A star of spectral type A. A stars have surface temperatures in the range 7,500-11,000 K and are white in colour. The most prominent features in their absorption line spectra are the Balmer lines due to hydrogen atoms. Lines of heavier elements, such as iron, are also noticeable at the cooler end of the temperature range. Examples of A stars are Sirius and Vega.

A rare type of asteroid characterized by a moderately high albedo and an extremely red colour. Strong absorption in the near-infrared spectrum is interpreted as indicating the presence of olivine.

Abbreviation for Anglo-Australian Observatory.

Abbreviation for American Astronomical Society.

Abbreviation for Anglo-Australian Telescope.

Abbreviation for American Association of Variable Star Observers.

A catalogue of 2,712 rich clusters of galaxies drawn up by George Abell from the Palomar Sky Survey photographs. He demonstrated that there are two types of galaxy cluster: a compact type that is regular in shape and a more spread-out, irregular type.

An imperfection in the imaging properties of a lens or mirror. The main aberrations are chromatic aberration, spherical aberration, coma, astigmatism, curvature of the field and distortion.

An apparent displacement in the observed position of a star. It is a result of the finite speed of the light travelling from the star, combined with the motion through space of the observer on Earth relative to the star, etc. Aberration arising from the Earth’s orbital motion around the Sun is termed annual aberration. The much smaller component that results from the daily rotation of the Earth is called diurnal aberration.

The erosion of a surface through a process such as vaporization or friction. For example, when a meteoroid enters a planetary atmosphere, ablation occurs because of friction between the surface and its gas molecules in the atmosphere.

A measure of the actual rate of energy output of a star or other celestial object as opposed to the apparent luminosity, which depends on the distance to the object.

The supposed brightness an object in space would be at a distance of 10 parsecs, or 33 light years; a measurement obtained by finding the objects apparent magnitude m and it’s parallax PI in arcseconds with the formula M = m + 5 + 5logPI. Absolute magnitude is used by astronomers to compare the brightness of different objects in space.

The magnitude a star would appear to have if it were at a standard distance of 10 parsecs. Absolute magnitudes are a method of comparing the actual luminosities of stars on an arbitrary scale. For an asteroid or comet, the absolute magnitude is the apparent magnitude it would have at zero phase angle and at a distance of 1 AU from both the Sun and Earth.

The point at which all molecular motion ceases and so, theoretically, the lowest possible temperature. It is the zero point of the Kelvin temperature scale used in science. The equivalent on the Celsius scale is -273.16°C.

A dark line or band in a spectrum that is caused by the absorption of electromagnetic radiation with a particular wavelength. Dark lines appear in the spectrum when a light source with a continuous spectrum is viewed through a cool gas. The wavelengths of these dark lines are identical to the wavelengths of emission lines from the same gas when heated. The spectra of the Sun and other stars have absorption lines. Most are Fraunhofer lines, but some arise in the cold interstellar gas along the line of sight and give clues to the physics and chemistry of our Galaxy’s interstellar medium. The spectra of remote quasars have absorption lines which provide information about intergalactic space.