The medium between the planets in the solar system composed of interplanetary dust, electrically charged particles from the Sun and neutral gas from the interstellar medium. The charged particles consist of electrons, protons and helium nuclei alpha particles streaming outwards from the Sun and forming the solar wind. Atoms of neutral hydrogen and helium gas are also present, replenished from the interstellar medium in the Sun’s vicinity. Under the influence of solar ionizing radiation, these atoms have a lifetime against ionization of only twenty days at the Earth’s distance from the Sun.

Fluctuations in the signal received from a distant radio source observed along a line of sight close to the Sun. The scintillation is caused by irregularities in the solar wind.

small particles in the interstellar medium. Interstellar dust particles range in size between 0.005 and 1 micrometre and are generally mixed in with gas in the interstellar medium. Though accounting for less than one per cent of the mass of the typical interstellar medium, the dust absorbs far more light and emits far more infrared radiation than the gas. It causes both interstellar extinction and interstellar reddening. Starlight scattered from dust particles creates reflection nebulae. The absorption of energy from starlight by dust raises its temperature to a few tens of degrees above absolute zero. At such temperatures, the dust emits thermal radiation peaking in intensity in the infrared. Dust heated to temperatures higher than about 1,500 K is destroyed. It is unlikely that all interstellar dust is composed of the same material. Graphite a common form of carbon and silicates of iron, aluminium, calcium and magnesium are thought to be among the commonest, though the broad spectral features produced by the dust are difficult to identify with certainty. Polarization effects suggest that at least some of the particles are not spherical. Most of the dust is thought to originate in the outflow of material from cool red giant stars. As the gas cools with increasing distance from the star, solid materials condense out. Infrared emission detected from such stars shows that they are indeed surrounded by dust shells. Grains of material may also condense within molecular clouds.

The dimming of light from distant stars by absorption and scattering by interstellar dust. The effect decreases with increasing wavelength. Extinction is less effective for red light than for blue, resulting in the phenomenon of interstellar reddening. The blue light from a star near the centre of the Galaxy is reduced in brightness by 25 magnitudes by the interstellar material along our line of sight. At infrared and radio wavelengths, which are longer than those of visible light, the interstellar medium is increasingly transparent. In the ultraviolet, extinction continues to increase towards shorter wavelengths; it has been studied down to 100 nanometres.

The diffuse material in the space within galaxies between individual stars, which are typically separated by several light years. In our own Galaxy, the mass of material in the interstellar medium is estimated to be at least one-tenth that in the stars. It is concentrated in the central region and in four spiral arms. In general, spiral galaxies have substantial amounts of interstellar material and elliptical galaxies little or none. There is a continuous interaction between stars and the interstellar medium, which is not uniform but consists of a number of diverse components: dark clouds of gas and dust, regions of ionized hydrogen and neutral hydrogen, molecular clouds, globules, a very hot dilute gas and high-energy cosmic ray particles. Interstellar clouds are the sites of star formation but are also enriched by material ejected by supernovae and other processes of mass loss from stars. Over distance scales of thousands of light years, the structure of the interstellar medium is probably dominated by the coalescing of supernova remnants. The thick shells surrounding them ultimately cool and condense into small clouds. Such clouds can interact, either coalescing or fragmenting on collision. See also: astration. Local Bubble.

Molecules present in the interstellar medium, especially in molecular clouds. They can survive only when shielded from the destructive effect of ultraviolet radiation from stars and so are found in dense interstellar or circumstellar clouds. Prior to 1963, CH methylidyne, CH+ and CN cyanogen were the only interstellar molecules known, having spectra in the visible range. Radio emission at 18 centimetres wavelength was recognized as being from hydroxyl OH in 1963. Over 90 different molecules have been identified since 1968, primarily by means of their characteristic spectra at millimetre wavelengths. Most are simple organic molecules.

The apparent reddening of light from distant stars by interstellar dust, which causes scattering. The extent to which light is scattered and absorbed in the interstellar medium depends strongly on wavelength: blue light is dimmed more than red light. As a result, the colours of stars viewed through interstellar material are altered and appear redder. The degree of reddening increases with the quantity of intervening matter. A similar effect in the Earth’s atmosphere causes the reddening of the Sun when it is close to the horizon.

The material between the galaxies in a cluster of galaxies. The ICM contains several components. The presence of tenuous hot gas is revealed through its X-ray emission. It typically consists of only one atom in 1,000 cm3 but accounts for an estimated 10 per cent of the total cluster mass. Diffuse radio emission from clusters is probably synchrotron radiation produced by high-energy particles in the ICM. Stars that have been torn from galaxies by the gravitational interactions between them are also found in intergalactic space. These observed components, and the galaxies themselves, still account for only 20 per cent of the mass of a typical cluster judged by applying gravitational theory to the motion of member galaxies. Though some of this unseen mass is almost certainly associated with galaxies, the behaviour of clusters suggests that there is also unseen matter in the ICM, but its nature is unknown. See also: dark matter.

The plane that includes the centre of mass of the solar system and is at a right angle to the angular momentum vector i.e. the spin axis of the solar system. This plane is the fundamental reference plane in computations of the dynamics of the solar system.

A collision between a photon and a high-energy electron in which some of the electron’s energy is transferred to the photon. See also: Compton effect.

A relationship between two physical quantities in which one declines in proportion to the reciprocal of the square of the other. Gravity is an example where the inverse square law operates. Mathematically, the attractive force F between two masses m and M can be expressed as F = GmM/r2 where r is the distance between the masses and G is Newton’s gravitational constant.

One of the four Galilean moons of Jupiter number I, the nearest to the planet and arguably the most remarkable. Its surface is brightly coloured - much of it a greenish yellow dappled with patches of orange and white. Eight active eruptive centres were identified in the images returned during the encounter of the Voyager 1 spacecraft, six of which were still active when Voyager 2 flew by four months later. Continuous monitoring from ground-based observatories, and comparisons between Voyager and Galileo images confirm the high level of continuous eruptive activity on Io. The eruptive centres appear as dark spots. Many are surrounded by roughly circular haloes of ejected material, and lava flows are also visible. The coloured crust is made of sulphur and solid sulphur dioxide. No impact craters are seen; any that were formed in Io’s early history have long since been covered by erupted material. Io is the only body in the solar system, apart from the Earth, definitely observed to be volcanically active, though Triton and Enceladus show evidence of likely activity, and Venus might also be active. Io’s activity was predicted on the basis of the strong tidal effects Jupiter has on the interior of Io. The satellite is surrounded by a thin atmosphere of sulphur dioxide and a ring of electrically charged particles - a plasma torus surrounds Jupiter, enclosing the orbit of Io. Data from Galileo indicates that Io has a substantial metallic, electrically conducting core.

An atom that has gained or lost one or more electrons and so is not electrically neutral.

One of the two distinct types of tail developed by comets as they near the Sun. The ion tail, also known as the gas tail or plasma tail, consists of ionized atoms and molecules that are emitting light by the mechanism of resonance fluorescence. In this process, energy is absorbed from sunlight then re-emitted later. The ion tail lies in the plane of the comet’s orbit. It is nearly straight but curves away from the direct radial line to the Sun by a few degrees. The ion tail is blown away from the comet by the solar wind and the effects of its magnetic field. See also: disconnection event.

The process in which electrons are removed from atoms or molecules through collisions between particles or by absorbing a photon. The electrically charged particles are positive ions.