A neutrino detector located 1 km underground at the Kamioka mine in Japan. It is the successor to the earlier Kamiokande Kamioka nucleon decay experiment. The main element of the detector is a tank containing 50,000 tonnes of water. Sensors record Cerenkov radiation in the form of visible light, emitted when high-velocity charged particles travel through the water. See also: neutrino astronomy.

A concentration of clusters of galaxies. About fifty are known, containing on average twelve rich galaxy clusters, though the largest have many more. These structures are hundreds of millions of light years across.

An automated photographic plate-processing facility located at the Royal Observatory, Edinburgh. It is the more powerful successor to a previous facility, COSMOS, which ceased operation in 1993. The name COSMOS is contrived from the parameters the instrument can measure: COordinates, Sizes, Magnitudes, Orientations and Shapes.

The reference plane of a system of coordinates used for expressing the positions of relatively nearby galaxies. It passes through the Sun, the centre of our Galaxy and the centre of the Virgo Cluster of galaxies. It is almost perpendicular to the galactic plane.

A member of the class of the largest, most luminous stars known. Supergiants can be up to 500 times larger than the Sun and many thousands of times more luminous. There are supergiants of all spectral types. They are massive stars mass greater than about ten times the Sun’s in an advanced state of stellar evolution. A supergiant is likely to become a supernova.

A pattern of large-scale convection cells on the Sun. They are best detected by the horizontal motions they produce in the photosphere away from the centre of the solar disc. They are virtually invisible in integrated white light.

Theories that attempted to incorporate a theory of gravity with all other forces. These theories were found to be flawed because they treated the most basic entities as points of zero size, and have been superseded by the more successful superstring theories.

The point in the orbit of either Mercury or Venus when the planet lies on the far side of the Sun as viewed from the Earth.

The same as upper culmination.

Any of the major planets whose orbits lie outside that of the Earth - Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Motion at a velocity that apparently exceeds that of light. The angular separation of the components of some double radio sources is increasing at a rate that is apparently equivalent to as much as ten times the speed of light when the distance of the source is taken into account. Speeds in excess of that of light, however, are physically impossible, as shown by Special Relativity. In reality, the effect is a purely geometrical one caused by one component travelling almost directly towards us along the line of sight at a velocity nearly as great as that of light. The phenomenon has been observed in the quasar 3C 273.

A very massive star. The term has no precise definition, but the most massive stars have up to about 100 times the mass of the Sun.

supernova pl. supernovae A catastrophic stellar explosion in which so much energy is released that the supernova alone can outshine an entire galaxy of billions of stars. In addition to the radiant energy produced, ten times as much energy goes into the kinetic energy of the material blown out by the explosion, and a hundred times as much is carried off by neutrinos. A supernova explosion occurs when an evolved massive star has exhausted its nuclear fuel. Under these circumstances, the core becomes unstable against collapse. Two distinct kinds of supernova are recognized, known as Type I and Type II. They are distinguished by the presence of hydrogen features in the spectrum of Type II supernovae which are absent from Type I. The light curves of Type I supernovae are all very similar: the luminosity increases steadily for about three weeks then declines systematically over six months or longer. The light curves of Type II supernovae are more varied. Type I supernovae are subdivided into Types Ia and Ib, according to thestrength of a particular silicon absorption line in the optical spectrum. The line is strong in Ia and weak in Ib. Type Ia supernovae are thought to be white dwarfs in binary systems, where mass transfer from the companion takes place. A wave of carbon burning through the newly acquired material could account for the energy released. The explosion may represent the total disintegration of the white dwarf. The nuclear reactions create about one solar mass of the unstable isotope 56Ni, which decays to 56Co and finally 56Fe over a period of months. This radioactive decay would take place at a rate consistent with the observed decline in light output. The difference in mechanism between Types Ia and Ib is not yet clear. Type II supernovae appear to be stars of eight solar masses or more that have run the course of stellar evolution and totally exhausted the nuclear fuel available in their cores. At this stage their structure is like that of an onion, consisting of concentric spherical shells in which different nuclear reactions are taking place. Once silicon burning starts in the central core, instability develops within a day because the iron created cannot fuse into heavier elements without an input of energy. In the absence of energy generation, the pressure balancing the weight of the overlying layers is removed. When the crunch comes, the core collapses in less than a second. The rate accelerates as iron nuclei break up and neutrons form. However, implosion cannot continue indefinitely. When the density of nuclear matter is reached, there is a sudden strong resistance to further pressure, the imploding material bounces back and an outward shock wave is generated. The outer layers of the star are blown outwards at thousands of kilometres per second, leaving the core exposed as a neutron star. The material ejected in the explosion forms an expanding supernova remnant. The neutron stars can be detected as pulsars through their radio emission and, in some cases, by pulsed light and X-ray emission as well. The explosion of supernovae serves to enrich the chemical composition of the interstellar medium from which subsequent generations of stars are created. Very old stars contain much lower quantities of the elements heavier than hydrogen and helium than are found in the Sun and solar system and many of these heavier elements can be created naturally only in the explosion of a supernova. Supernovae are fairly rare events: only five have been observed visually in our own Galaxy in the last thousand years. Others have taken place, and radio emission from their remnants has been detected, but the outbursts were concealed behind obscuring dust. However, Supernova 1987A in the nearby Large Magellanic Cloud provided an opportunity unprecedented in modern times, enabling astronomers to study a supernova at relatively close hand. About fifty supernovae are detected each year in galaxies beyond our own. See also: Crab Pulsar.

A supernova in the Large Magellanic Cloud discovered on 24 February 1987 when it was about sixth magnitude. It was the nearest and brightest supernova observed since 1604. The star that exploded was identified as a twelfth magnitude blue supergiant, known as Sanduleak -69° 202. Maximum magnitude, reached in mid-May, was near 2.8.

The expanding shell of material created by the ejection of the outer layers of a star that explodes as a supernova. Some supernova remnants are observable visually; others have been detected through their radio and X-ray emission. A shock wave precedes the ejected shell, colliding with and heating the interstellar gas. A reverse shock, moving inwards, is created, which heats the ejected material and the interstellar material, causing it to emit X-rays. Electrons accelerated by the shocks emit radio waves by the synchrotron radiation mechanism. The ejected material breaks up into clumps, so the radiation emitted from the shell often does not make up a uniform ring. A small proportion of supernova remnants, including the Crab Nebula, have a rather different appearance. In these, the synchrotron radiation coming from within the shell far outshines any from the shell itself. This type of supernova remnant has been termed a plerion. A continuing supply of electrons travelling at relativistic speeds is needed to account for the emission. In the Crab Nebula, the known pulsar can produce the electrons, but for plerions where no pulsar has been detected, it is assumed that we are observing at the wrong angle to pick up the pulses from the central pulsar. Some other well-known examples of supernova remnants are Cassiopeia A, Kepler’s Star, Tycho’s Star and the Cygnus Loop.