The prototype of a group of peculiar variable stars characterized by sudden and unpredictable dips in brightness of many magnitudes. R Coronae Borealis R CrB is normally magnitude 5.8, but every few years becomes up to nine magnitudes fainter when enshrouded in a dust cloud blown off by a strong stellar wind. About forty stars of similar type are known, typically supergiant stars of spectral type F or G.

A spectral type formerly used for stars now incorporated in the class of carbon stars.

A reference grid photographed by a separate exposure on to the same plate as astronomical images.

A process of nucleosynthesis thought to take place when there is a relatively high flux of neutrons. Heavy elements are built up by the capture of more than one neutron by a nucleus in rapid succession, before there has been time for an unstable intermediate isotope to decay. The r stands for rapid. This is the only means be which some of the heaviest known elements and particular isotopes could be made naturally. The r-process is believed to occur in the explosion of a supernova. See also: s-process.

A rare type of asteroid with moderately high albedo, of which 349 Dembowska is an example.

Abbreviation for right ascension.

Asteroid 2100, diameter 3 km, discovered by E. Helin in 1978. It is the largest known member of the Aten group of asteroids, whose orbits lie wholly within that of the Earth.

The use of pulsed radio signals in astronomical applications, such as the detection of meteor showers, the measurement of distances within the solar system and the mapping of surfaces of objects in the solar system. Radar signals transmitted by the 305-metre 1,000-foot radio telescope at Arecibo Observatory have successfully been used to map Venus and to characterize the size and shape of asteroids. The Magellan spacecraft, placed in orbit around Venus, used synthetic aperture radar to map the planet’s surface, which is concealed by opaque cloud. Radar is of fundamental importance in calibrating the distance scale within the solar system and thereby determining the value of the astronomical unit.

The velocity of an object relative to an observer as measured in the direction of the line of sight. To determine an object’s true velocity in space, it is necessary also to know the transverse velocity, which is across the line of sight. For stars, galaxies and other astronomical objects, the radial velocity is often much easier to determine than the transverse velocity because of the operation of the Doppler effect. See also: proper motion.

A graph in which the radial velocity of an object is plotted as a function of time. Such curves are used particularly in the analysis of binary star orbits, where the radial velocities of the components change with a regular periodic cycle.

A unit in which angles are measured. It is the angle subtended by an arc of a circle that has the same length as its radius. Thus, 2π radians are equivalent to 360° and one radian is approximately 57.30°.

The point on the celestial sphere from which the trails of meteors belonging to a particular meteor shower appear to radiate. Meteors entering the Earth’s atmosphere from a stream create trails that are almost parallel; the apparent divergence from a radiant is simply a perspective effect.

A ring-shaped region around a planet in which electrically charged particles electrons and protons are trapped, following spiral trajectories around the direction of the magnetic field of the planet. The radiation belts surrounding the Earth are known as the Van Allen belts. Similar regions exist around other planets with magnetic fields, such as Jupiter.

The period from one second to one million years after the Big Bang. During this interval, the behaviour of the universe was dominated by radiation i.e. photons. The era ended with the recombination epoch, when the temperature of the expanding universe had fallen to a few thousand degrees, enabling electrons and protons to form the first stable atoms.

The pressure exerted by a stream of photons when they are transferring momentum to matter. In astronomy, radiation pressure is important wherever the flux of radiation is extremely high, for example in the outer layers of a star. In the interstellar medium, radiation pressure on dust grains can be more important than the local gravitational field. Within the solar system, the pressure of radiation from the Sun acts to push the smallest particles outwards. Poynting-Robertson effect.