Abbreviation for charge-coupled device. A CCD is an electronic imaging device widely used in astronomical applications. The CCD consists of semiconducting silicon; when photons of light fall on it, free electrons are released. To preserve the pattern of light falling on the CCD, the photons are collected in a matrix of small picture elements pixels, which are defined by means of an array of electrodes, called gates, formed on the surface of the CCD. The electric charge in each pixel is then transferred to the ends of the rows by systematically changing the voltages on each gate so that the charge passes along as if on a conveyor belt. Finally, the electrons in each little packet of charge from individual pixels are counted and converted into a form in which the whole image can be stored in a computer or displayed on a television screen. CCDs are particularly useful in astronomy because they have all the qualities required in an astronomical detector: a highly efficient response to light so they can be used to pick up very faint objects, sensitivity over a broad spectrum range, low noise levels and a large dynamic range - that is, they can detect bright and faint objects simultaneously. Furthermore, the output is linear, - the number of electrons collected is in direct proportion to the number of photons received. This means that the image brightness is a direct measure of the real brightness of the object, a property not shared, for example, by photographic emulsions.

Abbreviation for Córdoba Durchmusterung.

A member of a class of giant elliptical galaxies with an extended halo of stars. The designation was introduced by W. Morgan in the late 1950s. A cD galaxy is often found to be the central galaxy in a rich cluster, and many are also radio emitters. They are five to ten times more luminous than typical elliptical galaxies and their mass may be as high as 1013 solar masses. A classic example is NGC 6616, which appears to have multiple nuclei within its envelope and is thought to be swallowing smaller galaxies in its vicinity.

Abbreviation for Centre de Données Astronomiques.

Abbreviation for cold dark matter. See also: dark matter.

Abbreviation for Centre de Données Stellaires, the former name of the Centre de Données Astronomiques.

Any system of coordinates that can be used to describe the position of an object on the celestial sphere. Different types of coordinates are used for different applications in astronomy. Those commonly used are equatorial coordinates, horizontal coordinates, ecliptic coordinates and galactic coordinates.

The great circle on the celestial sphere marking the boundary between the northern and southern hemispheres, and acting as the zero-mark for declination. It is the projection into space of the Earth’s equatorial plane.

A general term for the branches of astronomy dealing with the movements and consequent positions of astronomical objects, particularly the determination of orbits.

The two points on the celestial sphere about which the sky appears to rotate daily. Their positions are the directions in space towards which the Earth’s rotation axis points. The north celestial pole currently lies close to the star Polaris and the south pole is in the constellation Octans, unmarked by any bright star. Because of the effects of precession, the positions of the poles are not stationary but sweep out circles with radii of about 23° over a period of 25,800 years.

The sky considered as the inside of a hollow sphere in order to describe the positions and motions of astronomical objects. Any particular observer is located at the centre of his own celestial sphere. It is easy to imagine the sky as a hemispherical dome. Half the sky is always hidden from an observer on the Earth’s surface; which half of the celestial sphere can be seen by an observer depends on his latitude on the Earth and on the date and time. Measurements on the celestial sphere are made in angular measure degrees and do not depend on how far away the objects actually are.

English name for the constellation Centaurus.

A class of asteroids with orbits in the outer part of the solar system. Examples are Chiron and Pholus. Their orbits are within Neptune’s but they approach no closer to the Sun than Jupiter. Their orbits are unstable, and easily perturbed when they pass close to one of the giant planets.

A large southern constellation, lying in the Milky Way and very rich in stars. It is one of the ancient constellations recorded by Ptolemy c. AD 140. It contains a number of interesting objects, including the nearest star to the solar system, Proxima Centauri, and the finest and brightest of all globular star clusters, Omega Centauri.

A radio galaxy, identified with an elliptical galaxy, NGC 5128. At a distance of 15 million light years, it is the nearest radio galaxy and consequently one of the most studied. The visible galaxy is crossed by a thick dark lane of dust. The radio-emitting lobes jut out at right angles to the dust lane, extending across 7° of sky, equivalent to almost two million light years. It is also a strong source of X-rays.