
ultraviolet radiation from a star within the nebula may exsite the hydrogen atoms in the cloud and cause them to glow with a light of there own. The orion nebula is such an emmisions nebula

other nebula may be dark because the dust in the cloud blocks out light from the stars and the gasses behind them

this is known as m20 a relatively faint puff of nebulosity. William Herschel was the first to note that the nebula was cut into thirds by dark lanes. These apparent divisions have led to m20 being nicked named the trifid nebula.

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