A "star shell" was a form of artillery used a means of illuminating the battlefield during the hours of darkness, and also as a means of passing signals.

When fired the star shell, which contained a fuse, would burst while at a given height igniting a magnesium flare which burned while the shell, which also contained a parachute, gradually fell to earth.

When used in this way the flare would serve to light up a wide portion of the battlefield - and in particular the spread of No Man's Land which separated the two sets of enemy front-line trenches - thus identifying any enemy patrol or wiring activity caught within the flare's boundaries.

As soon as men working in No Man's Land at night in this manner - such activity was both common and feverish - spied the ascent of a flare they would invariably throw themselves prostrate to the ground in an effort to avoid enemy detection (and resultant artillery fire) before the flare was extinguished.

Star shells used as a means of passing signals between groups were however often sent up without parachutes.  These flares were sometimes multi-coloured, different colours serving to pass along a pre-given signal.

Photograph courtesy of Photos of the Great War website.