kitt Peak, opened in 1994. It is operated jointly by the University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories.

Abbreviation for Wolf-Lundmark-Melotte galaxy.

Wolf-Rayet star.