A star of spectral type G. G stars on the main sequence have temperatures in the range 4,900-6,000 K and are yellow in colour. Many absorption lines of neutral and ionized metals feature in the spectrum, and there are some molecular bands. The Sun is a typical dwarf G star; Capella is an example of a giant G star.

A subclass of the C-type asteroids, distinguished by strong ultraviolet absorption in the spectrum.

The notion that life on Earth regulates the composition of the lower atmosphere. Gaia was a Greek Earth goddess.

The central region of our Galaxy, which cannot be seen optically because it is obscured by dense concentrations of dust. Radio and infrared observations reveal a complex environment dominated by the radio source Sagittarius A. The compact radio source Sagittarius A* appears to mark the central point and is used as the origin for galactic coordinates. Within ten light years of the centre there is a ring of gas and dust, rotating at about 110 km/s and surrounding a massive object, probably a black hole, of 4 million solar masses.

An old term, no longer in common use, for an open cluster of stars.

A region around our Galaxy extending out to a radius of about 250,000 light years.

A spherical region around a spiral galaxy. The halo round our own Galaxy extends to a radius of about 50,000 light years. It is the region within which all the stars belonging to the galaxy are found, particularly the ancient globular clusters. The halo contains very hot gas that emits X-rays. The stars outside the disc of the galaxy but within the halo are the oldest, and record the size of the galaxy before much of it collapsed to a disc.

The great circle on the sky that includes the galactic centre and the densest parts of the Milky Way. It is inclined at about 63° to the celestial equator.

The poles of the galactic plane, i.e. the points at galactic latitudes 90° north and south. The north galactic pole is in the constellation Coma Berenices at RA 12h 51.4m and Dec. 27° 7.7' (epoch 2000.0). The diametrically opposite south pole is in the constellation Sculptor.

The time taken for the Sun to complete one orbit around the galactic centre, roughly 220 million years.

A satellite of Neptune 1989 N3 discovered during the flyby of Voyager 2 in August 1989.

A family of stars, held together by their mutual gravitational attraction, and with a distinct identity separating it from other galaxies. Galaxies cover a huge range of size and mass as well as exhibiting a variety of structures and properties. The smallest galaxies known are relatively nearby dwarf galaxies containing only 100,000 stars, fewer than in a typical globular cluster. At the other end of the scale, the most massive galaxy known, the giant elliptical M87, contains 3,000 billion solar masses, about 15 times more than our own Galaxy. Most galaxies can be categorized into a number of broad morphological types. Spiral galaxies are disc-shaped, with a central bulge, from which spiral arms appear to wind outwards. In barred spirals, a bar of stars extends out from the bulge and the arms appear to be attached to the ends of the bar. Spiral galaxies contain very luminous young stars and significant amounts of interstellar material concentrated in the arms. Most of the conspicuous galaxies in the sky are spirals, but the most numerous type is the elliptical galaxy. Both the smallest and largest galaxies are of this kind. They are thought to consist entirely of old stars with relatively little interstellar material. The three-dimensional shape of galaxies in the elliptical category can be spheroidal or virtually spherical. The third main group is that of irregular galaxies, which are neither spiral nor elliptical. These account for up to a quarter of all known galaxies. At visible wavelengths, irregular galaxies show no particular circular symmetry and look chaotic. A very small number of galaxies have unusual structure, often attributable to a gravitational interaction with another galaxy. Others emit exceptionally large amounts of energy and exhibit other evidence, such as variability, suggesting that unusual and violent processes are at work. Such active galaxies include Seyfert galaxies and radio galaxies. See also: Hubble classification.

cluster of galaxies.

The four largest moons of Jupiter, Io, Europa, Callisto and Ganymede, which were discovered telescopically by Galileo in 1610. They are easily observed with the aid of a small telescope or binoculars and their orbital motion around Jupiter can be readily followed.