A class of metallic meteorites containing between 6 and 17 per cent nickel by weight. Octahedrites contain two different forms of iron-nickel alloy, called kamacite and taenite. The intergrowth of crystals of the two types produces an octahedral form and a characteristic pattern, called the Widmanstätten figures, that shows up clearly on the surface of a slice of meteorite that has been polished and etched with dilute acid.

A faint and obscure constellation containing the south celestial pole. It was introduced in the mid-eighteenth century by Nicolas L. de Lacaille and contains only one star brighter than fourth magnitude.

English name for the constellation Octans.

An alternative term for eyepiece.

An O star with emission lines, mainly of helium and nitrogen, in its spectrum. See also: spectral class.

Abbreviation for object glass.

An astronomical source emitting microwave radiation characteristic of the hydroxyl OH molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar space and in the cool envelopes of evolved stars. The first detection of OH was in 1963. Four microwave spectral lines, near a wavelength of 18 centimetres, were seen in absorption from the direction of the galactic centre against the radio source Sagittarius A. In 1965 emission was detected from a source in the Orion Nebula in which the relative strengths of the four lines were not as predicted by theory and as seen in normal sources. A line at a frequency of 1,665 MHz was found to be fifty times too strong; this was explained in terms of maser action. The OH molecules absorb infrared radiation then re-radiate the energy at the particular frequency of the 1,665-MHz spectral line.

The question Why is the sky dark at night? In 1826, Heinrich W. M. Olbers 1758-1840 drew attention to the fact that, in a large universe of infinite age, filled more or less uniformly with stars, the sky should be a continuous blaze of light, because every line of sight from an observer would ultimately encounter a star. The paradox is that, despite this, the sky is dark at night. The resolution of the paradox lies in identifying an underlying assumption that is wrong. Since the paradox makes assumptions about the properties of the universe, it is clear that the darkness of the night sky is of cosmological importance. The paradox is resolved in the Big Bang cosmology: the finite age of the galaxies means that there has been insufficient time to fill the universe with light. Furthermore, the expansion of the universe leads to yet more dilution of the observed sky brightness from remote objects.

New Style Date.

Magnesium iron silicate, the most abundant mineral in chondritic meteorites.

Asteroid 2201, diameter 2.8 km, discovered in 1947 by H. Giclas then lost until recovered in 1979. It is in a highly elliptical Earth-crossing orbit and has a unique spectrum that does not resemble that of any other known asteroid, meteorite or comet. Its nature is not known, but it could be the dead nucleus of a comet that has ceased to be active.

The highest peak on Mars, and the largest volcano in the solar system. It rises to a height of 27 kilometres 17 miles above the datum level selected on the basis of atmospheric pressure.This gigantic shield volcano, 700 kilometres 435 miles across, is similar in nature to volcanoes on Earth but its volume is at least fifty times greater than its nearest terrestrial equivalent. The caldera at the summit is 90 kilometres 60 miles across and a cliff at least 4 kilometres high rings the mountain. Older volcanic rocks, fractured and eroded by the wind, surround the main peak, forming an area called the aureole. Olympus Mons is located to the north-west of the Tharsis region and was formerly known as Nix Olympica Olympic Snow because the clouds over the area appeared as a light-coloured spot to Earth-based observers.

A particularly bright globular cluster of stars in the southern constellation Centaurus. Its diameter, 620 light years, is the largest of any globular cluster known. Omega Centauri lies at a distance of 16,500 light years. Through a large telescope, it is seen to spread over as much as 1° of the sky. Its total magnitude is 3.6 and it is easily visible to the naked eye. A curious feature is its shape, which is distinctly elliptical with axes in the ratio 5:4. The name Omega Centauri is of a type normally given to single stars. The cluster was mistaken for a single star by early observers at Mediterranean latitudes, for whom it never rises more than about 10° above the horizon.

A luminous nebula in the constellation Sagittarius, also known as the Horseshoe Nebula and the Swan Nebula. It is 4,800 light years away and 27 light years in diameter. It is a region of ionized hydrogen, excited by a group of at least five hot stars. A dark dust cloud lies on the western edge of the luminous region. See also: molecular cloud.

The designation of the variable star more generally known as Mira.