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

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Orion Glossary

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Local Group

The grouping of galaxies in the neighbourhood of our galaxy. It contains about 40 known galaxies including the Milky Way and all the other galaxies within about three or four million light years.

local supercluster

The supercluster of galaxies centred on the rich Virgo cluster of galaxies and including the Local Group as an outlying constituent.


The amount of power emitted by a luminous object such as a star or galaxy, in the form of light or other electromagnetic radiation. It is related to the observed brightness of an object by the relationship: luminosity is proportional to brightness multiplied by distance squared. Luminosity may be measured in the unit of watts.