ESO Astronomical Glossary - E


An eclipse occurs when the moon blocks the light from the Sun (solar eclipse) or the Earth's shadow falls on the moon (lunar eclipse). Eclipses also occur in binary star systems, when one of the stars passes in front of the other.

Electromagnetic radiation
Electromagnetic radiation is a form of energy in the form of waves. Visible light is the most familiar form on Earth, but the radiation is seen over a very wide rane of energies, called the electromagnetic spectrum.

Electromagnetic spectrum
The electromagnetic spectrum denotes the full range of energies electromagnetic radiation is observed at. Many of these types of radiation are used in everyday life, such as radio waves, microwaves, and X-rays. In astronomy, the wavelength of radiation is often used to refer to the wave's energy as the two quantities are related in an inverse manner: the longer the wavelength, the lower the energy. The table below lists the types of radiation observed in order of descending energy.

Gamma rays below 0.01 nm
X-Rays 0.01 to 20 nm
Ultraviolet (UV) 20 - 400 nm
Visible light 400 - 700 nm
Infrared (IR) 700 nm - 0.3 mm
Radio 0.3 mm and longer

Radio waves form a very broad category, which includes the important submillimetre (0.3 - 1 mm) and microwave regions (1 mm - several cm). Some regions are further divided into sub-regions according to their proximity to the visible spectrum (e.g. near-infrared, near-ultraviolet) or according to energy (e.g. extreme ultraviolet, hard and soft X-rays)

Elliptical galaxy
As the name suggests, elliptical galaxies are galaxies that are elliptical, or oval, in shape. They do not show the distinctive features of spiral galaxies, and don't contain much gas and dust, implying that not many stars have formed inside them. Their centres are likely to house supermassive black holes.

Emission nebula
An emission nebula is a type of nebula that 'glows'. Emission nebulae are formed when energetic ultraviolet light from one or more very hot nearby stars heats the gas to such an extent that it begins to emit light of its own. This sets them apart from reflection nebulae, which simply reflect and scatter starlight. The main types of emission nebulae are planetary nebulae and HII regions.

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates telescopes at three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor.

An exoplanet (short for extrasolar planet) is a planet found outside of our solar system.

Extremely Large Telescope (ELT)
The Extremely Large Telescope (ELT) is a 39-metre telescope under construction by ESO at Cerro Armazones, 20 kilometres from the VLT. It will be the largest optical/near-infrared telescope in the world: “the world’s biggest eye on the sky”. The ELT programme was approved in 2012 and green light for construction was given at the end of 2014. First light is targeted for 2024.