The strongest radio source in the constellation Virgo, identified with the giant elliptical galaxy M87, which dominates the Virgo Cluster. The radio emission is associated with a jet 4,000 light years in extent and may well be caused by the accretion of matter on to a supermassive black hole in the nucleus of M87.

The nearest rich cluster of galaxies at a distance of about 50-60 million light years and the centre of the Local Supercluster. It covers 120 square degrees of sky and contains several thousand galaxies. It is an irregular cluster with no central condensation. The giant elliptical galaxy M87 is the most massive in the cluster. A total of sixteen of the brighter members are included in the Messier Catalogue.

The statement that, in a self-gravitating system in equilibrium e.g. a cluster of stars or galaxies, the gravitational potential energy is twice the kinetic energy of the member objects. From this theorem, the masses of galaxy clusters can be estimated from their size and the average velocities of their luminous members. The VALUES (obtained are nearly ten times greater than the total mass of visible galaxies, and this has given rise to the problem of missing mass in galaxy clusters. It indicates that there are large quantities of dark matter in clusters of galaxies.

A binary star in which the two components can be resolved as separate images with a telescope of appropriate size.

The magnitude of a celestial object measured over a wavelength band corresponding to the sensitivity of the human eye. See also: V .

Abbreviation for Very Large Array.

Abbreviation of Very Long Baseline Array.

Abbreviation for very-long-baseline interferometry.

Abbreviation for Very Large Telescope.

A small and faint southern constellation introduced in the 1603 atlas of Johann Bayer with the longer name Piscis Volans, which was later shortened. Its six main stars are third and fourth magnitude.

A material that readily evaporates, such as water.

One of a pair of almost identical planetary probes launched by the USA in 1977. The other was Voyager 2. The Voyager missions were possible only because of a chance favourable alignment of the outer planets, Jupiter, Saturn, Uranus and Neptune, occurring just once in more than a hundred years. Between them, the two spacecraft were able to explore these four planets, their environs and satellite systems. The gravitational sling-shot technique was used to accelerate the craft from one encounter to the next. The missions were immensely successful, making numerous discoveries and returning huge quantities of data as well as visual images. The instruments on the Voyagers consisted of two groups. One set was designed to sample the craft’s environment and these remained in operation constantly, even between planetary encounters. They measured magnetic field, low-energy charged particles, cosmic rays and the characteristics of the local plasma. The other instruments were a wide-angle 3° camera, a close-up 0.4° camera, a Michelson interferometer to analyse infrared emission from planetary atmospheres, an ultraviolet spectrometer, a photopolarimeter to measure light intensity and its state of polarization, and a detector for radio emission from planetary magnetospheres. The main communication dish was 3.7 metres in diameter and a plutonium-238 power source was used. Voyager 1 was launched on 5 September 1977. Its closest encounter with Jupiter was on 5 March 1979 at 350,000 kilometres 217,500 miles and that with Saturn was on 12 November 1980 at 124,000 kilometres 77,000 miles. It entered the jovian system close to Io and Callisto and the saturnian system near Titan, Rhea and Mimas. After the encounter with Saturn, it left the plane of the solar system to travel on into interstellar space. See also: Voyager Interstellar Mission.

One of a pair of planetary probes launched by the USA in 1977. It was virtually identical to Voyager 1, except that its power source was designed to last for much longer in order to survive the extended journey to Uranus and Neptune. Voyager 2 was launched on 20 August 1977. Its first encounter was with Jupiter on 9 July 1979, passing within 71,400 kilometres 44,000 miles of the planet. It passed close to Europa and Ganymede, complementing the coverage of the Galilean satellites obtained by Voyager 1. Saturn was reached in August 1981. Closest approach was on 25 August, at a distance of 101,000 kilometres 63,000 miles. The trajectory took the probe near the saturnian satellites Tethys and Enceladus. On 24 January 1986 Voyager 2 reached Uranus, which it passed at a distance of 107,000 kilometres 67,000 miles and the mission was completed with the Neptune and Triton encounter of 24 August 1989, when the craft passed within 48,000 kilometres 30,000 miles of the surface of Neptune. See also: Voyager Interstellar Mission.

The continued operation of the Voyager 1 and Voyager 2 spacecraft after their planetary encounters were completed. The power sources on both craft are expected to sustain operation until about 2020. As long as power is available, instruments on board continue to detect particles in the heliosphere, and to measure magnetic field. It is anticipated that they will gather the first scientific data from the region of the heliopause, where the solar wind merges with the interstellar medium.

Abbreviation for VLBI Space Observatory Programme. See also: very-long-baseline interferometry.