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

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Orion Glossary


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E

ecliptic

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.


exoplanet

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


exponent

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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