Wispy cloud-like structures seen above and below the plane of the Galaxy in the infrared maps of the sky produced by the Infrared Astronomical Satellite. They are thought to be caused by dust - probably micrometre-sized graphite and silicate particles - associated with relatively local neutral hydrogen clouds. The estimated temperature of the dust is 35 K. See also: infrared astronomy.

A galaxy that emits most of its energy typically more than 90 per cent in the infrared region of the spectrum. Such galaxies are thought to have unusually high rates of star formation and, for that reason, they are also described as starburst galaxies.

An orbiting infrared telescope launched by the European Space Agency on 17 November 1995. It was equipped to make observations in the waveband between 2.5 and 200 micrometres with a sensitivity much greater than that of its predecessor, the Infrared Astronomical Satellite, IRAS. The instruments on ISO were a camera, an imaging photopolarimeter and two spectrometers to cover shorter and longer wavelengths.

A star that is a source of infrared radiation. See also: infrared astronomy, Infrared Astronomical Satellite.

A telescope for astronomical observations in the infrared. See also: infrared astronomy, Infrared Astronomical Satellite.

A NASA infrared telescope located at the Mauna Kea Observatories in Hawaii, where it has been in operation since 1979 as a national facility for the USA. The main mirror is 3 metres 120 inches in diameter.

A mathematical expression describing the relative number of objects found in different ranges of mass for a cluster of stars or galaxies or any specified volume of space at the time of its formation.

The planets Mercury, Venus, Earth and Mars collectively. They are also known as the terrestrial planets.

The amount of radiative energy received from the Sun per unit area per unit time.

A region of the Hertzsprung-Russell diagram occupied by pulsating variable stars. The strip delineates combinations of luminosity and temperature corresponding to unstable states that result in regular pulsations of a star’s outer layers.

A collaborative project between France, Germany and Spain for studies in millimetre-wave astronomy. The institute operates a 30-metre 98.5-foot dish in the Sierra Nevada, Spain, and a four-dish interferometer located in France, south of Grenoble.

Acronym for International Gamma Ray Laboratory.

Brown-Twiss stellar interferometer.

A binary star system in which mass transfer between the components takes place. See also: contact binary, dwarf nova, mass transfer, Roche lobe.

Galaxies that are close enough for their mutual gravitational attraction to produce distortions of shape and structure. Most galaxies are in clusters, and gravitational or tidal interactions between pairs are not uncommon. They often result in the production of long wisps or filaments, forming bridges between them. Computer simulations have been used to demonstrate that interactions can indeed produce the distortions observed.