An informal expression for the central energy source powering an active galactic nucleus, radio galaxy or quasar. The energy source is generally thought to be a black hole accreting matter.

The imaginary north-south line bisecting the disc of a planet or a moon or the Sun as seen by an observer.

A mountain formed in the centre of an impact crater as the crust rebounds following the impact explosion. A crater may contain several peaks grouped together.

An institute at the University of Strasbourg in France, established in 1972 as the Centre de Données Stellaires CDS, which is devoted to the collection, critical evaluation and handling of stellar and other astronomical data.

The balancing point in a system of individual masses or in a solid object through which mass is distributed. See also: barycentre.

A type of pulsating variable star, named after the group’s prototype, Delta Cephei, which varies between magnitudes 3.6 and 4.3 in a period of 5.4 days. Cepheid variables have an unstable structure that causes them to pulse in and out. Their size may change by as much as 10 per cent during a cycle and the temperature varies too. As pressure builds up inside, the star expands until the pressure is released, rather like it might be through a valve. The star then contracts and the cycle starts again. Cepheids are luminous yellow giant stars that radiate ten thousand times as much energy as the Sun, so they can be seen at very great distances. In 1912, Henrietta Leavitt, working at Harvard College Observatory, noted a number of Cepheids in the Small Magellanic Cloud and plotted their light curves. It became clear to her that there was a relationship between the periods, typically between 3 and 50 days, and the average apparent brightness: the brighter the star, the longer its period. This is called the period-luminosity relation. The importance of this discovery lies in the fact that Cepheids can be used as distance indicators. All the stars in the Small Magellanic Cloud can be considered to be at roughly the same distance certainly in relation to the distance of the SMC itself, so the apparent magnitudes will differ from the absolute magnitudes by a constant factor. Once the distance to a single Cepheid variable was found by an independent method, the distances to all others could be deduced simply from measuring their periods. Two distinct varieties of Cepheid variable have been identified: the so-called classical Cepheids and Population II Cepheids, also commonly known as W Virginis stars. Their period-luminosity relations differ: for a given period, classical Cepheids are about two magnitudes brighter than W Virginis stars. This is a result of differences in mass and chemical composition. The lower mass of W Virginis stars results in a lower luminosity, but the effect is partially offset by the low abundance of elements heavier than helium metals in the stars of the old Population II. It is essential to distinguish whether a variable star is a classical Cepheid or a W Virginis star before its distance can be deduced. This is best achieved by determining the metal content from analysis of its spectrum.

A constellation close to the north celestial pole, taking its name from the legendary King of Ethiopia, husband of Cassiopeia and father of Andromeda. It is one of the ancient constellations recorded by Ptolemy c. AD 140 but is not conspicuous, having no star as bright as second magnitude.

Asteroid 1865, diameter 1.6 km, discovered in 1971 by L. Kohoutek when it made a close approach to the Earth.

Electromagnetic radiation caused by the shock wave created when electrically charged particles move through a medium at velocities greater than that of light in that medium.

The first asteroid to be discovered, found by Giuseppi Piazzi from Palermo, Sicily, on 1 January 1801. It is by far the largest asteroid, 940 km 585 miles in diameter, and its orbit lies in the main asteroid belt at a distance of 2.77 AU from the Sun. Its mass of 1.17×10 to the power of 21 is about one-third the entire mass of the asteroid belt. It reaches a maximum magnitude of 6.9, its albedo being only 9 per cent. Ceres rotates in just over 9 hours during which time its colour and brightness vary only slightly, suggesting that it is almost spherical and uniformly grey. Information from its spectrum indicates that the surface may be similar in composition to the carbonaceous chondrite meteorites.

An observatory in Chile, forming part of the National Optical Astronomy Observatories of the USA. The headquarters are at La Serena, 480 kilometres 300 miles north of Santiago. The mountain site, 70 kilometres 45 miles inland is at an altitude of 2,200 metres 7,200 feet. The largest instrument, the 4-metre 160-inch Victor M. Blanco Telescope, is a twin of that at Kitt Peak in Arizona. Among the six other instruments located at the observatory are 1.5-metre 60-inch, 1.0-metre 39-inch and 92-centimetre 36-inch reflectors. A 1.2-metre 47-inch radio telescope of the Universidad do Chile is also at the site.

A large constellation in the region of the celestial equator, supposed to represent the sea monster that threatened Andromeda, though normally translated as the whale. It was listed by Ptolemy c. AD 140. It lies in a rather sparse area of the sky and all but one of its stars are fainter than third magnitude. The most notable star is the variable Mira.

Abbreviation for Canada-France-Hawaii Telescope.

A giant star of spectral type G or K that shows particularly strong bands of the CH molecule in its spectrum.

A small, faint southern constellation, introduced probably by sixteenth-century navigators and included by Johann Bayer in his atlas Uranometria, published in 1603. None of its stars are brighter than fourth magnitude.