An A star that shows emission lines of hydrogen superimposed on the absorption lines in its spectrum. Such emission may arise from interactions between the two members of a close binary system.

An alternative, largely obsolete name for a stony meteorite.

The study of the physical and chemical processes in the upper atmosphere of theEarth, or of other planets.

A suspension of particles in a gas, such as mist in an atmosphere.

Alternative spelling of ether.

Abbreviation for asymptotic giant branch.

The time when the spring equinox, the point where the Sun crosses from south to north of the celestial equator, will lie in Aquarius. The crossover point gradually moves against the stars, owing to precession, Earth’s slow wobble in space. The crossover point lay in Aquarius over 2,000 years ago. It is now in Pisces. It will reach Aquarius nearly 600 years from now.

An alternative name for the star Hadar.

Abbreviation for the star catalogue Astronomische Gesellschaft Katalog. The first project, for cataloguing all stars between declinations -2° and +80° down to ninth magnitude, was initiated by F. W. Argelander in 1867. Seventeen observatories participated and the fifteen volumes of AGK1 listed 150,000 stars. An extension to declination -23° published in 1887 contained a further 50,000 stars. A second version was started in the 1920s, using photography rather than visual observations, and was published in 1951-58. Measurements of proper motions were included in the third version, AGK3. AGK positions are accurate positions, calculated relative to fundamental stars. See also:

Abbreviation for active galactic nucleus.

The largest meteorite displayed in any museum, housed at the Hayden Planetarium in New York City. It was found by Robert Peary in Greenland in 1897 and is of the iron type. Its weight is 31 tonnes.

Delta Scuti star.

English name for the constellation Antlia.

A proliferation of high-energy charged particles in the atmosphere, triggered by the collision of a cosmic ray particle with an atomic nucleus in the atmosphere. The first collision produces a number of secondary particles, all of which have considerable energy and may subsequently undergo further collisions.

The light produced and emitted by the Earth’s own atmosphere, excluding thermal radiation, aurorae, lightning and meteor trains. The spectrum of the airglow ranges from 100 nanometres to 22.5 micrometres. A major component is an emission line due to oxygen at 558 nanometres, which originates in a layer 30 to 40 kilometres thick at a typical height of 100 kilometres 60 miles. From space, the airglow appears as a ring of greenish light around the Earth.