A supposed spherical shell, surrounding the solar system at a distance of about 1 light year 50,000 AU, containing billions of comets with a total mass about that of the Earth. The cloud is invoked as a source for the comets, observed in the inner solar system, which could be deflected there by the gravitational influence of a passing star. The idea was first put forward by E. Öpik in 1932 and developed by J. Oort in the 1950s. The term Öpik-Oort cloud is sometimes used. There is no direct evidence for the existence of the cloud, other than the need to explain an origin for comets. If it exists, it is unclear how it formed, though a popular theory is that the incipient comets were formed near the present location of the outer planets and ejected to a much greater distance later.

The mass density in the plane of the Galaxy near the Sun’s locality, as calculated from the velocities and distribution of stars in relation to the gravitational field of the galactic disc. At one time it was believed that the mass density deduced in this way exceeded the amount that could be seen directly in the form of stars and interstellar material. However, the discovery of fainter stars and more interstellar material has brought the observed and calculated VALUES (of local mass density closer together, implying little, if any, hidden matter in the solar neighbourhood. The name is that of the Dutch astronomer, Jan Oort, distinguished for his work in the field of galactic dynamics.

Two empirical quantities contained in mathematical expressions derived by the Dutch astronomer, Jan Oort, for the radial velocity and proper motion of stars resulting from their orbital motion around the centre of the Galaxy, which is assumed to be circular. The expressions take the form ν = A r sin 2l and μ = 0.211B + A cos 2l where ν is radial velocity, μ proper motion, r distance from the Sun and l the star’s galactic longitude. A and B are Oort’s constants; generally accepted VALUES (are 15 kilometres per second per kiloparsec for A and -10 kilometres per second per kiloparsec for B.

A measure of the extent to which a material absorbs and scatters incident electromagnetic radiation. It is dependent on wavelength, though a mean value may be used to simplify calculations.

A type of star cluster containing several hundred to several thousand stars distributed in a region a few light years across. The member stars are much more spaced out than in globular clusters. Open clusters are relatively young, typically containing many hot, highly luminous stars. They are located within the disc of the Galaxy and so appear to lie within the Milky Way. Well-known open clusters include the Pleiades, the Hyades and the Jewel Box. See also: Trumpler classification.

A model of the universe in which expansion continues for ever and reaches infinity with a non-zero speed. See also: oscillating universe, closed universe.

A small satellite of Uranus discovered by the Voyager 2 spacecraft in 1986. Ophelia is one of two satellites that act as shepherds of the planet’s Epsilon ring the other being Cordelia.

A large constellation straddling the celestial equator. It was one of the 48 constellations listed by Ptolemy c. AD 140. The mythological figure of the serpent holder is sometimes identified with the healer Aesculapius. Though Ophiuchus is not traditionally a zodiacal constellation, the ecliptic passes through its southern part. It contains five stars of second magnitude and seven of third magnitude. Barnard’s Star also lies in Ophiuchus.

The position of one of the superior planets when it is opposite the Sun in the sky, i.e. when its elongation is 180°. At opposition planets are at full phase and reach their highest point in the sky at midnight. At the same time, they achieve their closest approaches to the Earth. As the orbits of the planets are elliptical rather than perfectly circular, some oppositions bring the planets closer to Earth than others. This effect is particularly marked with Mars.

The additional brightening of an asteroid observed at full phase in excess of the brightness that would be expected from measurements made at partial phases.

A measure of the degree of absorption that takes place when electromagnetic radiation travels along a specified path through a gaseous or dusty medium, such as the atmosphere of a star or planet.

A pair of stars that lie close to each other in the sky by chance, but are not physically associated with each other as with a true binary star.

interferometer.

The path followed by a body moving in a gravitational field. For bodies moving under the influence of a centrally directed force, without significant perturbation, the shape of the orbit must be one of the conic section family of curves - it must be a circle, an ellipse, a parabola or a hyperbola.

A set of parameters that together completely define the shape, orientation and timing of orbital motion. Those most commonly used for the orbits of planets and comets around the Sun are semimajor axis, perihelion distance, eccentricity, inclination, argument of perihelion, longitude of the ascending node and period. In order to determine the position of an object at a specific time, it is also necessary to know the time when a particular point in the orbit is reached, such as perihelion passage. Analogous elements are used to describe the orbits of stars in binary systems or of satellites natural or artificial orbiting planets. The standard orbital elements are detailed in the table and the illustration, which depicts an elliptical orbit in the solar system. Here P is the perihelion point, N the ascending node and the direction of the First Point of Aries.