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In the night sky: Orion
In the night sky: Orion

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Orion Glossary

Browse the glossary using this index

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The plane of the Earth's orbit about the Sun. Also the path of the Sun in the sky (on the celestial sphere) during the course of a year.

electromagnetic radiation

A type of radiation that includes visible light and travels through empty space at the speed of light. All forms of electromagnetic radiation consist of wave-like patterns of electric and magnetic disturbances but interact with matter (i.e. are emitted or absorbed) as a stream of particles, called photons.

electromagnetic spectrum

The entire range of electromagnetic radiation from radio waves through microwaves, infrared radiation, light, ultraviolet radiation, and X-rays to gamma rays.

electron degeneracy pressure

An effect of quantum physics that provides the support against gravity in a white dwarf star.

elliptical galaxy

A galaxy that appears as an elliptical distribution of stars (typically old stars).

emission line

A spectral line produced when atoms of a particular type emit photons with a specific energy.

emission spectrum

A spectrum containing emission lines. A pattern of narrow bright lines at specific wavelengths, generally superimposed on a dark background. The wavelengths of the lines are characteristic of the composition of the excited gas that produces them.

event horizon

The boundary around a black hole within which the escape velocity exceeds that of light. Note that this does not consist of a solid, physical surface.


A planet orbiting a star other than the Sun. Shortened version of extrasolar planet.


The power to which a number or quantity is raised. For example, 103 is read as 'ten to the power 3' and means 10 x 10 x 10, where the exponent (or index) is '3'.

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