hydrogen alpha.

A pair of open star clusters in the constellation Perseus. They are visible to the naked eye as faint hazy patches. Their names, of a type used for individual stars, were given before their nature was known. The two clusters are very similar in appearance and are less than one degree apart in the sky. They are 7,100 light years away and estimated to be only 50 light years apart. See also: open cluster.

The strongest lines in the visible spectrum of ionized calcium, lying in the violet at wavelengths of 393.4 and 396.8 nanometres. They are conspicuous features in the spectra of many stars, including the Sun. The designations H and K were given by Fraunhofer and are still commonly used. See also: Fraunhofer lines.

An interstellar cloud of neutral hydrogen.

An interstellar cloud of ionized hydrogen.

Abbreviation for hour angle.

The second-brightest star in the constellation Centaurus. It is a giant B star of magnitude 0.6.

A sinuous channel on the Moon, running across Palus Putredinis. It is close to the landing site of the Apollo 15 mission and is believed to be a collapsed lava tube. See also: Apollo programme, rille.

The period from 10 to the power of -35 to 10 to the power of -6 seconds after the primordial Big Bang. During this interval, the behaviour of the universe was dominated by the strong interaction, the force governing the interactions between unstable elementary particles known as hadrons, which are more massive than protons and neutrons.

Name given to the satellite Corsa-B.

The formation of an unwanted halo of scattered light, for example in a defective optical system or on a photographic film, by internal reflection of light from a bright object.

A Japanese radio astronomy satellite launched on 11 February 1997. It was the first satellite for mapping celestial radio sources. Before launch, HALCA was known as Muses-B. See also: very-long-baseline interferometry.

A name used between 1970 and 1980 for a group of observatories comprising the Palomar Observatory, the Mount Wilson Observatory, the Big Bear Solar Observatory and the Las Campanas Observatory.

The 5-metre 200-inch reflecting telescope at the Palomar Observatory. Work on the telescope began in the 1930s following the award of a grant to the California Institute of Technology from the Rockefeller Foundation. Completion was delayed by World War II. It was officially opened in 1948 and dedicated to the memory of George Ellery Hale 1868-1938, who had been the driving force behind the initiation of the project.

The most famous of all periodic comets. It travels in an elongated elliptical orbit around the Sun, returning to the Earth’s vicinity in the inner solar system every 76 years. From historical records it has been shown that Halley’s Comet has been observed for over 2,200 years. Edmond Halley 1656-1742, in whose honour the comet is named, was not the comet’s discoverer but the first person to realize the connection between the comet he saw in 1682 and certain other recorded appearances of comets separated by intervals of 76 years. He calculated the orbits of a number of comets, using Isaac Newton’s newly published theory of gravitation. Noticing the similarity between the orbits of the comets seen in 1531, 1607 and 1682, he went on to predict a return in 1758-9, which was duly observed after his death. The orbit of Halley’s Comet has a perihelion distance of 0.59 AU, between the orbits of Mercury and Venus. At its most distant, it travels beyond the orbit of Neptune. The orbit is inclined to the main plane of the solar system at an angle of 162° and the comet travels around its orbit in the direction opposite to the motion of the planets. The return of 1986 was very unfavourable for observation from Earth but several countries launched space probes to investigate the comet, with considerable success. The closest approach was made by the European Giotto probe, which passed within 605 kilometres 375 miles of the nucleus on 14 March 1986. The Soviet probes Vega 1 and Vega 2 surveyed the nucleus from distances of 8,890 and 8,030 kilometres 5,550 and 5,020 miles on 6 and 9 March 1986, and information they gathered was used for last-minute corrections to the course of Giotto. Two small Japanese probes were also deployed. The results demonstrated conclusively the existence of a solid nucleus, probably made of ice and dust. It has an irregular elongated shape, reminiscent of a potato, measuring 16 × 8 kilometres 9 × 5 miles. It is also dark, reflecting only 4 per cent of incident sunlight. The nucleus rotates slowly - once in 7.1 days, with a 3.7-day precession. On the side facing the Sun, temperatures as high as 350 K were measured, enough to melt ice, and escaping jets of material were observed. Two meteor showers, the Eta Aquarids and the Orionids, are associated with Halley’s Comet.