An unusual variable star, the spectral lines of which have a particularly characteristic profile interpreted in terms of an expanding envelope around the star. P Cygni is a recurrent nova. It was recorded as third magnitude in August 1600 and stayed at this brightness for six years before fading slowly. A second outburst occurred in about 1655, which was again followed by slow fading. It subsequently fluctuated in brightness around sixth magnitude and has been about fifth magnitude, with only small variations, since 1715. The lines in the spectrum of P Cygni are all double, consisting of a broad emission line with a narrower absorption line adjacent on the blue side. The absorption comes from starlight passing through surrounding shells of material, while the emission comes from the portions of the shells either side of the central star as viewed from the Earth. The emission and absorption components are displaced from each other by the Doppler effect because the shells are expanding. Detailed analysis has shown that there are three distinct shells, the outermost of which is pulsating with a 114-day period. Similar line profiles are observed in the spectra of other objects surrounded by expanding envelopes and are described as P-Cygni profiles.

A type of asteroid with low albedo, common in the outer main belt.

Abbreviation for position angle.

English name for the constellation Pictor.

A circular spot on an icy moon believed to be the ghost of a former impact crater.

Asteroid 2, mean diameter 533 km, discovered by Heinrich W. M. Olbers in 1802. It is the second-largest asteroid and of the carbonaceous type, which is similar to the largest asteroid, Ceres. Its orbit is at the unusually steep inclination of 35° to the plane of the solar system.

A class of stony-iron meteorite.

The observatory on Palomar Mountain in California where the 5-metre 200-inch Hale Telescope is sited. It is owned and operated by the California Institute of Technology Caltech. The other instruments at the observatory are the 1.2-metre 48-inch Oschin Telescope a Schmidt camera, a 46-centimetre 18-inch Schmidt camera and the 1.5-metre 60-inch reflector owned jointly by Caltech and the Carnegie Institution of Washington.

A photographic atlas of the whole sky north of declination -30° consisting of wide-field plates taken with the 1.2-metre 48-inch Schmidt camera the Oschin Telescope at Palomar Observatory. There are two plates, one taken in red light and one in blue, for each area of sky.

Literally swamp or marsh, a term used in the names of certain dark features on the Moon. Its use dates from a time when it was believed that the darker features on the Moon were of liquid water; this is now known to be untrue. In recognition of its use over a long period, the term has been retained in certain official names for features on the Moon. See also: mare.

A small satellite of Saturn 1981 S13 orbiting in the Encke Division in the planet’s ring system. It was found by Mark R. Showalter in 1990 from studies of images taken by the spacecraft Voyager 1 and Voyager 2. Its existence had been predicted as an explanation for observed structure in the rings around the Encke Division. Pan is about 20 kilometres in diameter.

A small satellite of Saturn, measuring about 110 × 70 kilometres, discovered by Voyager 2 in 1980.

A particular form of open curve belonging to the family of curves known as conic sections. It is encountered in astronomy in two main contexts. First, it is one of the shapes that can be taken by the orbit of a body moving under the influence of a central gravitational force, such as a comet when it approaches the Sun. Second, the surface obtained by rotating a parabola, a paraboloid, is commonly used as the shape for the primary mirror in a reflecting telescope. Images formed by such parabolic mirrors, in contrast to spherical mirrors, do not suffer from spherical aberration.

A mirror whose surface is figured to the shape of a paraboloid. See also: parabola.

The change in the relative positions of objects when they are viewed from different places. The actual angular shift measured when a viewpoint is changed is also described as trigonometric parallax. In the case of astronomical objects, such changes are measurable only for relatively nearby objects in relation to the more distant stars. However, the measurement of parallaxes, where possible, is important since it is one of the most direct methods of determining astronomical distances. In astronomy, the word parallax is often used synonymously with distance. The rotation of the Earth produces a diurnal parallax effect, and the Earth’s orbital motion around the Sun causes annual parallax. See also: statistical parallax.