The process by which the intensity of radiation decreases as it passes through a material medium. The energy lost by the radiation is transferred to the medium. Many physical processes observed in astronomy involve absorption, including absorption of light from celestial objects in the Earth’s atmosphere atmospheric extinction, interstellar extinction and absorption within the gaseous layers of a star, resulting in an absorption line spectrum.

A measure of the ability of a material to absorb radiation that passes through it. A high absorption coefficient indicates that the material absorbs radiation effectively. The absorption coefficient may depend strongly on the wavelength of the radiation, the temperature and other physical conditions.

A sharp dip in intensity over a narrow wavelength range in a continuous spectrum. In a spectrum produced by a typical spectrograph, in which the light passes through a narrow slit before being dispersed, absorption lines have the appearance of dark lines cutting across at right angles to the direction of dispersion. Absorption lines are a characteristic of the spectra of the majority of stars. In the case of the Sun, they are known as Fraunhofer lines. Atoms are able to absorb radiation at a number of precise wavelengths. The wavelengths at which absorption occurs are different for each chemical element, making it possible to identify the elements present in a star, or other celestial body, by analysing which spectral lines are present. The strength of the lines can be used to deduce the abundance of the elements, though not directly since the temperature, density and other physical circumstances greatly influence the strength of absorption lines in a spectrum. See also: emission line.

A continuous spectrum in which narrow absorption lines can be seen.

A dark interstellar cloud that absorbs the light from bright objects behind it. Absorption nebulae range in size from small globules to large clouds visible to the unaided eye, such as the Coalsack in the southern Milky Way. Absorption nebulae contain both dust and gas, and the temperatures in them are low enough for simple molecules to form. Much of the knowledge about these nebulae comes from observations of infrared and radio radiation, which, unlike visible light, can pass through them. Strong evidence has been found that the initial stages of star formation take place in such dark nebulae. See also: molecular cloud.

A spectrum produced when electromagnetic radiation is absorbed. When radiation from a hot source passes through cooler matter, radiation at certain wavelengths is absorbed, proucing dark absorption lines and bands.

A disc structure that forms around a spinning object, such as a star or black hole, when material falls on to it from a close companion in a binary star system. As stars evolve they enter a giant phase, when their size increases dramatically. In a binary system, the gravitational pull of the companion star on the bloated envelope of the giant may be stronger than the force holding the giant star and its envelope together. Under these circumstances, material flows across from one star to the other. The presence of an obscuring accretion disc may cause the star to be variable, as with Beta Lyrae stars, and this may be detectable from features in the spectrum. If the companion is spinning rapidly, as will a collapsed star a white dwarf, neutron star or black hole, a swirling accretion disc forms on to which the material falls. The kinetic energy of the falling matter is turned into heat, and X-rays are produced. A mechanism of this kind is thought to be responsible for the production of X-rays in objects such as Cygnus X-1. See also: Roche lobe, SS433.

Accretion is the increase in mass of a celestial body by the addition of matter or smaller objects. An accretion disk is a disk of material surrounding an accreting object. Accretion often occurs by mans of an accretion disk. Accretion disks occur in close binary star systems in which gas flows from one star to the other, and at the centers of galaxies. They play a prominent part in active galactic nuclei.

Abbreviation for Advanced Composition Explorer.

The brightest star in the constellation Eridanus. The name, which is of Arabic origin, means the end of the river and the star marks the southern extremity of the constellation at a declination of -57°. Achernar is a B star of magnitude 0.5.

Asteroid 588, diameter 116 km. Discovered by M.Wolf in 1906, it was the first of the Trojan asteroids to be identified.

A type of stony meteorite that crystallized from molten rock. The name indicates the absence of chondrules in this type of meteorite, in contrast with chondrites. Compared with chondrites, they have a greater abundance of calcium-rich minerals and much less of the metal and sulphide minerals.

A composite lens, made up of two elements of different kinds of glass, designed to reduce chromatic aberration. See also: apochromat.

A dark plain in the northern hemisphere of Mars, formerly known as Mare Acidalia.

Pressure waves in a fluid. Acoustic waves can propagate through the gaseous layers of stars including the Sun. They are produced where pressure is the dominant force restoring the balance when there is a small displacement of the gas.