Abbreviation for extremely luminous far-infrared sources. These are objects that are ten or a hundred times more luminous in infrared radiation than in visible light.

A galaxy with an ellipsoidal shape and no spiral structure. In most there is no evidence of interstellar matter nor sign of recent star formation. Elliptical galaxies differ only in mass and shape. Almost all their stars are older than about 10 to the power of 10 years, and much of their light comes from red giant stars. About 80 per cent of normal galaxies are elliptical.

A measure of the degree to which the shape of a planet, or other body, deviates from a perfect sphere. Planets and stars that are rotating tend to bulge at their equators to a degree that increases with speed of rotation and also depends on whether the body is solid or fluid. The shape taken up by the body is described as an oblate spheroid. A cross-section through the body that cuts through both poles is elliptical. The semiminor axis of the ellipse is the body’s polar radius, Rp, and the semimajor axis of the ellipse is its equatorial radius, Re. The ellipticity is defined as Re - Rp/ Re.

The second-brightest star in the constellation Taurus, marking the tip of one of the bull’s horns. Its name comes from the Arabic for the one butting with horns. It also represented the right foot of neighbouring Auriga, and formerly shared the alternative designation Gamma Aurigae. Elnath is a B star of magnitude 1.7.

The angular distance between the Sun and a planet or the Moon as viewed from the Earth, i.e. the angle Sun-Earth-Moon/planet. For the inferior planets, Mercury and Venus, elongation is restricted to a limited range. The maximum values, east and west, reached during each orbit are called greatest elongation. The greatest elongation for Mercury lies between 18° and 28° according to circumstances; the equivalent range for Venus is 45°-47°. Any elongation is possible for the planets further from the Sun than the Earth. An elongation of 90° is called quadrature, of 0° conjunction and of 180° opposition.

A shield volcano on Mars, one of several in the Elysium Planitia area.

A large volcanic plain on Mars, more than 5,000 kilometres 3,000 miles across.

The reappearance of a star, moon, planet or other body at the end of an occultation or eclipse.

A narrow wavelength range in a spectrum over which energy is emitted at a level above that of the surrounding continuum. Emission lines arise when transitions occur between different energy levels in the atoms or molecules of a gas, with a net release of electromagnetic energy. atom, continuous spectrum.

Any galaxy that displays emission lines in its spectrum. This is a sign of unusual activity in the nucleus of the galaxy or of very rapid star formation. active galactic nucleus.

A spectrum consisting solely of emission lines, or containing emission lines in addition to a continuous spectrum with or without absorption lines.

Any star that exhibits emission lines in its spectrum in addition to or in place of the more typical absorption lines. The symbol e in a spectral type e.g. B5e signifies the presence of emission lines. Wolf-Rayet stars, the hottest of all, have totally emission line spectra. Emission lines occur in the spectra of certain O stars and B stars and also, at the cooler end of the range, in some M stars and cool variable Mira stars.

A cloud of glowing gas in interstellar space. Interstellar clouds consist primarily of hydrogen, which can be excited or ionized by the action of the ultraviolet radiation from hot stars embedded within it. Energy is released by the recombination of ions and the collision of electrons with ionized atoms of heavier elements, such as oxygen and nitrogen, which are also present. The most intense radiation from hydrogen gives rise to the typical pinkish colour observed in ionized hydrogen clouds H II regions such as the Orion Nebula.

A satellite of Saturn, discovered by William Herschel in 1789. Voyager 2 returned images showing detail as fine as 2 kilometres. Large areas of the surface have no craters, and the density of craters in areas that do have them is relatively low. This is evidence that the surface of Enceladus has been completely remodelled by geological activity of some kind since it was first formed. Activity has almost certainly taken place within the last 100 million years; there is even a suggestion of current activity since the orbit of Enceladus coincides with Saturn’s faint E ring. Eruptions on the satellite could be the source of the material of the ring, though there is no direct evidence.

A dark gap in the bright A ring around Saturn. A Voyager 2 image showed a narrow, rather wavy ringlet within the Encke Division. In 1990, a small satellite, Pan, was found orbiting within the Division. See also: planetary rings