ESO Astronomical Glossary - A

The ESO Astronomical Glossary explains a large collection of frequently used terms in astronomy. This glossary is not intended to be complete. It includes terms that are related to ESO Telescopes and Instrumentation and which often appears in ESO Publications and Press Releases.


Absolute magnitude
The absolute magnitude of a star or other astronomical object is a measure of its intrinsic brightness. The scaling used by astronomers is defined such that a difference of 5 points on the magnitude scale corresponds to a factor of 100 difference in brightness. The lower the value, the brighter the object. The Sun has an absolute magnitude of +4.8. Sirius, the brightest star in the sky after the Sun, has an absolute magnitude of -1.6, almost a factor of 100 brighter. However, the Sun appears much brighter because of its proximity.

Accretion disc
A very common phenomenon in astronomy, accretion discs are formed when material around a massive object is flattened into a disc-shape. They form around many types of astronomical objects, such as protostars, active galactic nuclei and black holes, as surrounding gas and dust falls towards the object under the influence of gravity. Accretion discs are also often seen around the white dwarf companion of a cataclysmic variable.

Active galactic nucleus (AGN)
Active galactic nucleus, or active galaxy, is a collective term for all types of galaxies that have ultra-powerful central engines, or 'nuclei', such as quasars, blazars and Seyfert galaxies. Some AGN are among the most powerful energy sources in the entire Universe and can thus been seen to huge distances. Their engines are thought to be powered by material falling into a supermassive central black hole.

Active galaxy
See Active galactic nucleus.

Active optics
Active optics, pioneered by ESO in the mid-1980s with the 3.5-m New Technology Telescope (NTT), is a concept that allows the shape of the telescope mirrors to be monitored and readjusted during operation. The telescope mirrors are fitted with a set of piston-like actuators, that can nudge them back into shape when distortions occur. The introduction of active optics has allowed modern telescopes to deliver optimal performance in changing environmental conditions for all parts of the sky.

Adaptive optics
Adaptive optics (AO) is a mechanism by which astronomical images are corrected for effects of turbulence in atmosphere in real time during an observation. Technologically very demanding, the process relies on several hundred measurements of the disturbances in the incoming light each second, which are used to compute the adjustments needed to re-sharpen the image.

In astronomy the term afterglow indicates the remnant radiation of a gamma-ray burst. Whereas the burst itself usually lasts a matter of seconds, the afterglow, which is mostly seen as X-rays but also in other wavelength regions, fades gradually over the course of hours, days or weeks.

Albedo describes the amount of light reflected by an object when it is irradiated by the Sun, usually used for solar system bodies. An albedo of 1 indicates that all sunlight is reflected by the body; a body with albedo of 0 is perfectly black. The albedo of an asteroid, planet or satellite can give astronomers important information on the make-up of its surface.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the ESO Member States, North America and East Asia in cooperation with the Republic of Chile. ALMA, the largest astronomical project in existence, is a revolutionary telescope, comprising an array of 66 giant 12-metre and 7-metre diameter antennas observing at millimetre and submillimetre wavelengths. ALMA started scientific observations in 2011. It is located on the Chajnantor plateau of the Chilean Andes at an altitude of 5000 metres, one of the highest and driest places on Earth.

Andromeda Galaxy
The Andromeda Galaxy (also known in different catalogues as M31 or NGC 224) is the largest and closest major galaxy of the Local Group of galaxies. It is roughly twice the size of the Milky Way and can just be seen with the unaided eye.

Angular size
Angular size is the apparent width of an object as seen by an observer and can be expressed in degrees, arcminutes, or arcseconds. As a reference, the angular size of the Full Moon is 30 arcminutes, or one half of a degree. The same angular units are also often used to describe the separation of objects in the sky, as seen from Earth.

An antenna is a conductor that can transmit or receive radio waves. Radiotelescopes are antennas or a series of antennas, and function in a similar way to ordinary household radio aerials.

Antu is the name of the first Unit Telescope (UT1) of ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile. In the Mapuche language “Antu” means the Sun.

Apparent Magnitude
The apparent magnitude of an astronomical object, usually denoted by the symbol 'm', is a measure of its brightness as seen by an observer on Earth. The lower the numerical value of the apparent magnitude, the brighter an object appears. The Sun, for example, has an apparent magnitude of -26.8; Venus at its brightest one of -4.7.

Apparent Motion
The apparent motion is the observed motion of astronomical objects from Earth. The apparent motion is a combination of the intrinsic movement of the object, called its proper motion, and the movement of the Earth.

Atacama Pathfinder Experiment (APEX)
APEX is a submillimeter telescope using a single antenna, similar in design to those used in ALMA, located at the ALMA site in Chile. It is a collaboration of ESO, the Max Planck Institut für Radioastronomie (MPIfR) and the Onsala Space Observatory (OSO), and was inaugurated in 2005.

An arcminute is an angular size or distance unit. There are 60 arcminutes to a degree.

An arcsecond is an angular size or distance unit. There are 60 arcseconds to one arcminute and 3600 arcseconds to one degree. A 1 kilometer long line painted on the surface of the Moon would measure approximatey 1 arcsecond as seen from Earth.

An asteroid is a small, rocky body that orbits a star. The solar system is estimated to contain 1 to 2 million asteroids, some 130,000 of which have an official number. They form the largest group in the category of Small Solar System Bodies.

A relatively new area of research in astronomy, astrobiology is concerned with the origin, distribution and evolution of life in the Universe. It encompasses aspects of biology and geology as well as astronomy. It is sometimes referred to as exobiology.

Astronomical unit (AU)
The astronomical unit is a unit of length used by astronomers, usually to describe distances within planetary systems such as our Solar system. One AU is equal to 149,597,871 km, and corresponds to the average distance from the Earth to the Sun.

An atmosphere is a layer of gas that is retained around an astronomical body by its gravitational attraction. The term is most commonly used in relation to planets. The Earth has an atmosphere that separates us from outer space, as do several other solar system bodies, for example Jupiter, Venus and Saturn's large moon Titan. The outer gas layers of a star are also known as its atmosphere.

Auxiliary Telescope (AT)
The Auxiliary Telescope are four small moveable telescopes, each measuring 1.8 m in diameter, which are used in conjunction with the four VLT telescopes to form the VLT Interferometer (VLTI).