A double-lobed nebula surrounding a star in Cygnus, interpreted as an early stage in the formation of a planetary nebula. It was discovered by Rudolph Minkowski in 1946.

An alternative name for an asteroid. See also: major planet.

One of the three stars forming the belt of Orion. At magnitude 2.2, it is the seventh-brightest star in the constellation. It is actually an eclipsing binary and varies in brightness by 0.1 magnitude in a period of 5.7 days. There is also a seventh magnitude visual companion. The primary star is a supergiant O star. Mintaka, a name of Arabic origin, means the belt.

A unit of time equal to sixty seconds.

arc minute.

A Soviet space station, launched into Earth orbit in 1986.

The prototype of a class of long-period variable stars. The name is Latin for wonderful. Mira was the first variable star to be discovered: the Dutch astronomer David Fabricius noted it at third magnitude in 1596, but found it invisible to the naked eye a few months later. He noted it again at third magnitude in 1609. Mira is a giant M star that varies between second and tenth magnitudes in a period of about 332 days, though there are irregularities in the period and in the maximum and minimum brightnesses reached. It is shedding large amounts of gas and dust, which form a strong stellar wind. Ultraviolet and visible images taken with the Hubble Space Telescope have resolved Mira’s white dwarf companion, and have shown that the primary star has an elongated asymmetrical shape. It appears that material is being transferred to the white dwarf.

A member of a class of long-period variable stars, of which Mira is the prototype.

The second-brightest star in the constellation Andromeda. It is a giant M star of magnitude 2.1. The name, of Arabic derivation, means girdle.

A satellite of Uranus, the smallest of those known prior to the Voyager 2 encounter in 1986. It was discovered by Gerard Kuiper in 1948. The spacecraft passed Miranda at a distance of only 3,000 kilometres 1,800 miles, returning very detailed images of its surface. Though only 470 kilometres in diameter, Miranda has several contrasting types of terrain. Alongside cratered areas, typical of planets and satellites, there are large tracts of grooves and ridges. It seems unlikely that such variety could have been caused by geological activity in such a small satellite. One theory suggests that the satellite was once shattered by a massive impact into several parts that subsequently coalesced again.

The brightest star in the constellation Perseus. It is a yellow supergiant F star of magnitude 1.8. The name, of Arabic origin, means the elbow.

An element in an optical system, designed for the purpose of reflecting light or other electromagnetic radiation. The high degree of accuracy and reflectivity needed for astronomical mirrors used in optical telescopes is achieved by grinding and polishing to the required surface shape a piece of glass which is then coated with a thin layer of aluminium. Formerly, silver was used rather than aluminium, but it is less durable and oxidizes more quickly than aluminium. When light falls on any glass/air interface, part is reflected, part transmitted and part absorbed. Mirrors are designed to achieve the maximum reflection. Strong internal reflection may also take place at a surface in a glass prism. This phenomenon is used, for example, in prismatic binoculars.

The second-brightest star in the constellation Canis Major. It is a giant B star of magnitude 2.0 and the prototype of the class of the slightly variable Beta Canis Majoris stars. It changes in brightness by a few hundredths of a magnitude every six hours. This low level of variability cannot be detected by the naked eye alone.

Matter, as yet undetected, that would need to be present in the universe to stabilize it against infinite expansion. Missing mass, if it exists, must be in the form of dark matter.

A chaotic model for the early universe in which giant convulsions and oscillations allow light to circumnavigate the universe and transform a non-uniform universe into a uniform one. This was found not to work.