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

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Orion Glossary

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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'.


Of or pertaining to the Universe outside our own galaxy.

extrasolar planet

A planet orbiting a star other than the Sun. Also abbreviated to exoplanet.



A highly energetic form of solar activity, typically lasting a few hours, caused by the sudden release of magnetic energy in the corona.

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